Biostimulants in agriculture

Abstract

Biostimulants, which may be derived from a wide range of natural or synthetic processes, are now widely used in agriculture and yet the mode of action of these materials is not well understood. On the basis of available literature, and based upon the diversity of biostimulant responses highlighted in this focus issue, we hypothesize that biostimulants function by directly interacting with plant signaling cascades or act through stimulation of endophytic and non-endophytic bacteria, yeast and fungi to produce molecules of benefit to the plant. The benefit of the biostimulant is derived from the reduction in assimilates that are diverted to non-productive stress response metabolism.

Full text

MINI REVIEW
published: 27 August 2015
doi: 10.3389/fpls.2015.00671
Frontiers in Plant Science | www.frontiersin.org 1August 2015 | Volume 6 | Article 671
Edited by:
Ebrahim Hadavi,
Islamic Azad University, Iran
Reviewed by:
Victoria Fernandez,
Technical University of Madrid, Spain
Ioannis S. Minas,
Aristotle University of Thessaloniki,
Greece
*Correspondence:
Patrick Brown,
Department of Plant Sciences,
University of California, Davis, MS#2,
One Shields Avenue, Davis,
CA 95616, USA
phbrown@ucdavis.edu
Specialty section:
This article was submitted to
Crop Science and Horticulture,
a section of the journal
Frontiers in Plant Science
Received: 12 June 2015
Accepted: 13 August 2015
Published: 27 August 2015
Citation:
Brown P and Saa S (2015)
Biostimulants in agriculture.
Front. Plant Sci. 6:671.
doi: 10.3389/fpls.2015.00671
Biostimulants in agriculture
Patrick Brown 1*and Sebastian Saa 2
1Department of Plant Sciences, University of California, Davis, Davis, CA, USA, 2Escuela de Agronomía, Pontificia
Universidad Católica de Valparaíso, Quillota, Chile
Keywords: biostimulants, mode of action, plant signaling, stress response, endophytic microorganisms, microbial
extracts
The past decades have witnessed tremendous growth in the use of biostimulants in agriculture
and it is estimated that biostimulants will grow to $2 billion in sales by 2018 (Calvo et al., 2014).
Recognizing the need to establish a legal framework for the marketing and regulation of these
products the European biostimulants industry council (EBIC, 2012) defined plant biostimulants
as “containing substance(s) and/or micro-organisms whose function when applied to plants or the
rhizosphere is to stimulate natural processes to enhance/benefit nutrient uptake, nutrient efficiency,
tolerance to abiotic stress, and crop quality.”
There is a clear need to improve our understanding of biostimulant function so that the efficacy
of these materials can be improved and the industrial processes can be optimized. Determining
the function of this class of products, however, has proven to be immensely difficult (Khan et al.,
2009; Carvalhais et al., 2013; Rose et al., 2014). This is in large part due to the diversity of sources
of these materials and the complexity of the resulting product, which in most cases will contain
a significant number of poorly characterized molecules. Since biostimulants are derived from an
incredibly diverse set of biological and inorganic materials (Calvo et al., 2014) including microbial
fermentations of animal or plant feedstock, living microbial cultures, macro, and micro-alga,
protein hydrolysate, humic, and fulvic substances, composts, manures, food, and industrial wastes
prepared using widely divergent industrial manufacturing processes, it is illogical to assume that
there is a single mode of action.
The definition of biostimulants adopted by EBIC specifies that these materials should not
function by virtue of the presence of essential mineral elements, known plant hormones or
disease suppressive molecules. Accepting this definition, we hypothesize that biostimulants benefit
plant productivity by interacting with plant signaling processes thereby reducing negative plant
response to stress. This hypothesis recognizes the wealth of recent research demonstrating that
plant response to stress is regulated by signaling molecules that may be generated by the plant or
its associated microbial populations (Marasco et al., 2012; Bakker et al., 2014; Vandenkoornhuyse
et al., 2015). Biostimulants may either directly interact with plant signaling cascades or act through
stimulation of endophytic and non-endophytic bacteria, yeast, and fungi to produce molecules of
benefit to the plant (Figure 1). The benefit of the biostimulant is derived from the reduction in
assimilates that are diverted to non-productive stress response metabolism.
In this research topic the effects of biostimulants on plant productivity is examined in 10 research
papers. Colla et al. (2014), soil-applied a plant-derived protein hydrolosate and demonstrated
improved growth and nitrogen assimilation in seedlings of pea, tomato, and corn. The use of
giberrellic acid (GA) deficient mutants and classic auxin response treatments suggests this material
benefits plant growth by mimicking the actions of indole acetic acid (IAA) and GA.
Ertani et al. (2014) observed the effects of alfalfa hydrolosate (AH) and red grape extract (RG)
on nitrogen metabolism and growth of pepper plants (Capsicum chinensis). Significant, dose
dependent changes were observed in a wide range of sugars, phenols, and quarternary nitrogen
containing molecules. In almond grown under high nutrient supply conditions biostimulants
derived from either seaweed or microbial fermentation of cereal grains, had a marked positive
effect on shoot growth and leaf area (Saa et al., 2015). Under conditions of low nutrient supply

Brown and Saa Biostimulants in agriculture
FIGURE 1 | Non-lethal stress is experienced to varying degrees by all
crop plants resulting in a loss of productivity as assimilates are
diverted to stress response metabolism (top figure). It is hypothesized
that biostimulants interacting with plant signaling processes reduce the extent
of negative plant response to stress and increase the allocation of biomass to
the harvested yield component.
the benefit was less significant though there was a marked
increase in rubidium uptake (an analog for K uptake). A
differential response to the application of a nitrophenolate based
biostimulant (Przybysz et al., 2014) was observed with significant
and consistent growth and photosynthesis improvements under
drought and heavy metal stress (platinum) and inconsistent
growth benefit under non-stressed growth conditions.
Evidence that biostimulants may enhance macro nutrient
uptake has been reported previously (Calvo et al., 2014; Rose
et al., 2014) and have been ascribed to an effect on sink activity
or stimulation of nitrogen metabolism. Foliar application of
a biostimulant derived from microbial fermentation of cereal
grains (Tian et al., 2015) greatly enhanced the movement of
foliar applied zinc in sunflower. Using high resolution elemental
mapping techniques (µ-Xray Florescence) the movement of
Zn to the phloem following application of a combination of
biostimulant and zinc sulfate was elegantly demonstrated. This
research did not determine if the addition of the biostimulant
enhanced Zn uptake by increasing Zn movement through the leaf
surface and subsequent transport of Zn to the phloem, or if the
enhanced transport was a result of increased sink strength as was
observed when this same product was used in Almond (Saa et al.,
2015).
Vergnes et al. (2014) used foliar application of an essential
oil derived from Gaultheria procumbens and demonstrated
significant induced resistance on Arabidopsis leaves inoculated
with the fungal pathogen C. higginsianum. The authors
concluded that the essential oil from G. procumbens could be
a valuable natural source of methyl salicylic acid (MeSA) for
biocontrol applications. The application of salicylic acid (SA) has
been shown to have negative effects on plant productivity either
as a result of direct toxicity or changes in allocation of assimilates
to plant defense responses. This response was also observed by
Ghazijahani et al. (2014) who noted that the negative effects of
SA can be mitigated by co-application of citric acid.
Many biostimulants contain simple and complex
carbohydrates that when applied to plant may alter metabolism
by directly acting as a source of energy for endophytic and
non-endophytic microbial populations or acting as signaling
molecules. The complexity of the roles of carbohydrates in
plant immunity was reviewed by Trouvelot et al. (2014), who
suggested that carbohydrates activate defense reactions by
pathogen associated molecular patterns (PAMPs), microbe
associated molecular patterns (MAMPS), and damage associated
molecular patterns (DAMPs). The authors highlight the main
classes of carbohydrates that are involved in plant immunity
(beta-glucans, chitin, pectin) and discuss how the degree of
polymerization and types of oligosaccharides affects biological
activity. This review further suggests that carbohydrates in
biostimulants may act by beneficially manipulating plant
signaling cascades.
The great diversity of plant response to biostimulants
highlights the challenges faced by researchers. Many plant
responses to biostimulants cannot be explained by our current
understanding of plant processes and while this represents a
challenge, it also presents a great opportunity.
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Conflict of Interest Statement: 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 © 2015 Brown and Saa. This is an open-access article distributed under the
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