Mechanisms Underlying Graft Union Formation and Rootstock Scion Interaction in Horticultural Plants

Abstract

Grafting is a common practice for vegetative propagation and trait improvement in horticultural plants. A general prerequisite for successful grafting and long term survival of grafted plants is taxonomic proximity between the root stock and scion. For the success of a grafting operation, rootstock and scion should essentially be closely related. Interaction between the rootstock and scion involves complex physiological-biochemical and molecular mechanisms. Successful graft union formation involves a series of steps viz., lining up of vascular cambium, generation of a wound healing response, callus bridge formation, followed by vascular cambium formation and subsequent formation of the secondary xylem and phloem. For grafted trees compatibility between the rootstock/scion is the most essential factor for their better performance and longevity. Graft incompatibility occurs on account of a number of factors including of unfavorable physiological responses across the graft union, transmission of virus or phytoplasma and anatomical deformities of vascular tissue at the graft junction. In order to avoid the incompatibility problems, it is important to predict the same at an early stage. Phytohormones, especially auxins regulate key events in graft union formation between the rootstock and scion, while others function to facilitate the signaling pathways. Transport of macro as well as micro molecules across long distances results in phenotypic variation shown by grafted plants, therefore grafting can be used to determine the pattern and rate of recurrence of this transport. A better understanding of rootstock scion interactions, endogenous growth substances, soil or climatic factors needs to be studied, which would facilitate efficient selection and use of rootstocks in the future. Protein, hormones, mRNA and small RNA transport across the junction is currently emerging as an important mechanism which controls the stock/scion communication and simultaneously may play a crucial role in understanding the physiology of grafting more precisely. This review provides an understanding of the physiological, biochemical and molecular basis underlying grafting with special reference to horticultural plants.

Full text

fpls-11-590847 December 5, 2020 Time: 13:8 # 1
REVIEW
published: 10 December 2020
doi: 10.3389/fpls.2020.590847
Edited by:
Rosario Paolo Mauro,
University of Catania, Italy
Reviewed by:
Wenna Zhang,
China Agricultural University, China
Qiusheng Kong,
Huazhong Agricultural University,
China
Ana Pina,
Agrifood Research and Technology
Centre of Aragon (CITA), Spain
*Correspondence:
Parvaiz Ahmad
pahmad@ksu.edu.sa
Specialty section:
This article was submitted to
Crop and Product Physiology,
a section of the journal
Frontiers in Plant Science
Received: 06 August 2020
Accepted: 23 October 2020
Published: 10 December 2020
Citation:
Rasool A, Mansoor S, Bhat KM,
Hassan GI, Baba TR, Alyemeni MN,
Alsahli AA, El-Serehy HA, Paray BA
and Ahmad P (2020) Mechanisms
Underlying Graft Union Formation
and Rootstock Scion Interaction
in Horticultural Plants.
Front. Plant Sci. 11:590847.
doi: 10.3389/fpls.2020.590847
Mechanisms Underlying Graft Union
Formation and Rootstock Scion
Interaction in Horticultural Plants
Aatifa Rasool1, Sheikh Mansoor2, K. M. Bhat1, G. I. Hassan1, Tawseef Rehman Baba1,
Mohammed Nasser Alyemeni3, Abdulaziz Abdullah Alsahli3, Hamed A. El-Serehy4,
Bilal Ahmad Paray4and Parvaiz Ahmad3*
1Department of Fruit Science, Sher-e-Kashmir University of Agricultural Sciences and Technology of Kashmir, Srinagar, India,
2Division of Biochemistry, Faculty of Basic Science, Sher-e-Kashmir University of Agricultural Sciences and Technology
of Kashmir, Srinagar, India, 3Botany and Microbiology Department, College of Science, King Saud University, Riyad,
Saudi Arabia, 4Department of Zoology, College of Sciences, King Saud University, Riyad, Saudi Arabia
Grafting is a common practice for vegetative propagation and trait improvement
in horticultural plants. A general prerequisite for successful grafting and long term
survival of grafted plants is taxonomic proximity between the root stock and scion.
For the success of a grafting operation, rootstock and scion should essentially
be closely related. Interaction between the rootstock and scion involves complex
physiological-biochemical and molecular mechanisms. Successful graft union formation
involves a series of steps viz., lining up of vascular cambium, generation of a wound
healing response, callus bridge formation, followed by vascular cambium formation
and subsequent formation of the secondary xylem and phloem. For grafted trees
compatibility between the rootstock/scion is the most essential factor for their better
performance and longevity. Graft incompatibility occurs on account of a number
of factors including of unfavorable physiological responses across the graft union,
transmission of virus or phytoplasma and anatomical deformities of vascular tissue at
the graft junction. In order to avoid the incompatibility problems, it is important to predict
the same at an early stage. Phytohormones, especially auxins regulate key events in
graft union formation between the rootstock and scion, while others function to facilitate
the signaling pathways. Transport of macro as well as micro molecules across long
distances results in phenotypic variation shown by grafted plants, therefore grafting can
be used to determine the pattern and rate of recurrence of this transport. A better
understanding of rootstock scion interactions, endogenous growth substances, soil
or climatic factors needs to be studied, which would facilitate efficient selection and
use of rootstocks in the future. Protein, hormones, mRNA and small RNA transport
across the junction is currently emerging as an important mechanism which controls the
stock/scion communication and simultaneously may play a crucial role in understanding
the physiology of grafting more precisely. This review provides an understanding of the
physiological, biochemical and molecular basis underlying grafting with special reference
to horticultural plants.
Keywords: grafting, incompatibility, phytohormones, callus bridge, rootstock-scion
Frontiers in Plant Science | www.frontiersin.org 1December 2020 | Volume 11 | Article 590847

fpls-11-590847 December 5, 2020 Time: 13:8 # 2
Rasool et al. Grafting in Horticultural Plants
INTRODUCTION
Grafting has been performed in agriculture since the beginning of
civilization. Historical records have revealed that ancient Chinese
and Greeks have been practicing it since 1560 B.C. (Melnyk
and Meyerowitz, 2015). Since fruit and nut trees are difficult
to propagate by cuttings, grafting is used for their propagation.
Moreover, the superiority and quality of the grafted crops further
led to widespread adoption of this technique. It is a well-
established practice which makes it possible to physically join
two or more genetic entities in a single tree to influence the
productivity characters of a tree favorably and facilitates asexual
propagation in horticultural crops like apple, pear, plum and
cherry (Figures 1A–D;Kumari et al., 2015). A “de novo” formed
meristematic area must develop between scion and rootstock
for a successful graft union. The scion becomes the new shoot
system and the rootstock (under stock, stock) forms the root
system of the grafted plant. Scions are selected based on yield
related traits and are generally grafted over specific rootstocks
having the ability to survive the biotic and abiotic components
of the environment. Rootstock mostly influences scion vigor and
its water relations. Grafting is usually practiced in perennial
horticultural trees with the main aim to reduce vegetative
growth and shorten the juvenile period. Additional benefits of
grafting include dwarf tree structures to increase the planting
density per unit area and hence productivity simultaneously with
minimal investments in orchard cultural practices like pruning,
pest and foliar disease control. For an efficient root system to
develop the rootstock and scion compatibility plays a crucial
role (Goldschmidt, 2014;Warschefsky et al., 2016). However,
rootstock and scion compatibility vary to an extent that even
the closely related species might not be compatible therefore, it
becomes necessary to evaluate the compatibility before grafting
a particular scion into a rootstock (Lee et al., 2010;Guan
et al., 2012). Success of a grafting operation depends on the
strength of the union formed. Stronger unions result in successful
grafting operation. On the contrary, weaker unions lead to
graft failure and in adverse cases the trees may fall apart. Graft
union formation depends on a number of factors viz., molecular
pathways and physiological/biochemical responses. Lot of effort
has been put into studying the physiological mechanism of union
formation, causes and consequences of graft incompatibility
and also as to how molecules are being transferred across the
graft unions to reveal the mechanisms responsible for inducing
the phenotypic changes by grafting. In this review we not
only get an idea about the fundamental mechanism of graft
union formation, graft incompatibility: its types, mechanism and
causes, but it also makes some of the critical molecular and
physiological mechanisms associated with grafting much easier
for us to understand.
GRAFTING TECHNIQUES IN FRUIT
TREES
Grafting has become immensely important in view of improving
cultivation especially of fruit and vegetable crops. Besides this,
grafting tends to improve adaptability and resistance of plants
to different environments and stresses (Kumar et al., 2017).
The latter can be achieved by using a suitable rootstock.
However, not every grafting operation is successful. Apart from
a number of factors including stock scion combination used,
season, temperature, etc., the technique of grafting followed is
the most important one to determine the degree of grafting
success (Soleimani et al., 2010). Grafting techniques include
side, tongue, cleft, bark, and splice grafting methods (Figure 2).
Among these cleft grafting also known as wedge grafting is the
most commonly used method. Additionally, budding is also a
form of grafting in which the scion size is reduced to a small piece
of stem with one or more axillary bud attached to it. In general,
success of a grafting operation depends on combining anatomical
structures of the stock and scion, so much so that if there is
some dislocation of vascular elements, weak or distorted unions
may get formed eventually leading to graft failure (Wang, 2011).
Thus, the choice of an appropriate grafting method is critical
to ensure proper contact between stock and scion and to avoid
the formation of incompatible or weak graft unions. Depending
upon the existing environmental conditions the success of any
grafting method may vary from one crop to another. Maximum
success percentage i.e., 100% was obtained in mango by following
cleft grafting technique in the month of June or March. On
the other hand, Allan et al. (2010) reported that in papaya side
grafting brings about 80% success rate while Nguyen and Yen
(2018) recommended cleft grafting using 1-month old rootstocks
as the best method for maximum grafting success in papaya.
Maximum graft success in plum i.e., 9.67 out of 10 grafts was
achieved by following cleft grafting in April suggesting it to be
the commercial method for grafting (Mozumder et al., 2017).
In walnut wedge grafting was found comparatively superior to
tongue grafting in terms of sprouting percentage, graft union
success, and plant growth (Srivastava, 2012). The influence of
grafting technique on plant growth has been studied in peach
cultivar Shan-e-Punjab grafted on wild peach rootstock. Different
grafting methods including tongue grafting, chip budding and T
budding were followed. Tongue grafted plants showed maximum
sprouting percentage, graft success, plant height, girth and
number of branches. The results indicate that tongue grafting
is the best method of propagation for peach variety Shan-
e-Punjab (Sharma et al., 2018). Bud take and bud sprouting
were found to be earliest when T budding (with wood) was
performed in cherry. Also, maximum shoot length and the
highest number of leaves and lateral branches were obtained with
T budding (with wood) compared to the other two methods
i.e., T without wood budding and chip budding (Yazdani et al.,
2016). Graft take success in apple cultivars grafted on different
rootstocks was evaluated and it was found that the cultivar
Gala mast grafted on crab apple rootstock by means of bench
grafting showed maximum graft take success as well as prominent
growth (Fazal et al., 2014). Whip and cleft grafting methods
have been reported to be the most promising ones for the
asexual propagation of Jabuticabeira Acu grafted on rootstocks
belonging to other species of the same family (Cassol et al., 2017).
Depending upon the research purposes other grafting methods
like in vitro grafting have been introduced. Besides being time
Frontiers in Plant Science | www.frontiersin.org 2December 2020 | Volume 11 | Article 590847

fpls-11-590847 December 5, 2020 Time: 13:8 # 3
Rasool et al. Grafting in Horticultural Plants
FIGURE 1 | (A) Grafted Apple (Rootstock- M9, grafting method-Wedge grafting, Age-7 years); (B) Grafted Pear (Rootstock- Quince- C, grafting method- Wedge
grafting, Age-8 years); (C) Plum (Rootstock- Seedling, grafting method- Tongue grafting, Age-5 years); (D) Cherry (Rootstock- Gisela 5, Grafting method- Tongue
grafting, Age-6 years).
consuming, the planting material used in the conventional system
of plant propagation is not healthy. To overcome these problems
application of in vitro techniques is an effective alternative.
One such application is shoot tip grafting or micrografting.
Micrografting involves in vitro placing of shoot tip as an explant
on a decapitated rootstock grown from a seed (Hussain et al.,
2014). Micrografting protocols have been developed for many
fruit crops including grapes (Aazami and Bagher, 2010), walnut
(Wang et al., 2010), almond (Yıldırım et al., 2013), etc. Rafail
and Mosleh (2010) reported that in in vitro grafting of apple and
pear, homografting was relatively more successful compared to
heterografting and an increase in micrograft success was noticed
from 30 to 90% in pear (cv. Aly-sur on Calleryana pear) and
40 to 90% in apple (cv. Anna on MM106) with increasing
benzylaminopurine (BAP) concentration from 0 to 2.0 mg/L.
Patharnakh shoot tips were propagated in vitro on Kainth
rootstock. Graft success and vigor was found to be maximum
by following wedge grafting method and using 5–10 mm scions
and M2 (MS liquid media +20 g/l sucrose) media (Rehman and
Gill, 2014). Hetero-grafting allows the alteration of important
plant processes including water uptake, nutrient absorption,
hormonal signaling and enzyme activity. Cookson and Ollat
(2013) reported that it is heterografting and not homografting
which affects the gene expression pattern in shoot apical regions.
Stress response genes were upregulated at the graft interface of
heterografts compared to homograft’s indicating that the cells
at the graft interface have the ability to recognize and function
differently as soon as they come in contact with a self or non-self-
grafting partner (Cookson et al., 2014). Comparative analysis of
differentially expressed genes in homo and heterografted tomato
seedlings was carried out and it was found that in heterografts
healing process was slightly slow compared to homograft’s and
several genes involved in oxidative stress were significantly up-
regulated in the scion of heterografted tomato (Wang et al., 2019).
Gene expression studies in homo and heterografts of grapevine
revealed upregulation of genes involved in the synthesis of cell
wall, wound responses, hormone signaling and other metabolic
pathways in homograft’s, while in heterografts stress responsive
genes were up-regulated at the graft interface (Clemente Moreno
et al., 2014). Studies on efficiency of grafting techniques and time
of grafting have been conducted and standardized for different
areas (Ghosh and Bera, 2015). This information may help in
Frontiers in Plant Science | www.frontiersin.org 3December 2020 | Volume 11 | Article 590847

fpls-11-590847 December 5, 2020 Time: 13:8 # 4
Rasool et al. Grafting in Horticultural Plants
FIGURE 2 | Different types of grafting like side, tongue, cleft, bark, and splice.
determining ideal grafting time for quick multiplication of fruit
crops to enhance quality fruit production. In view of the same,
application of in vitro grafting techniques for propagation of fruit
crops can be considered a relatively sustainable alternative to
conventional methods of propagation. Micrografting technique
has a great scope in plant improvement and their large-scale
propagation. Production of virus free plants is one important
advantage of this technique due to which it finds application
in fruit crop propagation. In addition to this, prediction of
graft incompatibility has been possible through micrografting.
Grafting operation can be conducted at any time of the year
through micrografting. Due to an adequate number of advantages
this technology has great potential to be practically used by
researchers and nursery growers.
GRAFT UNION FORMATION
For grafting to be successful, a number of complex biochemical
and structural processes are involved. The latter result
in establishing a connection between the root-stock and
scion. Adhesion of parenchyma is the first step for union
formation followed by formation of vascular elements and their
differentiation into xylem and phloem. Formation of vascular
connection between the stock and scion during wound healing
is of utmost importance as the wound given to the stock and
scion during grafting causes disruption of the vascular system in
plants (Asahina and Satoh, 2015), hence connecting up of the
vascular system is required to facilitate water uptake as well as
to ensure nutrient transport to the graft junction. In addition
to this, vascular reconstruction enables macromolecules to be
transported across the graft union (Harada, 2010). This specifies
that vascular differentiation is imperative for grafting success
during the process of wound healing. Five histological stages
are reported to come about during graft union formation in
rootstock scion combinations: (1) formation and orientation
of necrotic layers, (2) callus cell proliferation, (3) formation
of callus bridge at the graft interface, (4) vascular cambium
formation and (5) vascular tissue reconstruction between the
stock and scion (Figure 3). Except for the outer cortex necrotic
layers tend to disappear by the cellular activities in the callus. In
most of the cases the portion of necrotic layers in the outer cortex
gets transformed into bark (Yildirim et al., 2010). The process
of graft union formation is temporally separated. Herbaceous
plants take relatively shorter time for the successful formation of
a graft union compared to trees. Time required for graft union
formation in the Arabidopsis micrograft system is 7–12 days
(Turnbull, 2010). On the contrary, trees take several months to
form a union at the graft interface (Olmstead et al., 2006). The
process of vascular reconnection during the formation of a graft
union has been studied in Arabidopsis thaliana. Attachment
of tissues on either side of the cut surfaces, establishment of a
connection between the phloem cells of rootstock and scion,
resumption of root growth, and connection between xylem
vessels were found to be temporally separated. It was found that
at first the two parts get attached to each other, this is followed by
Frontiers in Plant Science | www.frontiersin.org 4December 2020 | Volume 11 | Article 590847

fpls-11-590847 December 5, 2020 Time: 13:8 # 5
Rasool et al. Grafting in Horticultural Plants
FIGURE 3 | Represents stages of graft union formation: Stage 1, Parenchymatous tissue divides to form callus cells; Stage 2, Xylem vessel formation; Stage 3,
Formation of vascular cambium across the graft union linking the two partners; Stage 4, Secondary xylem and phloem dedifferentiate across the graft union
establishing sufficient vascular continuity for plant growth [Hartmann and Kester, 2002].
connecting up of phloem about 3 days after germination (DAG),
growth of the roots gets resumed at this stage around 5 DAG, and
at 7 DAG xylem vessels get re-joined. By analyzing the pattern of
cell division and tissue regeneration at the graft junction it was
observed that cells at first showed an asymmetrical pattern of cell
division and differentiation but as soon as a contact establishes
between the stock and scion, they tend to lose this asymmetry
and the vascular connection gets re-established (Melnyk et al.,
2015). Melnyk et al. (2018) in their subsequent study explained in
detail the differential expression and upregulation of many genes
during the union formation leading to vascular regeneration.
They described that genes are expressed asymmetrically at the
graft junction in Arabidopsis hypocotyl grafts. This differential
expression of genes was observed on account of abundant carbon
concentration in the scion and less carbon in the rootstock, till
the phloem was reconnected. At the graft union, genes associated
with the formation of vascular tissues were upregulated, thereby
activating a recognition mechanism between the stock and the
scion. Different stages of union formation at the graft interface in
tomato seedlings revealed that a number of structures appeared
to interconnect the stock and scion 8 DAG, vascular bridges
appeared at 11 DAG and connection between the root-stock
and scion got completely established 14 DAG (Fan et al., 2015).
On the other hand, histological stages of development of graft
union in spur type apple varieties grafted on different apple
rootstocks revealed that ample amounts of callus boomed in
all the stock-scion combinations. Formation of cambium and
reconnection of vascular cells was apparently successful in
90-day old grafts. Callus bridge filled the stock and scion gap
on 120th day and it continued for a few more days, following
which xylem and phloem strands bridged the union (Polat et al.,
2010). Also, study of changes at the graft union in cashew by
Mahunu et al. (2012) revealed that 30 DAG the necrotic layer
disappeared, coinciding with the enlargement of callus, while
adhesion of stock and scion occurred at 60 DAG. At 98 DAG
cambial connections and healed unions were visible. However,
in case of unsuccessful graft combinations at 98 days after
grafting, a gap between the cells of stock and scion was noticed
indicating that union formation is the key factor for graft success
and further growth of the grafted plant. Since grafting puts a
considerable amount of stress on plants, it is associated with
the stimulation of a number of wounding responses such as
production of ROS (reactive oxygen species), upregulation of
certain genes providing stress resistance, synthesis of enzymes
and other chemical substances. These compounds eventually
trigger the formation of wound induced callus. Furthermore,
Frontiers in Plant Science | www.frontiersin.org 5December 2020 | Volume 11 | Article 590847

fpls-11-590847 December 5, 2020 Time: 13:8 # 6
Rasool et al. Grafting in Horticultural Plants
the growing callus is sustained by production and stimulation
of specific metabolites. Study of transcriptional changes at the
graft interface in grapevine at 3 and 28 DAG revealed differential
expression of certain genes involved in the synthesis of cell wall
and formation of vascular elements. Expression of these genes
was particularly upregulated at the graft interface compared
to the rootstock which resulted in the identification of genes
specific to the graft interface (Cookson et al., 2013). To identify
the compounds involved in callus formation, Prodhomme
et al. (2019) conducted metabolite profiling in grapevine. The
study revealed increased production of amino acids (basic and
branched chain) as well as accumulation of stilbene compounds
at the graft interface. Additionally, the union formation was
associated with increased activity of two enzymes viz., PAL and
NI at the graft interface compared to the surrounding tissue. All
these metabolic modifications together support callus growth
and serve as a source for the identification of potential markers
for selection in breeding programs. Despite these findings, we
still lack the understanding of how the two components i.e., stock
and scion actually establish a physical connection, integration of
the vascular tissues, role of plasmodesmata in union formation,
and material exchange at the graft interface. Thus, advanced
research is needed to address these basic questions. This can be
done by using fluorescent markers and correlative light-electron
microscopy techniques (Gautier et al., 2019).
GENETIC LIMITS OF GRAFTING
A prerequisite for graft compatibility is taxonomic proximity.
Autografts are taxonomically quite close and thus are expected
to be always compatible. When grafting is performed within the
same species it forms compatible combinations, if the grafting
partners are from different species but the genus to which
they belong is same, grafts are more or less compatible, intra
familial grafts are rarely compatible, while inter familial grafts
are essentially unsuccessful due to incompatibility (Mudge et al.,
2009). Therefore, the taxonomic proximity between the grafting
partners is essential for the successful re-establishment of both
the rootstock and scion fused together. It is a very well-known
fact that presence of vascular cambium is a prerequisite for
grafting. Vascular cambium due to its meristematic activity
divides to form xylem and phloem during the secondary growth
resulting in increased plant diameter (Spicer and Groover, 2010).
Monocots cannot be grafted, moreover grafting of a monocot
plant onto a dicotyledonous plant is also not possible. This is
because vascular bundles in monocots are scattered and they lack
cambium, which is a basic requirement for graft union formation.
The parallel venation in the leaves of monocots indicates that the
veins do not interconnect to each other like they do in dicots.
Thus, it is the lack of vein connection in stem and leaves of
monocot plants which makes grafting in monocots an impossible
task (Figure 4). Plants that are closely related have a good chance
of successful union formation compared to the remotely related
ones. Such plants have less or no chance of successful graft union
formation. In order to achieve maximum success, grafting should
be performed between or within the clones (Kumar, 2011).
However, successful interfamilial graft combinations between
Nicotiana benthamiana (Nb) and Arabidopsis thaliana (At) have
been reported where the growth of Nb scion was slow but
distinct at the same time (Notaguchi et al., 2015). During the
course of time, plants have developed a specialized haustorium
that pierces into the host plant to derive nutrients by means of
tissue adhesion and this property of cell to cell adhesion can
be used to develop interfamilial grafts (Westwood et al., 2010).
Natural tendency for cell to cell adhesion with plants belonging
to different families including vegetables, fruits trees as well
as monocots is found in Nicotiana. Here, the reconstruction
of vascular structures follows the normal pattern as in case
of intrafamilial grafting. A transcriptomic study revealed the
upregulation of β-1,4-glucanases followed by graft adhesion in
inter as well as intra familial grafts. The use of Nicotiana stem,
an interscion produced tomato fruits on rootstocks belonging
to different families. Therefore, the mechanism of cell to cell
adhesion can be used to modify plant grafting techniques and
to develop graft combinations which are otherwise difficult to
obtain (Notaguchi et al., 2020).
GRAFT INCOMPATIBILITY: TYPES AND
DETERMINING TECHNIQUES
Graft incompatibility is generally referred to as inability of the
stock and scion to bind together to form a successful graft union.
Lack of compatibility between the rootstock and scion is the
major limiting factor in propagation of fruit trees, particularly
stone fruits (Zarrouk et al., 2006). Graft incompatibility is
therefore a critical issue for breeding rootstocks of fruit trees and
longevity of an orchard (Hossein et al., 2008). It leads to the
formation of unhealthy trees, breakage at the graft union and
premature death of grafted plants (Zarrouk et al., 2006). The
arrival of these symptoms could take a number of years (Guclu
and Koyuncu, 2012). Thus, to ensure a successful graft union the
selection of a mutually compatible scion/rootstock combination
is important (Goldschmidt, 2014).
Graft incompatibility is broadly categorized as: translocated
and localized (Zarrouk et al., 2006). In case of “translocated”
incompatibility, symptoms are observed at early stages of plant
development. Scion and root growth tend to terminate at a very
early stage, reduced carbohydrate translocation at the union,
shriveling of leaves, leaf chlorosis leading to leaf reddening and
early leaf drop are commonly observed symptoms. Translocated
incompatibility can be evaluated using the soil plant analysis
development (SPAD) chlorophyll meter which measures the
chlorophyll content and nitrogen status of plants. Low SPAD
index values indicate restricted carbohydrate assimilation and
nitrogen uptake due to translocated incompatibility (Zarrouk
et al., 2006). Significantly lower SPAD index values were observed
for rootstocks “Mirabolano 29C” and “Marianna 2624” in
the three scion cultivars, ultimately resulting in death of the
trees owing to incompatibility between the graft partners. The
rootstocks showing translocated graft incompatibility symptoms
like reddening of leaves, excessive leaf curling, leaf drop, etc.,
with scion cultivars died 5 months after planting. SPAD index
Frontiers in Plant Science | www.frontiersin.org 6December 2020 | Volume 11 | Article 590847

fpls-11-590847 December 5, 2020 Time: 13:8 # 7
Rasool et al. Grafting in Horticultural Plants
FIGURE 4 | It shows correlation between taxonomic proximity and graft compatibility. Grafting between closely related plants is comparatively more successful than
distantly related ones. Maximum grafting success is achieved by performing grafting within or between the clones. Grafting success goes on decreasing as the
plants become less related taxonomically.
values did not decrease in other scion/rootstock combinations
after 9 months of planting, indicative of their compatibility
(Figures 5A,B;Neves et al., 2017). Thus, SPAD chlorophyll meter
serves as an effective and non-destructive tool for the prediction
of incompatible graft combinations.
Localized incompatibility, on the other hand, leads to
malformation at the graft union due to physiological and
morphological changes taking place which eventually results in
impaired union formation and in severe cases the tree might
fall apart at the junction after some years of grafting (Errea,
1998). The alterations associated with localized incompatibility
include disruption of vascular cambium, lower rate of tissue
differentiation, lignification may not take place efficiently and
disruption of vascular continuity. These changes might cause the
graft union to rupture (Zarrouk et al., 2006, 2010;Pina et al., 2012,
2017). The most known example of localized incompatibility
is between pear and quince, when pear cultivars are used as
scion and quince as rootstock, prunasin, a cyanogenic glycoside
generally present in quince but absent in pear is translocated into
the phloem cells in pear scion, where the pear enzymes disrupt
the prunasin in the region of graft union, producing hydrocyanic
acid as one of the products of decomposition. Hydrocyanic acid
obstructs the actively dividing cambial cells at the graft union
and also disrupts phloem tissues at and above the graft union.
Restriction in water flow and mineral/metabolite translocation
across the union consequently kills quince phloem as well (Gur
et al., 1968). Early and correct forecast of graft incompatibility
is of utmost significance because the unwanted incompatible
combinations could be avoided while the desirable compatible
ones could be selected (Petkou et al., 2004;Gökbayrak et al.,
2007). Standardized methods for evaluation of compatibility
between the rootstock and scion would be of great use to
the breeders while using a particular rootstock and scion for
grafting (Pina et al., 2017). In several apricot combinations
grafted on prunus rootstocks, graft incompatibility resulted
in breakdown of the trees at the union years after planting,
therefore an early selection process could help in detecting a
comparatively compatible combination. Analysis of the phenol
content at the graft union can be used as a technique for
the estimation of graft incompatibility (Dogra et al., 2018).
Callus formation is of utmost importance for stock and scion
to be compatible and grafting to be successful. In grapevine
it was found that it is not the graft take rates but the status
of callus formation at 21 DAG which is an indicative of
compatibility between the stock and scion. Moreover, analysis
of leaf chlorophyll content can also serve as an efficient
means to estimate the compatibility (Tedesco et al., 2020).
The difference in the quality and quantity of phenol in the
stock and scion can point toward metabolic dysfunctions at
the graft union (Errea, 1998) and can serve as a biochemical
marker to predict graft incompatibility (Musacchi et al., 2000).
Histological studies of callus formation and cell alignment have
made it possible to predict the compatibility of any combination
way before the symptoms appear (Errea and Borruey, 2004).
Additionally, the findings of Guclu and Koyuncu (2012) revealed
that by using peroxidase activity as a technique it is possible
to predict graft incompatibility before the grafting operation is
conducted particularly in those combinations that might show
delayed incompatibility symptoms (Figure 6). Furthermore,
electrophoresis method and X-ray tomography can serve as
important tools for assessment of graft quality and success
(Dogra et al., 2018). The complex mechanisms involved in the
incompatibility reaction between the stock and scion have been
studied in many ways, however, still many processes remain
unclear therefore, advanced research is needed to completely
understand the physiology of graft incompatibility especially in
perennial plants.
ROLE OF PHENOLIC COMPOUNDS IN
GRAFT INCOMPATIBILITY
Incompatibility is allied with complex biochemical and
physiological interactions between the grafting partners
(Pereira et al., 2014). Pina and Errea (2008), suggested that
graft compatibility/incompatibility response could be related
to the protein UDP-glucose pyrophosphorylase. Phenolic
substances play a crucial part in plants and are one of the
Frontiers in Plant Science | www.frontiersin.org 7December 2020 | Volume 11 | Article 590847

fpls-11-590847 December 5, 2020 Time: 13:8 # 8
Rasool et al. Grafting in Horticultural Plants
FIGURE 5 | (A) SPAD values for Jade and Maciel (scion cultivars) grafted onto a range of clonal rootstocks. (B) SPAD values BRS-Kampai scions grafted on a range
of clonal rootstocks (Neves et al., 2017).
most important compounds that determine the rootstock-scion
interactions. Errea (1998) believed that quality and quantity
of phenols in rootstock-scion parts indicated why the rate
of metabolic activities is low at the graft union. Mng’omba
et al. (2008) presented that the combinations which are
apparently less compatible possessed high concentrations of
phenolic compounds than the compatible ones. r-coumaric
acid was present in a huge amount in relatively less compatible
combinations. Therefore, phenols particularly r-coumaric acids
and flavonoids resulted in weak union formation at the graft
junction. This is the peculiar sign of graft incompatibility.
Pear stock-scion combinations were found associated with
profuse amounts of arbutin in phloem above and below the
graft union, after that procyanidin B1 and chlorogenic acid.
Compatible combinations had greater arbutin levels above
the graft junction, whereas in the incompatible combinations
of “Williams” on quince MA high arbutin concentration
was recorded at the lower side of the graft union. In all the
cultivars under study, arbutin content was found to be highest
below the graft union specifying that it’s not just catechin and
procyanidin B1, but also arbutin and a number of flavonols may
possibly serve as a cause for graft incompatibility (Hudina et al.,
Frontiers in Plant Science | www.frontiersin.org 8December 2020 | Volume 11 | Article 590847

fpls-11-590847 December 5, 2020 Time: 13:8 # 9
Rasool et al. Grafting in Horticultural Plants
FIGURE 6 | Overgrowth above the union caused by blockage of photosynthates, translocating from the scion into the stock, when sweet cherry is grafted onto sour
cherry [(A) Age 10–12 years; (B) Age 7–8 years].
2014). Several studies have shown that phenolic compounds in
incompatible combinations move from vacuole to cytoplasm
and cause inhibition of lignification which is required during
early stages of establishment of scion–stock connections. The
cell wall of xylem vessels are dynamic in nature composed
of phenolic compounds (for example, lignins), minerals,
polysaccharides and proteins (Liu, 2012;Herrero et al., 2014).
These phenolic compounds can serve as important markers for
determining compatibility between different graft combinations
(Prabpree et al., 2018). The role of different cyanogenic
glycosides (CGs) in the incompatibility reaction of Prunus
has been evaluated in different graft combinations belonging
to prunus species viz., Prunus persica and Prunus mume and
ungrafted genotypes. The incompatible graft combinations
were found to have greater prunasin levels and activity of the
enzyme phenylalanine ammonia lyase (PAL) was also higher
in rootstock. Additionally, the scion and stock were found
to have a moderately higher concentration of total phenolic
compounds with high antioxidant activity. Differences in
concentration of CGs, primarily prunasin, was found to be
responsible for the incompatibility between Prunus persica and
Prunus mume (Pereira et al., 2018). Therefore, grafting between
such plants with great differences in CG concentrations may
end up in generating incompatibility reactions between the
partners (Pereira et al., 2015). Plant hormones, especially auxins
determine the compatibility of a rootstock-scion combination
by interacting with phenolic compounds. Incompatibility has
been associated with increased levels of phenolic compounds
above the graft union which adversely affect the auxin transport
(Errea, 1998). Low auxin concentration in incompatible
combinations in turn affect the differentiation of vascular tissues
and lignification (Aloni, 2010;Koepke and Dhingra, 2013).
All these changes will lead to the formation of weak unions
which may cause huge economic losses to the growers. More
information about the compounds responsible for inducing
graft incompatibility is needed. This knowledge is necessary for
the development of molecular markers for rootstock breeding
(Gainza et al., 2015).
HORMONAL CONTROL OF GRAFT
UNION
The development of successful grafts involves some fundamental
steps in the following pattern: phloem tissue reunion, root
growth, and xylem tissue reconnection (Melnyk et al., 2015).
The key event involved in the formation of a graft is the
joining of vascular tissues between the rootstock and scion.
Grafting first and foremost causes the discharge of pectins
from cells at the graft union which makes the rootstock and
scion adhere together. Dedifferentiated cells at the union form
callus at the graft interface, these cells then interdigitate and
form a connection via plasmodesmata. The vascular tissues
differentiate together with the callus at the grafting site
giving rise to phloem which is succeeded by reconnection of
xylem tissue (Jeffree and Yeoman, 1983;Melnyk et al., 2015;
Ribeiro et al., 2015). Plant hormones commonly known as
phytohormones regulate all phases of growth and development
in plants, from embryogenesis to reproductive development.
They standardize the response of plants to a wide range
Frontiers in Plant Science | www.frontiersin.org 9December 2020 | Volume 11 | Article 590847

fpls-11-590847 December 5, 2020 Time: 13:8 # 10
Rasool et al. Grafting in Horticultural Plants
FIGURE 7 | Hormonal signaling taking place at the graft interface during the grafting process. CKs, cytokinins; IAA, auxin; 534 JAs, Jasmonic acids; GAs,
Gibberellins.
of biotic and abiotic stresses. Additionally, phytohormones
regulate the physiological processes taking place at the site
of graft union. Phytohormones facilitate plants to combat the
stress induced by grafting. Aloni (1995) found that relatively
less concentration of indole-3-acetic acid (IAA) encouraged
phloem differentiation, but higher levels brought about the
differentiation of xylem. Similarly, in grafting trials, auxins
are an essential group of elements resulting in the formation
of compatible graft unions (Figure 7). Auxins are produced
from the vascular strands of the stock and scion, which bring
about the vascular tissue differentiation, hence working as
a growth regulating substance (Aloni, 1987;Mattsson et al.,
2003). Plant hormones are translocated from source to sink as
signal molecules influencing growth of cells and differentiation
of vascular tissues, especially at the graft crossing point
(Aloni, 2010). Kümpers and Bishopp (2015) demonstrated that
phytohormones regulate the complex physiological interaction
between scions and rootstocks in A. thaliana. Along these
lines, they may be well thought-out candidates in the scion–
rootstock communication both above and below the grafted
interface (Kondo et al., 2014). Nanda and Melnyk (2018)
inspected the function of eight plant hormones for the
period of grafting to determine their role in healing of cut
surfaces and vascular tissue differentiation in plants at the
graft interface (Melnyk, 2017). They concluded that every
known plant hormone controls the vascular tissue differentiation
during the period of graft union formation. Nonetheless,
auxin is the principal regulator of vascular differentiation
and other hormones augment its signaling pathway to make
satisfactory adjustments in this process. After the cut is
given, wound induced dedifferentiation 1 (WIND1) stimulates
cytokines which triggers the formation of callus at the graft
junction. Simultaneously, auxin gets transported basipetally and
due to the lack of vascular connectivity, its flow across the
union is hampered, resulting in its accumulation above the
graft interface and low concentration at the bottom junction.
Auxin accumulation, along with ethylene signaling, triggers
the expression of Arabidopsis NAC domain containing protein
71 (ANAC071) above the graft union, and simultaneously
inhibits the expression of RAP2.6L as well as Jasmonic acid
biosynthesis. Beneath the graft, the depleted auxin levels
trigger the biosynthesis of Jasmonic acid and expression of
RAP2.6L. The expression of ANAC071 and RAP2.6L prompts
cell division around the graft junction. Auxin, in collaboration
with gibberellins and cytokinins, stimulates differentiation
of cells, resulting in formation of vascular connection and
re-joining between both junctions, thereby restoring auxin
symmetry. Gibberellins, in collaboration with auxin, stimulate
emergence of tissues by means of cell expansion (Figure 7;
Nanda and Melnyk, 2018).
Frontiers in Plant Science | www.frontiersin.org 10 December 2020 | Volume 11 | Article 590847

fpls-11-590847 December 5, 2020 Time: 13:8 # 11
Rasool et al. Grafting in Horticultural Plants
EFFECT OF ROOTSTOCK-SCION
INTERACTION ON GROWTH, QUALITY
AND STRESS TOLERANCE IN PLANTS
Many reports recognized the interactions between several
parameters related to physiology of the grafted trees and their
fruit quality (Naor, 1998). These relationships are important since
they provide a source for choosing the most compatible graft
combination for specific environments and good fruit quality.
Choice of a suitable combination is fundamental for increased
production of trees, for the reason that the interaction between
the stock and scion effects translocation of water and minerals,
gas exchange in leaves, size of the plant, time and duration
of flowering, time and percentage of fruit set and quality of
fruits (Gonçalves et al., 2003). Therefore scion-stock combination
is a key factor to be well thought-out in orchards before
taking up the planting procedure. In grafted trees, the rootstock
mainly controls the plant size. Performance of the fruit trees is
fundamentally related to forming an optimum balance amongst
growths and fruiting. Excessive vegetative growth always reduces
the yield and deteriorates fruit quality (Mitre et al., 2012).
Thus, for successful fruit production it is equally important to
maintain an appropriate balance between the vegetative and
reproductive processes (Sharma et al., 2009). Grafting scions
on a less vigorous rootstock is the first and foremost step to
achieve this equilibrium. In case of apple dwarfing rootstocks,
it has been found that the sugar concentration is considerably
low in these rootstocks and cellular action in these rootstocks is
significantly reduced regardless of having huge starch reserves.
Additionally, in dwarfing rootstocks, the down-regulation of
MdAUX1 and MdLAX2 (auxin influx transporters) together
with increase in flavonoids concentration lead to the reduced
auxin movement which correlates to the dwarfing stature of
these rootstocks (Foster et al., 2017). Lately, in apple WRKY
transcription factor family has been found to be responsible for
dwarfing phenotype in M26 rootstock. MdWRKY9 on account
of its differential expression in dwarfing and non-dwarfing
rootstocks is considered as a candidate gene for controlling the
dwarfing phenotype (Zheng et al., 2018). In addition to growth,
many agronomically desirable qualities are also influenced by the
pattern of stock-scion interaction. Pal et al. (2017) established
that rootstock has a pronounced effect on growth as well as
fructification in cherry. Lesser number of fruiting branches were
formed in trees grafted on Mahaleb than those grafted on “Gisela
5.” A comparative evaluation of the two rootstocks indicated that
fruit yield was almost double where “Gisela 5” was used as a
rootstock compared to Mahaleb, in the first 3 years of fruiting.
Rootstocks differ in their ability to utilize soil resources and
transport these resources through the union to the scion. Owing
to their different root architectures, rootstocks of grapevine and
citrus differently take up phosphate and remobilize phosphorus
reserves (Zambrosi et al., 2012;Gautier et al., 2018). Greater
root length increases stomatal conductance in grafted grapevine
under the conditions of water deficit (Peccoux et al., 2018). The
influence of rootstocks on mineral concentration in scion leaf
and fruits, plant growth, yield potential and quality traits of two
apple cultivars was studied by Amiri et al. (2014). They found
that rootstocks exert their influence on scion yield, quality and
vigor by influencing the amount of minerals reaching the scion.
M.9 rootstocks were more effective in nitrogen (N), manganese
(Mn), and iron (Fe) uptake. The rootstock MM.106 was found
to have the highest uptake potential for phosphorus (P) uptake,
while M.9 had the lowest uptake potential for potassium (K) and
calcium (Ca). One of the chief historic benefits of rootstocks
is their ability to control diseases. These diseases can kill the
plants at an early stage before they become productive or cause
substantial impairment and yield reductions. The levels of disease
resistance in a given cultivar can vary according to the rootstock
onto which it is grafted (Cline et al., 2001). Jensen et al. (2012)
measured the influence of rootstock on fire blight resistance of
scion and observed that rootstocks predominantly influenced
gala scion’s susceptibility to fire blight. Rootstocks were found
to regulate the expression of different transcription factors. This
differential gene expression pattern could be associated with
variation in susceptibility. Besides increasing scion’s ability to
deal with biotic stresses, rootstocks can also improve tolerance to
a number of abiotic stresses. Amongst these, drought and high
salinity have a massive control on crop production; certainly,
they are one of the major aspects which restricts the plant
productivity and effect yield severely (Cramer et al., 2007;
Tsago et al., 2014). In grapevine the rootstocks “110R,” “1103P,”
and “99R” have been found to enhance efficiency of using
the water during critical stages of growth and as a result
are effective in battling the drought stress (Nimbolkar et al.,
2016). Grafting is the commercial method of propagation in
citrus where the rootstock influences numerous horticultural
traits, including tolerance to drought. A transcriptome analysis
conducted in Sweet orange where Rangpur lime was used as the
rootstock recommended that the rootstock was responsible for
inducing the drought tolerance in scion cultivar by up regulating
the transcription of genes associated to the cell wall, biotic
and abiotic stress resistance, antioxidant systems and soluble
carbohydrate, TFs, PKs, and ABA signaling pathway, and at
the same time by down regulating the activity genes involved
in the light reaction, metabolic processes and biosynthesis
of ethylene (Gonçalves et al., 2019). In case of temperate
fruits genetically diverse genotypes are available which serve as
potential rootstocks against abiotic stress (Cimen and Yesiloglu,
2016). The interaction between the scion–rootstock has been
reported to influence the quality and functioning in Prunus
avium. The rootstock has been found to influence the movement
of water and the process of photosynthesis in sweet cherry
trees whereas the scion chiefly exerts its influence on other
physical and chemical quality traits in cherry (Gonçalves et al.,
2006). Santarosa et al. (2016) showed that the physiological
interaction between rootstock-scion modified the vascular system
in grapevines by altering the xylem vessel. Rootstock-scion
combination that was highly vigorous had vessels with larger
diameters, xylem areas, and proportion of vessels with diameters,
greater than 50 µm. The effect of scion variety on rootstock
growth and development via signals stimulated in shoot has
been studied. These signals regulating root elongation and
development in model plants include metabolites, hormones,
Frontiers in Plant Science | www.frontiersin.org 11 December 2020 | Volume 11 | Article 590847

fpls-11-590847 December 5, 2020 Time: 13:8 # 12
Rasool et al. Grafting in Horticultural Plants
peptides, HY5, microRNA 156, and microRNA 399. More study
needs to be done to understand how scions modify phenotype of
the rootstocks (Gautier et al., 2019). Tietel et al. (2020) studied
the reciprocal effect between rootstock and scion, further they
proposed that the interactions between the two could possibly
be organ, time or distance dependent. Rootstock affected fruit
yield and biochemical parameters of the fruit in relations with
the scions. Overall distribution of 6 out of 53 and 14 out of
55 basic metabolites in the sap of scion and rootstock, was
controlled by the rootstock, whereas 42 and 33 were affected by
the rootstock-scion interaction, correspondingly.
MOLECULAR RESPONSES AT THE
GRAFT INTERFACE
Presently, besides its application in horticulture, grafting has
become more prominent as an important domain for research,
chiefly regarding signaling mechanisms associated with rootstock
and scion interaction. Recent studies have laid emphasis on
detecting the transport of molecules particularly proteins and
RNAs across long-distances. Proteins and RNAs transport across
long-distances via phloem and their possible role in generating
signals between the organs has turned to be a major field of
research lately (Harada, 2010;Kehr and Buhtz, 2013).
Movement of Genetic Components
Stegemann and Bock (2009) validated the movement of genetic
material through the graft union and found that plastid genes
travel small distances across the graft union. However, Stegemann
et al. (2012) later found that the intact plastid genome moves
across the graft union at the molecular level. Many genes on
account of their role in hormones signaling are responsible for
successful graft union formation. In Arabidopsis, it was found
that the genes involved in wound healing and cleaning up of
cellular debris were over expressed during the development
of graft union (Yin et al., 2012). In grape vine, formation of
the graft union triggered the differential pattern of expression
of genes participating in secondary metabolism, modification
of cell wall, and cell signaling (Cookson et al., 2013). The
movement of the nuclear genome between the stock and scion
led to the development of fertile alloploid plants (Fuentes et al.,
2014). Phenotypic changes induced by grafting have led to the
discovery of endogenous factors responsible for these changes.
Initially the study was confined to anatomical aspects, nutrient
and hormonal movement but now the latest technologies have
shown that some mobile RNA molecules are transported via
phloem tissue in the form of genetic material to complete
the entire growth and development process. miRNAs, are a
group of non-coding RNAs with roughly 22 nucleotides that
control gene expression. These miRNAs, bring about either
degradation of or inhibit translation of their target mRNAs
(Bartel, 2004). Li et al. (2014) identified miRNAs in heterografts
of vegetables, and observed that miRNAs played a critical role
in controlling physiological processes in heterografts. Mo et al.
(2018) identified miRNAs linked with graft union formation
in Pecan and noticed the involvement of miRS26 in formation
of callus and miR164, miR156, miRS10, miR160, and miR166
were found to be associated with differentiation of vascular
bundles. These results suggest the role of miRNA in the successful
graft union formation of pecan. Zhang et al. (2012) evaluated
the movement of gibberellic acid insensitive (GAI) through a
pear graft union. The results show that only 4 to 10 days after
micro-grafting, Pyrus-GAI could be transported endogenously
and not just this but it could also be transported to a scion
of 10–50-cm height in a 2-year-old tree. The results serve as a
basis for improving rootstocks and controlling scion properties
in trees by the application of portable mRNA to fruit tree
grafting. Agronomically significant traits such as compatibility,
short juvenile period, dwarf phenotypes and antivirus can be
transferred to the scion by making use of the transmissible mRNA
in a transgenic rootstock (Rivera-Vega et al., 2011). Transport
of GAI-mRNA in both directions between stock and scion has
been reported in apple (Malus×domestica cv. Fuji and Malus
xiaojinensis). Transport of GAI mRNA across the union took
place 5 days after grafting, which points out the movement
of GAI mRNA in both upward and downward direction (Xu
et al., 2010). This study can serve as the basis for using RNA
transport and its influence on properties of fruit trees. The study
of complex rootstock-scion interactions in fruit trees is necessary
for firming up resistance, higher yield and improving quality of
fruits (Li et al., 2012). miRNAs have been known to influence
several important stages of development like flowering time,
response to various hormonal signaling, morphology of plant
organs such as stem, leaves and roots (Pant et al., 2008;Xing
et al., 2016). Moreover, miRNAs are also found to influence the
interaction between the stock and scion. Earlier studies have
revealed that miR398, miR395, and miR399 in the phloem are
greatly linked with stress, and that the latter two are capable of
moving from scions to rootstocks (Buhtz et al., 2008). Maturity
in grafted avocado was found to be controlled by scion whereas
rootstock encourages the successful union formation along with
its influence on miRNA and mRNA profusion in the scion.
The large amount of miR172, miR156 and the miR156 target
gene SPL4, was directly associated with the scion and rootstock
maturity in avocado (Ahsan et al., 2019). By comparing the
expression of miRNAs in leaves and roots of homografts of
cucumber seedlings, it was found that the expression of most
miRNAs in the leaves and roots of heterografted seedlings
altered vigorously: prompted under regular conditions and down
regulated after some time of drought stress, and then again
up regulated after 24 h of drought stress. These outcomes are
valuable for the purposeful analysis of miRNAs in the facilitation
of grafting induced drought tolerance. An et al. (2018) conducted
a Gene Ontology study which showed that miRNAs which
are expressed differentially throughout grafting regulated genes
involved in a number of processes, including biosynthesis of
cellular compounds and metabolism. Various studies revealed
that miRNA172 can travel from source to sink in Nicotiana (Kasai
et al., 2010) and that miRNA156 functions as a signal which is
mobile through the phloem to affect important traits in potato
(Bhogale et al., 2014). Lower expression levels of microRNAs
(Vvi-miRNA159 and Vvi-miRNA166) were detected in more
compatible combination compared to the less compatible. These
Frontiers in Plant Science | www.frontiersin.org 12 December 2020 | Volume 11 | Article 590847

fpls-11-590847 December 5, 2020 Time: 13:8 # 13
Rasool et al. Grafting in Horticultural Plants
microRNAs target the key TFs which promote plant growth
and development (Assunção et al., 2019). It could therefore be
said that it is the changes in RNA abundance brought about
by grafting across the union that leads to differences in gene
expression pattern resulting in changed phenotype. In cherry
Zhao et al. (2020) detected more than 2 million sRNAs in
each scion, among them 21-nt sRNA were the most abundant
followed by 24-nt sRNA. On the other hand, 3000 sRNAs were
transported from the scion into the rootstock. Out of these
the most abundant ones were 24-nt sRNA followed by 21-nt
sRNA. The study of sRNAs transported across the graft union
provides a better understanding about their role in rootstock-
scion interactions. Thus, the technique of grafting involves
transport of genetic material across the union which facilitates
further study of grafting mechanisms, rootstock scion interaction
and the potential role of grafting in evolving new plant species
(Wang et al., 2017).
Movement of Proteins
The phloem sap is known to contain mobile proteins which move
across the graft union between the rootstock and scion. Long
distance transport of proteins in plants across the graft union
influences their growth and development by regulating some
important processes, for example adventitious root formation
in Arabidopsis is induced by the transport of Arabidopsis
translationally controlled tumor protein 2 (AtTCTP2) protein
across the graft union (Toscano-Morales et al., 2016). Besides
their role in regulating plant growth and development, long
distance transfer of proteins also plays an important role in
combating different types of stresses. Proteomic analysis in
cucumber scions grafted on Momordica rootstocks in response
to heat stress revealed accumulation of 77 different proteins
associated with important processes like photosynthesis, energy
metabolism and synthesis of nucleic acids which eliminated
the inhibitory effect of heat stress on scion growth (Xu et al.,
2018). Buhtz et al. (2010) have shown that xylem vessels which
generally transport water and solutes of low molecular weight
contain proteins, even though at lesser concentrations compared
to phloem. It has been reported that during transport from
the site of production to sink tissues within the plants some
proteins are able to bind with mRNAs as chaperones. These
proteins can aid the transport and protect mRNAs from getting
degraded. Duan et al. (2015) reported that polypyrimidine tract
binding protein 3 (PbPTB3), which falls in PTB family of
proteins and binds to a number of mRNAs in pear variety,
Du Li (Pyrus betulaefolia) gets transported to long distances
in the phloem and this process of transport is rather complex
in nature controlled by the form of a tree, environmental
conditions and nutrient concentration of the tree (Figure 8).
There are other proteins which can move to long distances and
regulate important cellular functions. For example, cyclophilin
protein (SICyp1), a phloem-mobile protein was found to move
from the scion to the stock through phloem. This transport
accompanied by augmented auxin levels, eventually helped in
promoting the root growth (Spiegelman et al., 2015). Paultre
et al. (2016) reported protein trafficking in Arabidopsis thaliana
micrografts, which displayed fluorescent protein-tagged signal
peptides originally expressed in scion only, in the roots of
rootstocks. This indicates extensive movement of proteins from
the scion cells to the root cells. The study distinctly supports the
existence of long-distance mRNAs and proteins transport, which
can modify the physiological and morphological development
of plants. The three TFs, VviLBD4, VviHB6, and VviERF3
involved in cambium activity, growth, and differentiation were
found to be expressed differently between the two heterograft.
A Transcriptomics study revealed that after 80 days of grafting
expression of TF VviLBD4 is relatively high in compatible
combination, together with VviHB6 and VviERF3. Since these
TFs play a crucial role in maintaining the activity of cambial cells,
growth, and tissue differentiation suggesting that in compatible
combinations at 80 days after grafting, the transcription factors
regulating cambial activity, cell differentiation and proliferation
of callus are expressed at a higher rate. Furthermore, the results
indicate that in the more compatible combination the expression
of the above mentioned three TFs is substantially less reduced
from 21 days after grafting to 80 days after grafting than in
the less compatible ones (Assunção et al., 2019). In the same
context, VviLBD4, VviERF3, and VviHB6 could serve as markers
for the estimation of compatibility at the 80 DAG. Compatibility
between the rootstock and scion is largely governed by the
formation of callus cells at the union, which in turn is under the
control of specific proteins. Differential expression of proteins at
the graft union influences major biological functions including
flavonoid synthesis. The up-regulation of proteins involved in
flavonoid biosynthesis play a major role in callus formation
during the healing of graft union (Xu et al., 2017) Another study
reported enhancement of callus formation by the upregulation
of the plant plasma membrane intrinsic protein (PIP1B), an
aquaporin which increases water content of cells and promotes
cell elongation resulting in successful union formation (Zheng
et al., 2010). Studying mobility of proteins during grafting has
shown that proteins are able to move from the companion
cells of the shoot into the root cells and thereby regulating
important physiological processes of plants (Paultre et al., 2016).
This evidence will help us to improve our understanding
of problems associated with grafting, successful graft union
and perpetuation of horticultural crops, where the scion and
stock material significantly affect the successful propagation and
operative costs.
EPIGENETICS IN GRAFTING
The cells surrounding the graft intersection have to regulate
under different metabolic, hormonal and redox conditions.
These peculiar conditions might trigger epigenetic modifications
and regulations which may play a role in graft healing. Some
of the studies have shown that the epigenetic regulators are
governed by redox status of cells (Shen et al., 2016;Berger
et al., 2018) and are also associated with signals produced by
hormones (Yamamuro et al., 2016). Plants face several kinds of
stresses in the field conditions and these stresses have been also
found to induce epigenetic modifications. Which contribute to
plants adaptability, memory and productive response towards
these stresses (Annacondia et al., 2018). There might be
key epigenetic processes like DNA methylation and histone
Frontiers in Plant Science | www.frontiersin.org 13 December 2020 | Volume 11 | Article 590847

fpls-11-590847 December 5, 2020 Time: 13:8 # 14
Rasool et al. Grafting in Horticultural Plants
FIGURE 8 | Schematic representation of signals exchanged between the rootstock and scion in grafted plants. DNA transfer across the graft union; Movement of
sRNA across the graft union; Long distance transfer of miRNA and mRNA molecules; Hormonal signaling across the union; Movement of proteins and other
metabolites through the graft union.
posttranslational modifications (PTM’S) governing some main
events in successful graft, root stock interaction and healing
(Probst and Mittelsten Scheid, 2015;Bilichak and Kovalchuk,
2016;Annacondia et al., 2018), Brassica rapa leaves were shown
to undergo DNA methylation changes when subjected to the
caterpillar Pieris brassicae (Kellenberger et al., 2016). Another
example of the epigenetic modifications has been found to be
induced in sweet potato caused by leaf crushing, which was
found to induce small interfering RNAs (siRNAs), that were
responsible to cause LbMYB1 gene methylation RNA directed
DNA methylation (RdDM) (Lin et al., 2013;Berger et al.,
2018). Jasmonic acid (JA) linked with histone deacetylation and
acetylation is a stress hormone produced at the graft interface.
Histone deacetylases (HDACs) are induced by JA. Which suggest
that PTMs and transcriptome reprogramming have a role in
wound healing (Yamamuro et al., 2016;Zhang et al., 2016, 2017;
Berger et al., 2018).
The discovery of non-autonomous RNA has made
a significant contribution in understanding epigenetic
modifications (Molnar et al., 2010;Bai et al., 2011;Tamiru
et al., 2018;Gaut et al., 2019). In Arabidopsis, SRNAs were found
to move from shoot to root and vice versa and were found to be
linked with DNA methylation as they are the key components
of the RdDM pathway (Law and Jacobsen, 2010). In genome
wide methylation data large numbers (thousands) of methylated
DNA bases were found within roots of methylation mutants
Frontiers in Plant Science | www.frontiersin.org 14 December 2020 | Volume 11 | Article 590847

fpls-11-590847 December 5, 2020 Time: 13:8 # 15
Rasool et al. Grafting in Horticultural Plants
(Lewsey et al., 2016;Gaut et al., 2019) signifying that sRNAs
not only traverse graft junctions but also affect methylation
in destination tissues. Epigenetic modifications and regulations
have been suggested to play a role in callogenesis, a process
which involves modifications in adult somatic cells from
less differentiated states recuperating their capability for
proliferation. Different models have been constantly deciphered
to understand mechanisms of callogenesis and all suggest that it
is also dependent on epigenetic regulations (Ikeuchi et al., 2013;
de la Paz Sanchez et al., 2015;Birnbaum and Roudier, 2017;
Lee and Seo, 2018).
CONCLUSION
Despite the fact that grafting technology has improved
enormously, the long-standing survival of grafted plants is still
to a certain extent unpredictable due to the incompatibility
problems. However, a prior selection method can serve as an
efficient means to foresee the future of a compatible combination
way before any external symptoms appear. The effect of
rootstock-scion interactions pertaining to growth, attainment
of reproductive potential, fruit set, yield efficiency and quality
characteristics of fruit crops is complex and poorly understood.
A healthier understanding of rootstock scion interactions would
aid in more effective selection and use of rootstocks in future. The
latest technology of silencing transmissible RNA and its potential
to regulate growth and stress responses has provided new
opportunities to understand stock-scion relationships. On the
basis of the results of different studies discussed in this review, it is
now possible to make use of mobile mRNAs in grafting systems of
fruit trees. Undeniably, there is great scope for the development
of transgenic rootstocks, in fruit trees that carry transportable
mRNAs which regulate key horticultural traits, such as disease
and stress resistance properties or dwarfing growth habit. This
kind of approach would not only improve the characteristics of
scion using transmissible mRNA from a transgenic rootstock, but
also might shun some of the disagreements regarding transgenic
fruit production. The epigenetic modifications occurring at the
graft site is one of the most important yet unexplored fields.
Understanding methylation patterns, epigenetic markers and
maintenance of these changes in different perennial crops is a
very important field to explore in future research.
AUTHOR CONTRIBUTIONS
AR and SM: conceptualization. AR and TB: software. GIH, KMB,
MNA, PA, and AJ: validation. AR and SM: writing—original
draft preparation. AR, SM, KMB, GIH, BAP, AAA, and PA:
writing—review and editing. AR, SM, GIH, HAE-S, BAP, and PA:
supervision. All authors contributed to the article and approved
the submitted version.
ACKNOWLEDGMENTS
The authors extend their appreciation to the Deputyship for
Research & innovation “Ministry of Education” in Saudi Arabia
for funding this research work through the Project number
IFKSURP-126.
REFERENCES
Aazami, M. A., and Bagher, M. H. (2010). In vitro micro-grafting of some Iranian
grapevine cultivars. Rom. Biotechnol. Lett. 15, 5576–5580.
Ahsan, M. U., Hayward, A., Alam, M., Bandaralage, J. H., Topp, B., Beveridge,
C. A., et al. (2019). Scion control of miRNA abundance and tree maturity in
grafted avocado. BMC Plant Biol. 19:382. doi: 10.1186/s12870-019-1994- 5
Allan, P., Clark, C., and Laing, M. (2010). Grafting papayas (Carica
papaya L.). Acta Hortic. 851, 253–258. doi: 10.17660/actahortic.2010.
851.38
Aloni, R. (1987). Differentiation of vascular tissues. Ann. Rev. Plant Physiol. 38,
179–204. doi: 10.1146/annurev.pp.38.060187.001143
Aloni, R. (1995). “The induction of vascular tissues by auxin and cytokinin,” in
Plant hormones, ed. P. J. Davies (Dordrecht: Springer), 531–546. doi: 10.1007/
978-94-011-0473- 9_25
Aloni, R. (2010). “The induction of vascular tissues by auxin,” in Plant hormones,
ed. P. J. Davies (Dordrecht: Springer), 485–518. doi: 10.1007/978-1- 4020-2686-
7_22
Amiri, M. E., Fallahi, E., and Safi-Songhorabad, M. (2014). Influence of
rootstock on mineral uptake and scion growth of ‘golden delicious’ and
‘royal gala’ apples. J. Plant Nutr. 37, 16–29. doi: 10.1080/01904167.2013.
792838
An, N., Fan, S., Yang, Y., Chen, X., Dong, F., Wang, Y., et al. (2018). Identification
and characterization of miRNAs in self-Rooted and grafted Malus reveals
critical networks associated with flowering. Int. J. Mol. Sci. 19:2384. doi: 10.
3390/ijms19082384
Annacondia, M. L., Magerøy, M. H., and Martinez, G. (2018). Stress response
regulation by epigenetic mechanisms: changing of the guards. Physiol. Plant
162, 239–250. doi: 10.1111/ppl.12662
Asahina, M., and Satoh, S. (2015). Molecular and physiological mechanisms
regulating tissue reunion in incised plant tissues. J. Plant Res. 128, 381–388.
doi: 10.1007/s10265-015- 0705-z
Assunção, M., Santos, C., Brazão, J., Eiras-Dias, J. E., and Fevereiro, P.
(2019). Understanding the molecular mechanisms underlying graft success in
grapevine. BMC Plant Biol. 19:396.
Bai, S., Kasai, A., Yamada, K., Li, T., and Harada, T. (2011). A mobile
signal transported over a long distance induces systemic transcriptional gene
silencing in a grafted partner. J. Exp. Bot. 62, 4561–4570. doi: 10.1093/jxb/
err163
Bartel, D. P. (2004). MicroRNAs: genomics, biogenesis, mechanism, and function.
Cell 116, 281–297.
Berger, M. M., Gallusci, P., and Teyssier, E. (2018). “Roles of epigenetic mechanisms
in grafting and possible applications,” in Advances in Botanical Research, Vol.
88, ed. J. A. Callow (Cambridge, MA: Academic Press), 203–246. doi: 10.1016/
bs.abr.2018.10.003
Bhogale, S., Mahajan, A. S., Natarajan, B., Rajabhoj, M., Thulasiram, H. V.,
and Banerjee, A. K. (2014). MicroRNA156: a potential graft-transmissible
microRNA that modulates plant architecture and tuberization in Solanum
tuberosum ssp. andigena.Plant Physiol. 164, 1011–1027. doi: 10.1104/pp.113.
230714
Bilichak, A., and Kovalchuk, I. (2016). Transgenerational response to stress in
plants and its application for breeding. J. Exp. Bot. 67, 2081–2092. doi: 10.1093/
jxb/erw066
Birnbaum, K. D., and Roudier, F. (2017). Epigenetic memory and cell fate
reprogramming in plants. Regeneration 4, 15–20. doi: 10.1002/reg2.73
Buhtz, A., Pieritz, J., Springer, F., and Kehr, J. (2010). Phloem small RNAs, nutrient
stress responses, and systemic mobility. BMC Plant Biol. 10:64. doi: 10.1186/
1471-2229-10-64
Frontiers in Plant Science | www.frontiersin.org 15 December 2020 | Volume 11 | Article 590847

fpls-11-590847 December 5, 2020 Time: 13:8 # 16
Rasool et al. Grafting in Horticultural Plants
Buhtz, A., Springer, F., Chappell, L., Baulcombe, D. C., and Kehr, J. (2008).
Identification and characterization of small RNAs from the phloem of Brassica
napus. Plant J. Cell Mol. Biol. 53, 739–749. doi: 10.1111/j.1365-313x.2007.
03368.x
Cassol, D. A., Pirola, K., Dotto, M., Citadin, I., Mazaro, S. M., and
Júnior, A. W. (2017). Grafting technique and rootstock species for the
propagation of Plinia cauliflora. Cienc. Rural 47. doi: 10.1590/0103-8478cr201
40452
Cimen, B., and Yesiloglu, T. (2016). “Rootstock breeding for abiotic stress tolerance
in citrus,” in Abiotic and Biotic Stress in Plants-Recent Advances and Future
Perspectives, eds A. Shanker and C. Shanker (London: IntechOpen). doi: 10.
5772/62047
Clemente Moreno, M. J., Hevin, C., Ollat, N., and Cookson, S. J. (2014).
Developments at the graft interface in homo-and hetero-grafts: gene expression
and histological changes during the first month after grafting. Plant Signal.
Behav. 9, 2997–3008. doi: 10.4161/psb.28852
Cline, J. A., Hunter, D. M., Bonn, W. G., and Bijl, M. (2001). Resistance of the
Vineland series of apple root-stocks to fire blight caused by Erwinia amylovora.
J. Am. Pomol. Soc. 55, 218.
Cookson, S. J., Clemente Moreno, M. J., Hevin, C., Nyamba Mendome, L. Z.,
Delrot, S., Magnin, N., et al. (2014). Heterografting with nonself rootstocks
induces genes involved in stress responses at the graft interface when compared
with autografted controls. J. Exp. Bot. 65, 2473–2481. doi: 10.1093/jxb/eru145
Cookson, S. J., Clemente Moreno, M. J., Hevin, C., Nyamba Mendome, L. Z.,
Delrot, S., Trossat-Magnin, C., et al. (2013). Graft union formation in grapevine
induces transcriptional changes related to cell wall modification, wounding,
hormone signalling, and secondary metabolism. J. Exp. Bot. 64, 2997–3008.
doi: 10.1093/jxb/ert144
Cookson, S. J., and Ollat, N. (2013). Grafting with rootstocks induces extensive
transcriptional re-programming in the shoot apical meristem of grapevine.
BMC Plant Biol. 13:147. doi: 10.1186/1471- 2229-13-147
Cramer, G. R., Ergül, A., Grimplet, J., Tillett, R. L., Tattersall, E. A., Bohlman, M. C.,
et al. (2007). Water and salinity stress in grapevines: early and late changes
in transcript and metabolite profiles. Funct. Integr. Genomic 7, 111–134. doi:
10.1007/s10142-006-0039-y
de la Paz Sanchez, M., Aceves-García, P., Petrone, E., Steckenborn, S., Vega-Le on,
R., Alvarez-Buylla, E. R., et al. (2015). The impact of Polycomb group (PcG) and
Trithorax group (TrxG) epigenetic factors in plant plasticity. New Phytol. 208,
684–694. doi: 10.1111/nph.13486
Dogra, K., Kour, K., Kumar, R., Bakshi, P., and Kumar, V. (2018). Graft-
incompatibility in horticultural crops. Int. J. Curr. Microbiol. App. Sci. 7,
1805–1820. doi: 10.20546/ijcmas.2018.702.218
Duan, X., Zhang, W., Huang, J., Hao, L., Wang, S., Wang, A., et al. (2015).
PbWoxT1 mRNA from pear (Pyrus betulaefolia) undergoes long−distance
transport assisted by a polypyrimidine tract binding protein. New Phytol. 210,
511–524. doi: 10.1111/nph.13793
Errea, P. (1998). Implications of phenolic compounds in graft incompatibility
in fruit tree species. Sci. Hortic. 74, 195–205. doi: 10.1016/s0304-4238(98)
00087-9
Errea, P., and Borruey, C. (2004). “Early detection of graft compatibility in
apricot/Prunus combinations,” in Proceedings of the I International Symposium
on Rootstocks for Deciduous Fruit Tree Species, Vol. 658, (Zaragoza), 555–558.
doi: 10.17660/actahortic.2004.658.84
Fan, J., Yang, R., Li, X., Zhao, W., Zhao, F., and Wang, S. (2015). The processes of
graft union formation in tomato. Horticult. Environ. Biotechnol. 56, 569–574.
doi: 10.1007/s13580-015- 0009-1
Fazal, R., Abdur, R., Mohammad, M., and Nadia, B. (2014). Response of apple
cultivars to graft take success on apple rootstock. J. Adv. Lab Res. Biol. 4, 78–84.
Foster, T. M., McAtee, P. A., Waite, C. N., Boldingh, H. L., and McGhie,
T. K. (2017). Apple dwarfing rootstocks exhibit an imbalance in carbohydrate
allocation and reduced cell growth and metabolism. Hortic. Res. 4, 1–13. doi:
10.1038/hortres.2017.9
Fuentes, I., Stegemann, S., Golczyk, H., Karcher, D., and Bock, R. (2014).
Horizontal genome transfer as an asexual path to the formation of new species.
Nature 511, 232–235. doi: 10.1038/nature13291
Gainza, F., Opazo, I., and Muñoz, C. (2015). Graft incompatibility in plants:
metabolic changes during formation and establishment of the rootstock/scion
union with emphasis on Prunus species. Chilean J. Agric. Res. 75, 28–34. doi:
10.4067/s0718-58392015000300004
Gaut, B. S., Miller, A. J., and Seymour, D. K. (2019). Living with two genomes:
grafting and its implications for plant genome-to-genome interactions,
phenotypic variation, and evolution. Annu. Rev. Genet. 53, 195–215. doi: 10.
1146/annurev-genet- 112618-043545
Gautier, A., Cookson, S. J., Hevin, C., Vivin, P., Lauvergeat, V., and
Mollier, A. (2018). Phosphorus acquisition efficiency and phosphorus
remobilization mediate genotype-specific differences in shoot phosphorus
content in grapevine. Tree Physiol. 38, 1742–1751. doi: 10.1093/treephys/
tpy074
Gautier, A. T., Chambaud, C., Brocard, L., Ollat, N., Gambetta, G. A., Delrot, S.,
et al. (2019). Merging genotypes: graft union formation and scion–rootstock
interactions. J. Exp. Bot. 70, 747–755. doi: 10.1093/jxb/ery422
Ghosh, S. N., and Bera, B. (2015). Studies on standardization of propagation
methods of some minor fruit crops in India. IJMFM AP 1, 31–36.
Gökbayrak, Z., Söylemezoðlu, G., Akkurt, M., and Celik, H. (2007). Determination
of grafting compatibility of grapevine with electrophoretic methods. Sci. Hort.
113, 343–352. doi: 10.1016/j.scienta.2007.04.008
Goldschmidt, E. E. (2014). Plant grafting: new mechanisms, evolutionary
implications. Front. Plant Sci. 5:727. doi: 10.3389/fpls.2014.00727
Gonçalves, B., Moutinho-Pereira, J., Santos, A., Silva, A. P., Bacelar, E., Correia,
C., et al. (2006). Scion–rootstock interaction affects the physiology and fruit
quality of sweet cherry. Tree Physiol. 26, 93–104. doi: 10.1093/treephys/
26.1.93
Gonçalves, B., Santos, A., Silva, A. P., Moutinho-Pereira, J., and Torres-Pereira,
J. M. G. (2003). Effect of pruning and plant spacing on the growth of
cherry rootstocks and their influence on stem water potential of sweet cherry
trees. J. Hortic. Sci. Biotech. 78, 667–672. doi: 10.1080/14620316.2003.11
511682
Gonçalves, L. P., Camargo, R. L. B., Takita, M. A., Machado, M. A., dos Soares,
Filho, W. S., et al. (2019). Rootstock-induced molecular responses associated
with drought tolerance in sweet orange as revealed by RNA-Seq. BMC Genomics
20:110. doi: 10.1186/s12864-019-5481-z
Guan, W., Zhao, X., Hassell, R., and Thies, J. (2012). Defense mechanisms involved
in disease resistance of grafted vegetables. HortScience 47, 164–170. doi: 10.
21273/hortsci.47.2.164
Guclu, S. F., and Koyuncu, F. (2012). A method for prediction of graft
incompatibility in sweet cherry. Not. Bot. Horti Agrobo. 40, 243–246. doi: 10.
15835/nbha4017560
Gur, A., Samish, R. M., and Lifshitz, E. (1968). The role of cyanogenic glycoside of
the quincein the incompatibility between pear cultivars and quince rootstocks.
Hortic. Res. 8, 113–134.
Harada, T. (2010). Grafting and RNA transport via phloem tissue in horticultural
plants. Sci. Hortic. 125, 545–550. doi: 10.1016/j.scienta.2010.05.013
Hartmann, H. T., and Kester, D. E. (2002). Plant propagation: principles and
practices. New Jersey, NJ: Prentice-Hall.
Herrero, J., Carrasco, A. E., and Zapata, J. M. (2014). Arabidopsis thaliana
peroxidases involved in lignin biosynthesis: in silico promoter analysis and
hormonal regulation. Plant Physiol. Biochem. 80, 192–202. doi: 10.1016/j.
plaphy.2014.03.027
Hudina, M., Orazem, P., Jakopic, J., and Stampar, F. (2014). The phenolic content
and its involvement in the graft incompatibility process of various pear
rootstocks (Pyrus communis L.). J. Plant Physiol. 171, 76–84. doi: 10.1016/j.
jplph.2013.10.022
Hussain, G., Wani, M. S., Mir, M. A., Rather, Z. A., and Bhat, K. M. (2014).
Micrografting for fruit crop improvement. Afr. J. Biotechnol. 13, 2474–2483.
doi: 10.5897/ajb2013.13602
Ikeuchi, M., Sugimoto, K., and Iwase, A. (2013). Plant callus: mechanisms of
induction and repression. Plant Cell Online 25, 3159–3173. doi: 10.1105/tpc.
113.116053
Jeffree, C. E., and Yeoman, M. M. (1983). Development of intercellular connections
between opposing cells in a graft union. New Phytol. 93, 491–509. doi: 10.1111/
j.1469-8137.1983.tb02701.x
Jensen, P. J., Halbrendt, N., Fazio, G., Makalowska, I., Altman, N., Praul, C., et al.
(2012). Rootstock-regulated gene expression patterns associated with fire blight
resistance in apple. BMC Genomics 13:9. doi: 10.1186/1471-2164-13-9
Frontiers in Plant Science | www.frontiersin.org 16 December 2020 | Volume 11 | Article 590847

fpls-11-590847 December 5, 2020 Time: 13:8 # 17
Rasool et al. Grafting in Horticultural Plants
Kasai, A., Kanehira, A., and Harada, T. (2010). miR172 can move long
distances in Nicotiana benthamiana.Open Plant Sci. J. 4, 1–6. doi: 10.2174/
1874294701004010001
Kehr, J., and Buhtz, A. (2013). “Endogenous RNA constituents of the phloem
and their possible roles in long-distance signalling,” in Phloem. Molecular Cell
Biology, Systemic Communication, Biotic Interactions, eds G. A. Thompson
and A. J. E. van Bel (Hoboken, NJ: Wiley), 186–208. doi: 10.1002/9781118382
806.ch9
Kellenberger, R. T., Schl€uter, P. M., and Schiestl, F. P. (2016). Herbivore-
Induced DNA demethylation changes floral signalling and attractiveness to
pollinators in Brassica rapa.PLoS One 11:e0166646. doi: 10.1371/journal.pone.
0166646
Koepke, T., and Dhingra, A. (2013). Rootstock scion somatogenetic interactions
in perennial composite plants. Plant Cell Rep. 32, 1321–1337. doi: 10.1007/
s00299-013-1471-9
Kondo, Y., Tamaki, T., and Fukuda, H. (2014). Regulation of xylem cell fate. Front.
Plant Sci. 5:315. doi: 10.3389/fpls.2014.00315
Kumar, G. N. M. (2011). Propagation of Plants by Grafting and Budding. Pullman,
WA: Washington State University Extension, 496.
Kumar, P., Rouphael, Y., Cardarelli, M., and Colla, G. (2017). Vegetable grafting as
a tool to improve drought resistance and water use efficiency. Front. Plant Sci.
8:1130. doi: 10.3389/fpls.2017.01130
Kumari, A., Kumar, J., Kumar, A., Chaudhury, A., and Singh, S. P. (2015).
Grafting triggers differential responses between scion and rootstock. PLoS One
10:e0124438. doi: 10.1371/journal.pone.0124438
Kümpers, B. M., and Bishopp, A. (2015). Plant grafting: making the
right connections. Curr. Biol. 25, R411–R413. doi: 10.1016/j.cub.2015.
03.055
Law, J. A., and Jacobsen, S. E. (2010). Establishing, maintaining and modifying
DNA methylation patterns in plants and animals. Nat. Rev. Genet. 11, 204–220.
doi: 10.1038/nrg2719
Lee, J. M., Kubota, C., Tsao, S. J., Bie, Z., Echevarria, P. H., Morra,
L., et al. (2010). Current status of vegetable grafting: diffusion, grafting
techniques, automation. Sci. Hortic. 127, 93–105. doi: 10.1016/j.scienta.2010.
08.003
Lee, K., and Seo, P. J. (2018). Dynamic epigenetic changes during plant
regeneration. Trends Plant Sci 23, 235–247. doi: 10.1016/j.tplants.2017.11.009
Lewsey, M. G., Hardcastle, T. J., Melnyk, C. W., Molnar, A., Valli, A.,
Urich, A. M., et al. (2016). Mobile small RNAs regulate genome-
wide DNA methylation. PNAS 113, E801–E810. doi: 10.1073/pnas.1515
072113
Li, C., Li, Y., Bai, L., Zhang, T., He, C., Yan, Y., et al. (2014). Grafting−responsive
miRNAs in cucumber and pumpkin seedlings identified by high−throughput
sequencing at whole genome level. Physiol. Plant 151, 406–422. doi: 10.1111/
ppl.12122
Li, F., Lei, H., Zhao, X., Tian, R., and Li, T. (2012). Characterization of three
sorbitol transporter genes in micropropagated apple plants grown under
drought stress. Plant Mol. Biol. Rep. 30, 123–130. doi: 10.1007/s11105-01
1-0323-4
Lin, J. S., Lin, C. C., Li, Y. C., Wu, M. T., Tsai, M. H., Hsing, Y. I. C., et al.
(2013). Interaction of small RNA-8105and the intron of IbMYB1 RNA regulates
IbMYB1 family genes through secondary siRNAs and DNA methylation after
wounding. Plant J. 75, 781–794. doi: 10.1111/tpj.12238
Liu, C. J. (2012). Deciphering the enigma of lignification: precursor transport,
oxidation, and the topochemistry of lignin assembly. Mol. Plant. 5, 304–317.
doi: 10.1093/mp/ssr121
Mahunu, J. K., Adjei, P. Y., and Asante, A. K. (2012). Anatomical studies on graft
formation in cashew (Anacardium occidentale L.). Agricult. Biol. J. North Am. 3,
150–153. doi: 10.5251/abjna.2012.3.4.150.153
Mattsson, J., Ckurshumova, W., and Berleth, T. (2003). Auxin signaling in
Arabidopsis leaf vascular development. Plant Physi. 131, 1327–1339. doi: 10.
1104/pp.013623
Melnyk, C. W. (2017). Plant grafting: insights into tissue regeneration. Regeneration
4, 3–14. doi: 10.1002/reg2.71
Melnyk, C. W., Gabel, A., Hardcastle, T. J., Robinson, S., Miyashima, S., Grosse,
I., et al. (2018). Transcriptome dynamics at Arabidopsis graft junctions reveal
an inter tissue recognition mechanism that activates vascular regeneration.
Proc. Natl. Acad. Sci.U. S. A. 115, E2447–E2456. doi: 10.1073/pnas.1718
263115
Melnyk, C. W., and Meyerowitz, E. M. (2015). Plant Grafting. Curr. Biol. 25,
183–188. doi: 10.1016/j.cub.2015.01.029
Melnyk, C. W., Schuster, C., Leyser, O., and Meyerowitz, E. M. (2015). A
developmental framework for graft formation and vascular reconnection in
Arabidopsis thaliana.Curr Biol. 25, 1306–1318. doi: 10.1016/j.cub.2015.03.032
Mitre, V., Mitre, I., Sestras, A. F., and Sestras, R. E. (2012). Effect of roots pruning
upon the growth and fruiting of apple trees in high density orchards. Bull.
UASMV Hortic. 69.
Mng’omba, S. A., du Toit, E. S., and Akinnifesi, F. K. (2008). The relationship
between graft incompatibility and phenols in Uapaca kirkiana Müell Arg. Sci.
Hortic. 117, 212–218. doi: 10.1016/j.scienta.2008.03.031
Mo, Z., Feng, G., Su, W., Liu, Z., and Peng, F. (2018). Identification of
miRNAs associated with graft union development in pecan [Carya
illinoinensis (Wangenh.) K. Koch]. Forests 9:472. doi: 10.3390/f9
080472
Molnar, A., Melnyk, C. W., Bassett, A., Hardcastle, T. J., Dunn, R., and Baulcombe,
D. C. (2010). Small silencing RNAs in plants are mobile and direct epigenetic
modification in recipient cells. Science 328, 872–875. doi: 10.1126/science.
1187959
Mozumder, S. N., Haque, M. I., Shahiduzzaman, M., and Goswami, B. K. (2017).
Effect of grafting time and methods on success of plum Propagation. Cell
88:1497.
Mudge, K., Janick, J., Scofield, S., and Goldschmidt, E. E. (2009). A history of
grafting. Hortic. Rev. 35, 437–493.
Musacchi, S., Masia, A., and Fachinello, J. (2000). Variation of some
enzymatic activities in relationship to scion/stock compatibility in pear/quince
combinations. Acta Hort. 596, 389–392. doi: 10.17660/actahortic.2002.
596.61
Nanda, A. K., and Melnyk, C. W. (2018). The role of plant hormones during
grafting. J. Plant Res. 131, 49–58. doi: 10.1007/s10265- 017-0994-5
Naor, A. (1998). Relations between leaf and stem water potentials and stomatal
conductance in three field-grown woody species. J. Hortic. Sci. Biotech. 73,
431–436. doi: 10.1080/14620316.1998.11510995
Neves, T. R., Mayer, N. A., and Ueno, B. (2017). Graft incompatibility in Prunus
spp. preceded by SPAD index reduction. Semin. Cienc. Agrar. 38, 635–648.
doi: 10.5433/1679-0359.2017v38n2p635
Nguyen, V. H., and Yen, C. R. (2018). Rootstock age and grafting season
affect graft success and plant growth of papaya (Carica papaya L.) in
greenhouse. Chil. J. Agric. Res. 78, 59–67. doi: 10.4067/s0718-58392018000
100059
Nimbolkar, P. K., Shiva, B., and Rai, A. K. (2016). Rootstock breeding for abiotic
stress tolerance in fruit crops. Int. J. Agric. Environ. Biotechnol. 9, 375–380.
doi: 10.5958/2230-732x.2016.00049.8
Notaguchi, M., Higashiyama, T., and Suzuki, T. (2015). Identification of
mRNAs that move over long distances using an RNA-Seq analysis of
Arabidopsis/Nicotiana benthamiana heterografts. Plant Cell Physiol. 56, 311–
321. doi: 10.1093/pcp/pcu210
Notaguchi, M., Kurotani, K. I., Sato, Y., Tabata, R., Kawakatsu, Y., Okayasu,
K., et al. (2020). Cell-cell adhesion in plant grafting is facilitated
by β-1, 4-glucanases. Science 369, 698–702. doi: 10.1126/science.
abc3710
Olmstead, M. A., Lang, N. S., Ewers, F. W., and Owens, S. A. (2006). Xylem vessel
anatomy of sweet cherries grafted onto dwarfing and nondwarfing rootstocks.
J. Am. Soc. Hortic. Sci. 131, 577–585. doi: 10.21273/jashs.131.5.577
Pal, M. D., Mitre, I., Asãnicã, A. C., Sestra?, A. F., Peticilã, A. G., and Mitre,
V. (2017). The influence of rootstock on the growth and fructification of
cherry cultivars in a high-density cultivation system. Not. Bot. Horti Agrobo.
45, 451–457. doi: 10.15835/nbha45210826
Pant, B. D., Buhtz, A., Kehr, J., and Scheible, W. R. (2008). MicroRNA399 is a
long−distance signal for the regulation of plant phosphate homeostasis. Plant
J. 53, 731–738. doi: 10.1111/j.1365-313x.2007.03363.x
Paultre, D. S. G., Gustin, M. P., Molnar, A., and Oparka, K. J. (2016). Lost
in transit: long-distance trafficking and phloem unloading of protein signals
in Arabidopsis homografts. Plant Cell 28, 2016–2025. doi: 10.1105/tpc.16.
00249
Frontiers in Plant Science | www.frontiersin.org 17 December 2020 | Volume 11 | Article 590847

fpls-11-590847 December 5, 2020 Time: 13:8 # 18
Rasool et al. Grafting in Horticultural Plants
Peccoux, A., Loveys, B., Zhu, J., Gambetta, G. A., Delrot, S., Vivin, P., et al. (2018).
Dissecting the rootstock control of scion transpiration using model-assisted
analyses in grapevine. Tree Physiol. 38, 1026–1040. doi: 10.1093/treephys/
tpx153
Pereira, I. D. S., Fachinello, C. J., Antunes, C. E. L., Campos, D. Â., and Pina, A.,
(2014). Incompatibilidade de enxertia em Prunus.Cienc. Rural 44, 1519–1526.
doi: 10.1590/0103-8478cr20131074
Pereira, I. D. S., Pereira, J. F. M., Campos, ÂD., Petri, J. L., and Couto, M. (2015).
Glicosídeos cianogênicos e compatibilidade de enxertia em pereiras asiáticas
enxertadas sobre Pyrus calleryana e P. betulifolia. Pesqui. Agropecu. Bras. 50,
854–859. doi: 10.1590/s0100-204x2015000900015
Pereira, I. D. S., Pina, A., Antunes, L. E. C., Campos, ÂD., and Fachinello, J. C.
(2018). Genotypic differences in cyanogenic glycosides levels of compatible
Prunus persica P. persica and incompatible P. persica P. mume combinations.
Bragantia 77, 1–12. doi: 10.1590/1678- 4499.2016367
Petkou, D. E. M. E. T. R. I. O. S., Diamantidis, G. R. I. G. O. R. I. O. S., and
Vassilakakis, M. (2004). Anionic peroxidase iso-form profiles from calli and
barks of pear cultivars and of the quince rootstock EM. J. Biol. Res. 2, 51–55.
Pina, A., Cookson, S. J., Calatayud, A., Trinchera, A., and Errea, P. (2017).
“Physiological and molecular mechanisms underlying graft compatibility,” in
Vegetable Grafting Principles and Practices, eds G. Colla, F. P. érez-Alfocea, and
D. Schwarz (Wallingford: CABI), 132–154. doi: 10.1079/9781780648972.0132
Pina, A., and Errea, P. (2008). Influence of graft incompatibility on gene expression
and enzymatic activity of UDP-glucose pyrophosphorylase. Plant Sci. 174,
502–509. doi: 10.1016/j.plantsci.2008.01.015
Pina, A., Errea, P., and Martens, H. J. (2012). Graft union formation and cell-to-
cell communication via plasmodesmata in compatible and incompatible stem
unions of Prunus spp. Sci. Hortic. 143, 144–150. doi: 10.1016/j.scienta.2012.
06.017
Polat, M., Dolgun, O., Yildirim, A., A¸skin, M. A., and Gökbayrak, Z. (2010). Graft
union formation of spur apple varieties grafted on different rootstocks. J. Food
Agri. Environ. 8, 490–493.
Prabpree, A., Sangsil, P., Nualsri, C., and Nakkanong, K. (2018). Expression profile
of phenylalanine ammonia-lyase (PAL) and phenolic content during early
stages of graft development in bud grafted Hevea brasiliensis.Biocatal. Agric.
Biotechnol. 14, 88–95. doi: 10.1016/j.bcab.2018.02.010
Probst, A. V., and Mittelsten Scheid, O. (2015). Stress-induced structural changes
in plant chromatin. Curr. Opin. Plant Biol. 27, 8–16. doi: 10.1016/j.pbi.2015.
05.011
Prodhomme, D., Fonayet, J. V., Hévin, C., Franc, C., Hilbert, G., de Revel, G.,
et al. (2019). Metabolite profiling during graft union formation reveals the
reprogramming of primary metabolism and the induction of stilbene synthesis
at the graft interface in grapevine. BMC Plant Biol. 19:599. doi: 10.1186/s12870-
019-2055-9
Rafail, S. T., and Mosleh, M. S. (2010). “Factors involved in micropropagation
and shoot tip grafting of apple (Malus x domestica Borkh.) and pear (Pyrus sp.
L.). Tropentag,” in World Food System–A Contribution from Europe September,
14-16, Zurich.
Rehman, H. U., and Gill, M. I. S. (2014). In vitro shoot tip grafting of Patharnakh
pear on Kainth rootstock. Vegetos 27, 363–369. doi: 10.5958/2229-4473.2014.
00058.5
Ribeiro, L. M., Nery, L. A., Vieira, L. M., and Mercadante-Simões, M. O. (2015).
Histological study of micrografting in passionfruit. Plant Cell Tissue Organ.
Cult. 123, 173–181. doi: 10.1007/s11240-015-0824-1
Rivera-Vega, L., Mamidala, P., Koch, J. L., Mason, M. E., and Mittapalli, O. (2011).
Evaluation of reference genes for expression studies in ash (Fraxinus spp.). Plant
Mol. Biol. Rep. 30, 242–245. doi: 10.1007/s11105- 011-0340-3
Santarosa, E., de Souza, P. V. D., de Araujo Mariath, J. E., and Lourosa, G. V. (2016).
Physiological interaction between rootstock-scion: effects on xylem vessels in
Cabernet Sauvignon and Merlot grapevines. Am. J. Enol. Viticult. 67, 65–76.
doi: 10.5344/ajev.2015.15003
Sharma, R., Singh, P. N., Dimri, D. C., Misra, K. K., and Singh, N. (2018). Effect of
different methods of propagation on peach cv. Shan-e-Punjab in Tarai condition
of Uttarakhand. Int. J. Chem. Stud. 6, 3220–3223.
Sharma, S., Rehalia, A. S., and Sharma, S. D. (2009). Vegetative growth restriction
in pome and stone fruits–a review. Agric. Rev. 30, 13–23. doi: 10.1094/
9780890545010.003
Shen, Y., Issakidis-Bourguet, E., and Zhou, D. X. (2016). Perspectives on the
interactions between metabolism, redox, and epigenetics in plants. J. Exp. Bot.
67, 5291–5300. doi: 10.1093/jxb/erw310
Soleimani, A., Hassani, V., and Rabiei, D. (2010). Effect of different techniques on
walnut (J. regia L) grafting. J. Food Agric. Environ. 8, 544–546.
Spicer, R., and Groover, A. (2010). Evolution of development of vascular cambia
and secondary growth. New Phytol. 186, 577–592. doi: 10.1111/j.1469-8137.
2010.03236.x
Spiegelman, Z., Ham, B. K., Zhang, Z., Toal, T. W., Brady, S. M., Zheng, Y.,
et al. (2015). A tomato phloem−mobile protein regulates the shoot−to−root
ratio by mediating the auxin response in distant organs. Plant J. 83, 853–863.
doi: 10.1111/tpj.12932
Srivastava, K. K. (2012). Effect of different environments, grafting methods and
time on sprouting, graft success and plant growth of walnut (J. regia). Indian J.
Agric. Sci. 82, 1022–1026.
Stegemann, S., and Bock, R. (2009). Exchange of genetic material between
cells in plant tissue grafts. Science 324, 649–651. doi: 10.1126/science.117
0397
Stegemann, S., Keuthe, M., Greiner, S., and Bock, R. (2012). Horizontal transfer of
chloroplast genomes between plant species. Proc. Natl. Acad. Sci. U. S. A. 109,
2434–2438. doi: 10.1073/pnas.1114076109
Tamiru, M., Hardcastle, T. J., and Lewsey, M. G. (2018). Regulation of genome-
wide DNA methylation by mobile small RNAs. New Phytol. 217, 540–546.
doi: 10.1111/nph.14874
Tedesco, S., Pina, A., Fevereiro, P., and Kragler, F. (2020). A phenotypic
search on graft compatibility in grapevine. Agronomy 10:706. doi: 10.3390/
agronomy10050706
Tietel, Z., Srivastava, S., Fait, A., Tel-Zur, N., Carmi, N., and Raveh, E. (2020).
Impact of scion/rootstock reciprocal effect on metabolomics of fruit juice and
phloem sap in grafted Citrus reticulata.PLoS One 15:e0227192. doi: 10.1371/
journal.pone.0227192
Toscano-Morales, R., Xoconostle-Cázares, B., Martínez-Navarro, A. C., and Ruiz-
Medrano, R. (2016). AtTCTP2 mRNA and protein movement correlates with
formation of adventitious roots in tobacco. Plant Signal. Behav. 11:e1071003.
doi: 10.1080/15592324.2015.1071003
Tsago, Y., Andargie, M., and Takele, A. (2014). In vitro selection of sorghum
(Sorghum bicolor (L) Moench) for polyethylene glycol (PEG) induced drought
stress. Plant Sci. Today 1, 62–68. doi: 10.14719/pst.2014.1.2.14
Turnbull, C. G. N. (2010). Grafting as a research tool. Methods Mol. Biol. 655,
11–26. doi: 10.1007/978-1-60761-765-5_2
Wang, G., Li, X., Chen, Q., and Tian, J. (2010). Studies on factors affecting the
microshoot grafting survival of walnut. Acta Hort. 861, 327–331. doi: 10.17660/
actahortic.2010.861.45
Wang, J., Jiang, L., and Wu, R. (2017). Plant grafting: how genetic exchange
promotes vascular reconnection. New Phytol. 214, 56–65. doi: 10.1111/nph.
14383
Wang, H., Zhou, P., Zhu, W., and Wang, F. (2019). De novo Comparative
Transcriptome Analysis of Genes Differentially Expressed in the Scion of
Homografted and Heterografted Tomato Seedlings. Sci. Rep. 9:20240. doi:
10.1038/s41598-019-56563-z
Wang, Y. (2011). Plant grafting and its application in biological research. Chinese
Sci. Bull. 56, 3511–3517. doi: 10.1007/s11434-011-4816-1
Warschefsky, E. J., Klein, L. L., Frank, M. H., Chitwood, D. H., Londo, J. P., von
Wettberg, E. J., et al. (2016). Rootstocks: diversity, domestication, and impacts
on shoot phenotypes. Trends Plant Sci. 21, 418–437. doi: 10.1016/j.tplants.2015.
11.008
Westwood, J. H., Yoder, J. I., Timko, M. P., and dePamphilis, C. W. (2010). The
evolution of parasitism in plants. Trends Plant Sci. 15, 227–235. doi: 10.1016/j.
tplants.2010.01.004
Xing, L., Zhang, D., Zhao, C., Li, Y., Ma, J., An, N., et al. (2016). Shoot bending
promotes flower bud formation by mi RNA-mediated regulation in apple
(Malus domestica Borkh.). Plant Biotechnol. J. 14, 749–770. doi: 10.1111/pbi.
12425
Xu, D., Yuan, H., Tong, Y., Zhao, L., Qiu, L., Guo, W., et al. (2017). Comparative
proteomic analysis of the graft unions in hickory (Carya cathayensis) provides
insights into response mechanisms to grafting process. Front. Plant Sci. 8:676.
doi: 10.3389/fpls.2017.00676
Frontiers in Plant Science | www.frontiersin.org 18 December 2020 | Volume 11 | Article 590847

fpls-11-590847 December 5, 2020 Time: 13:8 # 19
Rasool et al. Grafting in Horticultural Plants
Xu, H., Zhang, W., Li, M., Harada, T., Han, Z., and Li, T. (2010). Gibberellic
acid insensitive mRNA transport in both directions between stock and scion
in Malus.Tree Genet. Genomes 6, 1013–1019. doi: 10.1007/s11295- 010-0309-7
Xu, Y., Yuan, Y., Du, N., Wang, Y., Shu, S., Sun, J., et al. (2018). Proteomic analysis
of heat stress resistance of cucumber leaves when grafted onto Momordica
rootstock. Hortic. Res. 5:53. doi: 10.1038/s41438-018-0060- z
Yamamuro, C., Zhu, J. K., and Yang, Z. (2016). Epigenetic modifications and plant
hormone action. Mol. Plant 9, 57–70. doi: 10.1016/j.molp.2015.10.008
Yazdani, Z., Jafarpour, M., and Shams, M. (2016). Effect of scion source budding
method and graft union height on sweet cherry budding compatibility on
Mahaleb rootstock. Trends Appl. Sci. Res. 6, 1–4. doi: 10.3923/thr.2016.1.4
Yildirim, A. N., Polat, M., Dolgun, O., Askin, M. A., Gokbayrak, Z., and San,
B. (2010). Graft formation in some spur and vigorous apple varieties grafted
on Ottawa 3 rootstock: a histological investigation. J. Food Agric. Environ. 8,
512–514.
Yıldırım, H., Akdemir, H., Süzerer, V., Ozden, Y., and Onay, A. (2013). In vitro
micrografting of the almond cultivars “Texas”, “Ferrastar” and “Nonpareil”.
Biotechnol. Biotechnol. Eq. 27, 3493–3501. doi: 10.5504/bbeq.2012.0087
Yin, H., Yan, B., Sun, J., Jia, P., Zhang, Z., Yan, X., et al. (2012). Graft-union
development: a delicate process that involves cell–cell communication between
scion and stock for local auxin accumulation. J. Exp. Bot. 63, 4219–4232. doi:
10.1093/jxb/ers109
Zambrosi, F. C. B., Mattos, D., Boaretto, R. M., Quaggio, J. A., Muraoka, T., and
Syvertsen, J. P. (2012). Contribution of phosphorus (32 P) absorption and
remobilization for citrus growth. Plant Soil. 355, 353–362. doi: 10.1007/s11104-
011-1107-1
Zarrouk, O., Gogorcena, Y., Moreno, M. A., and Pinochet, J. (2006). Graft
compatibility between peach cultivars and Prunus rootstocks. HortScience 41,
1389–1394. doi: 10.21273/hortsci.41.6.1389
Zarrouk, O., Testillano, P. S., Risueño, M. C., Moreno, M. Á, and Gogorcena, Y.
(2010). Changes in cell/tissue organization and peroxidase activity as markers
for early detection of graft incompatibility in peach/plum combinations. J. Am.
Soc. Hortic. Sci. 135, 9–17. doi: 10.21273/jashs.135.1.9
Zhang, F., Qi, B., Wang, L., Zhao, B., Rode, S., Riggan, N. D., et al. (2016).
EIN2-dependent regulation of acetylation of histone H3K14 and non-canonical
histone H3K23 in ethylene signalling. Nat. Commun. 7:13018. doi: 10.1038/
ncomms13018
Zhang, F., Wang, L., Qi, B., Zhao, B., Ko, E. E., Riggan, N. D., et al. (2017). EIN2
mediates direct regulation of histone acetylation in the ethylene response. Proc.
Natl. Acad. Sci. U. S. A. 114, 10274–10279. doi: 10.1073/pnas.1707937114
Zhang, W. N., Gong, L., Ma, C., Xu, H. Y., Hu, J. F., Harada,
T., et al. (2012). Gibberellic acid-insensitive mRNA transport in
Pyrus. Plant Mol. Biol. Rep. 30, 614–623. doi: 10.1007/s11105-01
1-0365-7
Zhao, D., Zhong, G. Y., and Song, G. Q. (2020). Transfer of endogenous small
RNAs between branches of scions and rootstocks in grafted sweet cherry trees.
PLoS One. 15:e0236376. doi: 10.1371/journal.pone.0236376
Zheng, B. S., Chu, H. L., Jin, S. H., Huang, Y. J., Wang, Z. J., Chen, M., et al. (2010).
cDNA-AFLP analysis of gene expression in hickory (Carya cathayensis) during
graft process. Tree Physiol. 30, 297–303. doi: 10.1093/treephys/tpp102
Zheng, X., Zhao, Y., Shan, D., Shi, K., Wang, L., Li, Q., et al. (2018). MdWRKY9
overexpression confers intensive dwarfing in the M26 rootstock of apple
by directly inhibiting brassinosteroid synthetase MdDWF4 expression. New
Phytol. 217, 1086–1098. doi: 10.1111/nph.14891
Conflict of Interest: The authors declare that the research was conducted in the
absence of any commercial or financial relationships that could be construed as a
potential conflict of interest.
Copyright © 2020 Rasool, Mansoor, Bhat, Hassan, Baba, Alyemeni, Alsahli,
El-Serehy, Paray and Ahmad. This is an open-access article distributed under the
terms of the Creative Commons Attribution License (CC BY). The use, distribution
or reproduction in other forums is permitted, provided the original author(s) and
the copyright owner(s) are credited and that the original publication in this journal
is cited, in accordance with accepted academic practice. No use, distribution or
reproduction is permitted which does not comply with these terms.
Frontiers in Plant Science | www.frontiersin.org 19 December 2020 | Volume 11 | Article 590847