Emerging connections between small RNAs and phytohormones

Small RNAs (sRNAs), mainly including miRNAs and siRNAs, are ubiquitous in eukaryotes. sRNAs mostly negatively regulate gene expression via (post-)transcriptional gene silencing through DNA methylation, mRNA cleavage, or translation inhibition. The mechanisms of sRNA biogenesis and function in diverse biological processes, as well as the interactions between sRNAs and environmental factors, like (a)biotic stress, have been deeply explored. Phytohormones are central in the plant’s response to stress, and multiple recent studies highlight an emerging role for sRNAs in the direct response to, or the regulation of, plant hormonal pathways. In this review, we discuss recent progress on the unraveling of crossregulation between sRNAs and nine plant hormones

Different adaptation strategies of two citrus scion/rootstock combinations in response to drought stress

Scion/rootstock interaction is important for plant development and for breeding programs. In this context, polyploid rootstocks presented several advantages, mainly in relation to biotic and abiotic stresses. Here we analyzed the response to drought of two different scion/rootstock combinations presenting different polyploidy: the diploid (2x) and autotetraploid (4x) Rangpur lime (Citrus limonia, Osbeck) rootstocks grafted with 2x Valencia Delta sweet orange (Citrus sinensis) scions, named V/2xRL and V/4xRL, respectively. Based on previous gene expression data, we developed an interactomic approach to identify proteins involved in V/2xRL and V/4xRL response to drought. A main interactomic network containing 3,830 nodes and 97,652 edges was built from V/2xRL and V/4xRL data. Exclusive proteins of the V/2xRL and V/4xRL networks (2,056 and 1,001, respectively), as well as common to both networks (773) were identified. Functional clusters were obtained and two models of drought stress response for the V/2xRL and V/4xRL genotypes were designed. Even if the V/2xRL plant implement some tolerance mechanisms, the global plant response to drought was rapid and quickly exhaustive resulting in a general tendency to dehydration avoidance, which presented some advantage in short and strong drought stress conditions, but which, in long terms, does not allow the plant survival. At the contrary, the V/4xRL plants presented a response which strong impacts on development but that present some advantages in case of prolonged drought. Finally, some specific proteins, which presented high centrality on interactomic analysis were identified as good candidates for subsequent functional analysis of citrus genes related to drought response, as well as be good markers of one or another physiological mechanism implemented by the plants. (Résumé d'auteur

The molecular mechanisms of brassinosteroid-regulated drought stress response in Arabidopsis thaliana

Brassinosteroids (BRs) are involved in diverse developmental processes such as cell elongation, vascular differentiation, senescence and stress response. The mechanisms and regulatory networks of BR-regulated plant growth and development have been well described for the past decade with the identification of receptors, kinases and central transcription factors involved in BR signaling. Recent studies revealed BRs also extensively participated in plant response to environmental stresses, although the mechanisms of BR-regulated stress response is largely unknown. Coordination of plant growth and stress response requires integration of multiple signaling output through hormonal crosstalk. Studies of BR signaling pathway and BR-mediated physiological responses indicate there are intensive interactions between BRs and other phytohormones such as auxin, abscisic acid, jasmonic acid and ethylene. This study aims to unravel the function and regulatory mechanisms of BRs in abiotic stresses, particularly drought stress, through the investigation of the crosstalk between BR and drought/ABA signaling pathways. Through genetic, genomic and biochemical assays, we identified a transcription factor RESPONSIVE TO DESSICATION 26 (RD26) that mediates the the crosstalk between BR and ABA signaling pathways, and proposed a regulatory model that coordinates plant growth and stress response

Functions and mechanism of WRKY transcription factors in Brassinosteroid-regulated plant growth and stress response

Brassinosteroids (BRs), a family of plant steroid hormones, play important role in plant growth and development. BR signal is perceived by receptors BRI1 and BAK1, which transduce the hormonal signal to downstream transcription factors BES1/BZR1 to regulate the expression of thousands of BR target genes. BRs also play a vital role in both abiotic and biotic stress responses. However, the molecular mechanisms by which BRs regulate stress responses have just begun to be revealed. Here, we characterized three group III transcription factors, WRKY46, WRKY 54 and WRKY70, which are involved in BR-regulated plant growth and drought stress by cooperating with BES1 to positively regulate BR target genes and negatively control drought-responsive genes. The stability of WRKY54 is regulated through phosphorylation by BIN2, a GSK3-like kinase that plays a negative role in BR-regulated plant growth. Moreover, we also characterized another three group III transcription factors, WRKY30, WRKY 41 and WRKY53, in bacterial defense response, which likely act through MAP kinase cascade-regulated Pathogen Associated Molecular Patterns (PAMP)-Triggered Immunity pathway. Our results thus revealed that group III WRKYs integrate the signals from BRs, drought and bacterial pathogens to regulate the expression of downstream growth-related and stress-responsive genes, which significantly advances our understanding of BR regulation of stress responses

BRASSINOSTEROID-MEDIATED STRESS TOLERANCE: hormone pathways, genes and function

Brassinosteroids (BRs) are naturally occurring plant steroid derivatives that play crucial roles in plant development and also promote tolerance to a range of abiotic stresses. Although much has been learned about their roles in plant development, the mechanisms by which BRs control plant stress responses and regulate stress-responsive gene expression are not fully known. It is also likely that the stress tolerance conferring ability of BRs is in part due to their interactions with other stress hormones. In the present study the stress tolerance effects of BR, interactions of BR with other plant hormones, and global genomic responses of BR in mediating stress tolerance were explored. Treatment with 24-epibrassinolide (EBR), a BR, enhanced dehydration tolerance o f Arabidopsis thaliana (Arabidopsis), and this effect involved changes in the expression of dehydration-responsive genes. Study of EBR effects on the basic thermotolerance and salt tolerance of a collection of Arabidopsis mutants either deficient in or insensitive to abscisic acid (ABA), ethylene (ET), jasmonic acid (JA) and salicylic acid (SA), indicated that BR exerts anti-stress effects both independently as well as through interactions with other hormones. This study uncovered a critical role for NPR1 (NONEXPRESSOR OF PATHOGENESIS-RELATED GENES1), a master regulator of SA-mediated defense responses, in BR-mediated increase in stress tolerance. Yet another finding was that ABA inhibits BR effects during stress, and that BR shares transcriptional targets with other hormones. Whole-genome transcriptome analysis of BR-treated and untreated Arabidopsis seedlings under no-stress and heat stress revealed majority of the BR response genes to be related to stress tolerance, signal transduction and metabolism. Analysis of T-DNA insertion mutants of four BR response genes indicated that WRKY17, WRKY33, ACP5 and BRRLK have stress-related functions. As a final confirmation of a role for BR in stress tolerance, the AtDWF4 gene encoding a BR biosynthesis enzyme was overexpressed in seeds of Arabidopsis. Preliminary studies of transgenic seedlings showed an increase in cold tolerance and the ability to overcome ABA-induced inhibition o f germination. in In summary, the present study provides novel insights into the mechanism of BR- mediated stress tolerance by identifying genes and hormone interactions involved in this proce

To grow or survive: Plants modulate Brassinosteroid-regulated transcription factor BES1 during drought to balance growth and stress responses

Understanding how plants balance growth and stress responses is essential to optimize crop yield in an ever-changing environment. Brassinosteroids (BRs) regulate plant growth and stress responses, including that of drought. BRs signal to control the activities of the BES1/BZR1 family transcription factors (TFs), which in turn mediate the expression of more than 5,000 BR-responsive genes. The network through which BES1 regulates the large number of target genes and the factors that modulate BES1 during stress are only beginning to be understood. In this thesis, I investigated several mechanisms that converge on BES1 to balance BR-regulated growth and stress responses. First, BES1 is degraded by selective autophagy during stress. BES1 interacts with the ubiquitin receptor protein DSK2 and is targeted to the autophagy pathway during stress via the interaction of DSK2 with ATG8, a ubiquitin-like protein directing autophagosome formation and cargo recruitment. DSK2 is phosphorylated by the GSK3-like kinase BIN2, a negative regulator in the BR pathway. BIN2 phosphorylation of DSK2 flanking its ATG8 interacting motifs (AIMs) promotes the interaction of DSK2 with ATG8, thereby targeting BES1 for degradation under stress conditions. Accordingly, loss-of-function dsk2 plants accumulate BES1, have altered global gene expression profiles, and have compromised responses to drought and fixed-carbon starvation stresses. In addition, BES1 interacts with other TFs to coordinate growth and drought responses. RD26 is induced by drought and inhibits the activity of BES1 on target gene promoters during drought conditions. In contrast, under growth promoting conditions BES1 cooperates with a large network of TFs including WRKY46/54/70 to inhibit drought responses, thereby enabling BR-regulated growth. To more fully characterize the BR-regulatory network, we used genome-wide chromatin immunoprecipitation (ChIP), transcriptome and TF interactome datasets to identify 657 BR-related Transcription Factors (BR-TFs). We then took an integrated approach involving computational modeling, phenomics and functional genomics to study the networks through which BRs, BES1/BZR1 and BR-TFs function. Initially, 11,760 publicly available microarray datasets were used to build comprehensive gene regulatory networks (GRNs). BR-TFs are significantly enriched for BR and drought target genes in the GRNs, suggesting that these TFs function in growth and stress responses. BR-TFs were prioritized for functional studies using NEST (Network Essentiality Scoring Tool). Next, we developed BR response assays to conduct BR phenomics experiments for over 300 BR-TFs using more than 1000 knockout or overexpression lines. These studies identified numerous BR-TF mutants that displayed altered BR responses, allowing us to characterize the function of PLATZ and HMG as A/T-rich binding TFs that oppositely regulate BR-responsive gene expression. Finally, BR and drought phenomics experiments in soil-grown plants using time-lapse imaging and a robotic phenotyping system revealed that tcp mutants have increased BR-regulated growth and improved survival during drought compared to wild-type. These studies provide a paradigm for network-based discovery and characterization of hormone response pathways through the integration of genomics, network analysis and phenomics. Taken together, BES1 is emerging as a critical hub for BR-drought crosstalk, allowing plants to efficiently balance growth and stress responses

Crosstalk of the Brassinosteroid Signalosome with Phytohormonal and Stress Signaling Components Maintains a Balance between the Processes of Growth and Stress Tolerance

Brassinosteroids (BRs) are a class of phytohormones, which regulate various processes during plant life cycle. Intensive studies conducted with genetic, physiological and molecular approaches allowed identification of various components participating in the BR signaling—from the ligand perception, through cytoplasmic signal transduction, up to the BR-dependent gene expression, which is regulated by transcription factors and chromatin modifying enzymes. The identification of new components of the BR signaling is an ongoing process, however an emerging view of the BR signalosome indicates that this process is interconnected at various stages with other metabolic pathways. The signaling crosstalk is mediated by the BR signaling proteins, which function as components of the transmembrane BR receptor, by a cytoplasmic kinase playing a role of the major negative regulator of the BR signaling, and by the transcription factors, which regulate the BR-dependent gene expression and form a complicated regulatory system. This molecular network of interdependencies allows a balance in homeostasis of various phytohormones to be maintained. Moreover, the components of the BR signalosome interact with factors regulating plant reactions to environmental cues and stress conditions. This intricate network of interactions enables a rapid adaptation of plant metabolism to constantly changing environmental conditions

Functional Analysis of Two Brassinosteroid Responsive, Putative Calmodulin-Binding Proteins 60 (CBP60S) in Arabidopsis Thaliana

Brassinosteroids (BRs) have remarkable ability to increase stress tolerance in plants. Investigations to understand the molecular mechanisms underlying BR-mediated stress tolerance resulted in identification of genes belonging to the family calmodulin binding protein X (CBPX). The present study was focused on studying the role of CBPX1 and CBPX2 in BR mediated stress tolerance and functional characterization using a reverse genetic approach. The upregulation of CBPX1 and CBPX2 by BR and stress noted in publicly available AtGenexpress datasets and by qRT-PCR analysis strongly suggests that these are BR responsive genes and functional analysis of T-DNA insertion mutants showed salt stress related functions in A. thaliana. The T-DNA insertion mutants cbpx1 and cbpx2 were sensitive to salt stress compared to WT, while CBPX1 OE lines showed increased salt tolerance. The results of the present study have revealed two new stress related genes, CBPX1 and CBPX2 that displayed increased expression in response to BR and salt stress, and also play an important role in conferring salt stress tolerance to plants. CBPX1 also plays an important role in determining the root length in A. thaliana

Selective Autophagy of BES1 Mediated by DSK2 Balances Plant Growth and Survival

Plants encounter a variety of stresses and must fine-tune their growth and stress-response programs to best suit their environment. BES1 functions as a master regulator in the brassinosteroid (BR) pathway that promotes plant growth. Here, we show that BES1 interacts with the ubiquitin receptor protein DSK2 and is targeted to the autophagy pathway during stress via the interaction of DSK2 with ATG8, a ubiquitin-like protein directing autophagosome formation and cargo recruitment. Additionally, DSK2 is phosphorylated by the GSK3-like kinase BIN2, a negative regulator in the BR pathway. BIN2 phosphorylation of DSK2 flanking its ATG8 interacting motifs (AIMs) promotes DSK2-ATG8 interaction, thereby targeting BES1 for degradation. Accordingly, loss-of-function dsk2 mutants accumulate BES1, have altered global gene expression profiles, and have compromised stress responses. Our results thus reveal that plants coordinate growth and stress responses by integrating BR and autophagy pathways and identify the molecular basis of this crosstalk

Environmental, developmental, and genetic factors controlling root system architecture

A better understanding of the development and architecture of roots is essential to develop strategies to increase crop yield and optimize agricultural land use. Roots control nutrient and water uptake, provide anchoring and mechanical support and can serve as important storage organs. Root growth and development is under tight genetic control and modulated by developmental cues including plant hormones and the environment. This review focuses on root architecture and its diversity and the role of environment, nutrient, and water as well as plant hormones and their interactions in shaping root architecture