[Show abstract][Hide abstract] ABSTRACT: Multiple plant developmental processes, such as lateral root development, depend on auxin distribution patterns that are in part generated by the PIN-formed family of auxin-efflux transporters. Here we propose that AUXIN RESPONSE FACTOR7 (ARF7) and the ARF7-regulated FOUR LIPS/MYB124 (FLP) transcription factors jointly form a coherent feed-forward motif that mediates the auxin-responsive PIN3 transcription in planta to steer the early steps of lateral root formation. This regulatory mechanism might endow the PIN3 circuitry with a temporal 'memory' of auxin stimuli, potentially maintaining and enhancing the robustness of the auxin flux directionality during lateral root development. The cooperative action between canonical auxin signalling and other transcription factors might constitute a general mechanism by which transcriptional auxin-sensitivity can be regulated at a tissue-specific level.
[Show abstract][Hide abstract] ABSTRACT: PIN proteins are auxin export carriers that direct intercellular auxin flow and in turn regulate
many aspects of plant growth and development including responses to environmental
changes. The Arabidopsis R2R3-MYB transcription factor FOUR LIPS (FLP) and its paralogue
MYB88 regulate terminal divisions during stomatal development, as well as female
reproductive development and stress responses. Here we show that FLP and MYB88
act redundantly but differentially in regulating the transcription of PIN3 and PIN7 in
gravity-sensing cells of primary and lateral roots. On the one hand, FLP is involved in
responses to gravity stimulation in primary roots, whereas on the other, FLP and MYB88
function complementarily in establishing the gravitropic set-point angles of lateral roots.
Our results support a model in which FLP and MYB88 expression specifically determines the
temporal-spatial patterns of PIN3 and PIN7 transcription that are closely associated with their
preferential functions during root responses to gravity.
[Show abstract][Hide abstract] ABSTRACT: Strigolactones are important rhizosphere signals that act as phytohormones and have multiple functions, including modulation of lateral root (LR) development. Here, we show that treatment with the strigolactone analog GR24 did not affect LR initiation, but negatively influenced LR priming and emergence, the latter especially near the root-shoot junction. The cytokinin module ARABIDOPSIS HISTIDINE KINASE3 (AHK3)/ARABIDOPSIS RESPONSE REGULATOR1 (ARR1)/ARR12 was found to interact with the GR24-dependent reduction in LR development, because mutants in this pathway rendered LR development insensitive to GR24. Additionally, pharmacological analyses, mutant analyses, and gene expression analyses indicated that the affected polar auxin transport stream in mutants of the AHK3/ARR1/ARR12 module could be the underlying cause. Altogether, the data reveal that the GR24 effect on LR development depends on the hormonal landscape that results from the intimate connection with auxins and cytokinins, two main players in LR development.
Journal of Experimental Botany 10/2015; DOI:10.1093/jxb/erv478 · 5.53 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Functional analyses of MADS-box transcription factors in plants have unraveled their role in major developmental programs (e.g; flowering and floral organ identity), as well as in stress-related developmental processes such as abscission, fruit ripening and senescence. Over-expression of the OsMADS26 gene in rice (Oryza sativa) has revealed a possible function related to stress response (Lee et al., 2008b). Here we show that OsMADS26 down-regulated plants exhibit enhanced resistance against two major rice pathogens, Magnaporthe oryzae and Xanthomonas oryzae. Despite this enhanced resistance to biotic stresses, OsMADS26 down-regulated plants also displayed enhanced tolerance to water deficit. These phenotypes were observed both in controlled and field conditions. Interestingly, alteration of OsMADS26 expression has no strong impact on plant development. Gene expression profiling revealed that a majority of genes miss-regulated in over-expresser and down-regulated OsMADS26 lines compared to control plants are associated to biotic or abiotic stress response. Altogether, our data indicate that OsMADS26 acts as an upstream regulator of stress-associated genes and thereby as a hub to modulate the response to various stresses in the rice plant.
[Show abstract][Hide abstract] ABSTRACT: ] The leaf is the major functional part of the shoot performing the bulk of photosynthetic activity. Its development is relatively plastic allowing the plant to adapt to environmental changes by modifying leaf size and anatomy. Moreover, a leaf is made up of various distinct cell layers, each having specialized functions. To understand functional adaptation and the development of the leaf it is essential to obtain cross sections throughout leaf development and at maturity (Kalve et al., 2014). Here, we describe a protocol for transverse sectioning of Arabidopsis thaliana leaves using resin embedding. This protocol provides a reliable platform to yield high quality images of cross sections allowing study of development of various tissue layers across the transversal axis of the leaf. As this method is an adaptation of the protocol developed for the Arabidopsis root tip by Beeckman and Viane (1999) and De Smet et al. (2004), it can easily be modified to accommodate other organs and species.
[Show abstract][Hide abstract] ABSTRACT: The leaf is the major functional part of the shoot performing the bulk of photosynthetic activity. Its development is relatively plastic allowing the plant to adapt to environmental changes by modifying leaf size and anatomy. Moreover, a leaf is made up of various distinct cell layers, each having specialized functions. To understand functional adaptation and the development of the leaf it is essential to obtain cross sections throughout leaf development and at maturity (Kalve et al., 2014). Here, we describe a protocol for transverse sectioning of Arabidopsis thaliana leaves using resin embedding. This protocol provides a reliable platform to yield high quality images of cross sections allowing study of development of various tissue layers across the transversal axis of the leaf. As this method is an adaptation of the protocol developed for the Arabidopsis root tip by Beeckman and Viane (1999) and De Smet et al. (2004), it can easily be modified to accommodate other organs and
[Show abstract][Hide abstract] ABSTRACT: Fucoid zygotes have been extensively used to study cell polarization and asymmetrical cell division. Fertilized eggs are responsive to different environmental cues (e.g., light, gravity) for a long period before the polarity is fixed and the cells germinate accordingly. First, it is commonly believed that the direction and sense of the polarization vector are established simultaneously as indicated by the formation of an F-actin patch. Secondly, upon reorientation of the zygote, a new polar gradient is formed and it is assumed that the position of the future rhizoid pole is only influenced by the latter. Here we tested these two hypotheses investigating photopolarization in Fucus zygotes by reorienting zygotes 90° relative to a unilateral light source at different time points during the first cell cycle. We conclude that fixation of direction and sense of the polarization vector is indeed established simultaneously. However, the experiments yielded a distribution of polarization axes that cannot be explained if only the last environmental cue is supposed to determine the polarization axis. We conclude that our observations, together with published findings, can only be explained by assuming imprinting of the different polarization vectors and their integration as a vectorial sum at the moment of axis fixation. This way cells will average different serially perceived cues resulting in a polarization vector representative of the dynamic intertidal environment, instead of betting exclusively on the perceived vector at the moment of axis fixation.
[Show abstract][Hide abstract] ABSTRACT: Variations in size and shape of multicellular organs depend on spatio-temporal regulation of cell division and expansion. Here, cell division and expansion rates were quantified relative to the three spatial axes in the first leaf pair of Arabidopsis thaliana. The results show striking differences in expansion rates: the expansion rate in the petiole is higher than in the leaf blade; expansion rates in the lateral direction are higher than longitudinal rates between 5 and 10 days after stratification, but become equal at later stages of leaf blade development; and anticlinal expansion co-occurs with, but is an order of magnitude slower than periclinal expansion. Anticlinal expansion rates also differed greatly between tissues: the highest rates occurred in the spongy mesophyll and the lowest in the epidermis. Cell division rates were higher and continued for longer in the epidermis compared with the palisade mesophyll, causing a larger increase of palisade than epidermal cell area over the course of leaf development. The cellular dynamics underlying the effect of shading on petiole length and leaf thickness were then investigated. Low light reduced leaf expansion rates, which was partly compensated by increased duration of the growth phase. Inversely, shading enhanced expansion rates in the petiole, so that the blade to petiole ratio was reduced by 50%. Low light reduced leaf thickness by inhibiting anticlinal cell expansion rates. This effect on cell expansion was preceded by an effect on cell division, leading to one less layer of palisade cells. The two effects could be uncoupled by shifting plants to contrasting light conditions immediately after germination. This extended kinematic analysis maps the spatial and temporal heterogeneity of cell division and expansion, providing a framework for further research to understand the molecular regulatory mechanisms involved.
[Show abstract][Hide abstract] ABSTRACT: Photoassimilates such as sugars are transported through phloem sieve element cells in plants. Adapted for effective transport,
sieve elements develop as enucleated living cells. We used electron microscope imaging and three-dimensional reconstruction
to follow sieve element morphogenesis in Arabidopsis. We show that sieve element differentiation involves enucleation, in which the nuclear contents are released and degraded
in the cytoplasm at the same time as other organelles are rearranged and the cytosol is degraded. These cellular reorganizations
are orchestrated by the genetically redundant NAC domain–containing transcription factors, NAC45 and NAC86 (NAC45/86). Among
the NAC45/86 targets, we identified a family of genes required for enucleation that encode proteins with nuclease domains.
Thus, sieve elements differentiate through a specialized autolysis mechanism.
[Show abstract][Hide abstract] ABSTRACT: Coordination of cell division and pattern formation is central to tissue and organ development, particularly in plants where
walls prevent cell migration. Auxin and cytokinin are both critical for division and patterning, but it is unknown how these
hormones converge upon tissue development. We identify a genetic network that reinforces an early embryonic bias in auxin
distribution to create a local, nonresponding cytokinin source within the root vascular tissue. Experimental and theoretical
evidence shows that these cells act as a tissue organizer by positioning the domain of oriented cell divisions. We further
demonstrate that the auxin-cytokinin interaction acts as a spatial incoherent feed-forward loop, which is essential to generate
distinct hormonal response zones, thus establishing a stable pattern within a growing vascular tissue.
[Show abstract][Hide abstract] ABSTRACT: Overall root architecture is the combined result of primary and lateral root growth and is influenced by both intrinsic genetic programs and external signals. One of the main questions for root biologists is how plants control the number of lateral root primordia and their emergence through the main root. We recently identified SKP2B as a new early marker for lateral root development. Here, we took advantage of its specific expression pattern in a cell sorting and transcriptomic approach to generate a lateral root specific cs-SKP2B dataset that represents the endogenous genetic developmental program. We first validated this dataset by showing that many of the identified genes have a function during root growth or lateral root development. Importantly, genes encoding PEROXIDASES were highly represented in our dataset. We thus next focused on this class of enzymes and showed, using genetic and chemical inhibitor studies, that peroxidase activity and ROS signalling is specifically required during lateral root emergence, but intriguingly not for primordium specification itself.
[Show abstract][Hide abstract] ABSTRACT: The essential role of auxin for cell proliferation in plants is well known. Both auxin signaling and cell cycle regulation have been studied elaborately, but less is known about the connection between these processes. Recent studies report on the first molecular pathways that have been found to directly link auxin levels to the regulation of cell cycle activity. Here, we discuss the general effect of auxin on cell cycle progression and then zoom in on the interplay between auxin and the cell cycle during root development in Arabidopsis thaliana. At the root tip, an auxin gradient maintains the correct organization of the ground tissue layers and controls the size of the root apical meristem. During auxin-induced lateral root initiation LATERAL ORGAN BOUNDARIES-DOMAIN transcription factors are upregulated and control reactivation of the cell cycle and cell specification, both of which are needed for proper lateral root initiation. Auxin-induced lateral root initiation-like pathways are also involved in cell cycle reactivation during the formation of nematode feeding sites, nitrogen-fixing nodules and callus tissue, pointing to the existence of one common auxin–cell cycle module to initiate new organs in plants.
Auxin and Its Role in Plant Development, 04/2014: pages 119-141; , ISBN: 978-3-7091-1525-1