Tomato BRASSINOSTEROID INSENSITIVE1 is required for systemin-induced root elongation in Solanum pimpinellifolium but is not essential for wound signaling

Structure-Function Analysis of Volatile (Z)-3-Fatty Alcohols in Tomato

Plants emit green leaf volatiles (GLVs) in response to biotic and abiotic stress. Receiver plants perceive GLVs as alarm cues resulting in activation of defensive or protective mechanisms. While this is well documented, it is not known how GLVs are perceived by receiver cells and what the structural determinants are for GLV activity. We tested whether the carbon chain length in (Z)-3-fatty alcohols with four to nine carbons and the double bonds in six-carbon alcohols contribute to bioactivity. In Solanum peruvianum suspension-cultured cells we found that (Z)-3-fatty alcohols, except (Z)-3-butenol, induce medium alkalinization and MAP kinase phosphorylation, two signaling responses often tied to the perception of molecular patterns that function in plant immunity and resistance to herbivores. In tomato (S. lycopersicum) seedlings, we found that (Z)-3-fatty alcohols induce inhibition of root growth. In both signaling and physiological responses, (Z)-3-octenol and (Z)-3-nonenol had a higher bioactivity than (Z)-3-heptenol and (Z)-3-hexenol, with (Z)-3-butenol only being active in root growth assays. Bioactivity correlated not only with chain length but also with lipophilicity of the fatty alcohols. The natural GLVs (E)-2-hexenol and the saturated 1-hexanol exhibited a higher bioactivity in pH assays than (Z)-3-hexenol, indicating that the presence and position of a double bond also contributes to bioactivity. Our results indicate that perceiving mechanisms for (Z)-3-fatty alcohols show a preference for longer chain fatty alcohols or that longer chain fatty alcohols are more accessible to receptors.

Dressed Up to the Nines: The Interplay of Phytohormones Signaling and Redox Metabolism During Plant Response to Drought

Plants must effectively respond to various environmental stimuli to achieve optimal growth. This is especially relevant in the context of climate change, where drought emerges as a major factor globally impacting crops and limiting overall yield potential. Throughout evolution, plants have developed adaptative strategies for environmental stimuli, with plant hormones and reactive oxygen species (ROS) playing essential roles in their development. Hormonal signaling and the maintenance of ROS homeostasis are interconnected, playing indispensable roles in growth, development, and stress responses and orchestrating diverse molecular responses during environmental adversities. Nine principal classes of phytohormones have been categorized: auxins, brassinosteroids, cytokinins, and gibberellins primarily oversee developmental growth regulation, while abscisic acid, ethylene, jasmonic acid, salicylic acid, and strigolactones are the main orchestrators of environmental stress responses. Coordination between phytohormones and transcriptional regulation is crucial for effective plant responses, especially in drought stress. Understanding the interplay of ROS and phytohormones is pivotal for elucidating the molecular mechanisms involved in plant stress responses. This review provides an overview of the intricate relationship between ROS, redox metabolism, and the nine different phytohormones signaling in plants, shedding light on potential strategies for enhancing drought tolerance for sustainable crop production.

WOX1 controls leaf serration development via temporally restricting BRASSINAZOLE RESISTANT 1 and CUP SHAPED COTYLEDON 3 expression in Arabidopsis

Leaves have evolved shape diversity, ranging from simple leaves with a smooth margin to complicated shapes with toothed/serrated, lobed, and dissected leaves with leaflets. In the model plant Arabidopsis thaliana with simple leaves producing a serrated margin, boundary regulatory factor genes CUP SHAPED COTYLEDON 2 (CUC2) and CUC3 play important roles in promoting leaf initiation and maintenance of serration. Stem cell-related WUSCHEL-RELATED HOMEOBOX1 (WOX1) and PRESSED FLOWER (PRS)/WOX3 are also essential for leaf margin morphogenesis, but the role of WOX1 and PRS as well as the relationships between CUCs and WOXs for tooth development are unclear. In this study, we found that WOX1, but not PRS, prevents overproduction of number of teeth and excessive tooth size by limiting CUC3 expression to a moderate level in a temporally regulated manner. We also revealed that BRASSINAZOLE RESISTANT 1 (BZR1), a known regulator of plant development including boundary regions, is involved in WOX1 negative regulation of tooth development by repressing CUC3 expression during the initiation/early stage of tooth development. WOX1 parallelly limits BZR1 and CUC3 expression from the late stage of the first two teeth, while it restricts CUC3 activity in a BZR1-dependent manner from the initiation/early stage of subsequently developed teeth. This study uncovers a new mechanism for WOX1 in fine-tuning the leaf margin geometry.

Another inner truth of shaking: Water migration and transformation-advanced physicochemical alterations in tea leaves

Shaking, essential in oolong tea production, is becoming an innovative method to impart floral fragrance. Research on shaking primarily concentrates on biological underpinnings, including modifications in gene expression and stress-triggered enzymatic catalysis, and consequent physicochemical properties. Water phase and distribution, reshaped by shaking and affected the biological and physicochemical alterations of tea leaves, is always ignored. This work utilized TEM, LF-NMR, UPLC-QqQ-MS, and GC-TOF-MS to explore physicochemical alterations during shaking. Results revealed shaking induced stomatal opening, water migration from stems to leaf veins, and a reduction in free water, transformed into bound water. Mechanical stimulation disrupted cell microstructures, including vacuoles, chloroplasts, and cell walls, releasing precursors and enzyme substrates. Shaking triggered intracellular physicochemical reactions that decreased polyphenols, amino acids, chlorophyll, and carotenoids, while increasing organic acids and sugars. Also catalyzed the synthesis of aromatic compounds like (E)-nerolidol, β-ionone epoxide, and α-farnesene, shaping the floral-fruity aroma and mellow taste of tea.

SERKs serve as co-receptors for SYR1 to trigger systemin-mediated defense responses in tomato

Systemin, the first peptide hormone identified in plants, was initially isolated from tomato (Solanum lycopersicum) leaves. Systemin mediates local and systemic wound-induced defense responses in plants, conferring resistance to necrotrophic fungi and herbivorous insects. Systemin is recognized by the leucine-rich-repeat receptor-like kinase (LRR-RLK) receptor SYSTEMIN RECEPTOR1 (SYR1), but how the systemin recognition signal is transduced to intracellular signaling pathways to trigger defense responses is poorly understood. Here, we demonstrate that SERK family LRR-RLKs function as co-receptors for SYR1 to mediate systemin signal transduction in tomato. By using chemical genetic approaches coupled with engineered receptors, we revealed that the association of the cytoplasmic kinase domains of SYR1 with SERKs leads to their mutual trans-phosphorylation and the activation of SYR1, which in turn induces a wide range of defense responses. Systemin stimulates the association between SYR1 and all tomato SERKs (SlSERK1, SlSERK3A, and SlSERK3B). The resulting SYR1-SlSERK heteromeric complexes trigger the phosphorylation of TOMATO PROTEIN KINASE 1B (TPK1b), a receptor-like cytoplasmic kinase that positively regulates systemin responses. Additionally, upon association with SYR1, SlSERKs are cleaved by the Pseudomonas syringae effector HopB1, further supporting the finding that SlSERKs are activated by systemin-bound SYR1. Finally, genetic analysis using Slserk mutants showed that SlSERKs are essential for systemin-mediated defense responses. Collectively, these findings demonstrate that the systemin-mediated association of SYR1 and SlSERKs activates defense responses against herbivorous insects.

Green leaf volatiles co-opt proteins involved in molecular pattern signalling in plant cells

The green leaf volatiles (GLVs) Z-3-hexen-1-ol (Z3-HOL) and Z-3-hexenyl acetate (Z3-HAC) are airborne infochemicals released from damaged plant tissues that induce defenses and developmental responses in receiver plants, but little is known about their mechanism of action. We found that Z3-HOL and Z3-HAC induce similar but distinctive physiological and signaling responses in tomato seedlings and cell cultures. In seedlings, Z3-HAC showed a stronger root growth inhibition effect than Z3-HOL. In cell cultures, the two GLVs induced distinct changes in MAP kinase (MAPK) activity and proton fluxes as well as rapid and massive changes in the phosphorylation status of proteins within 5 min. Many of these phosphoproteins are involved in reprogramming the proteome from cellular homoeostasis to stress and include pattern recognition receptors, a receptor-like cytoplasmic kinase, MAPK cascade components, calcium signaling proteins and transcriptional regulators. These are well-known components of damage-associated molecular pattern (DAMP) signaling pathways. These rapid changes in the phosphoproteome may underly the activation of defense and developmental responses to GLVs. Our data provide further evidence that GLVs function like DAMPs and indicate that GLVs coopt DAMP signaling pathways.

Identification of Functional Brassinosteroid Receptor Genes in Oaks and Functional Analysis of QmBRI1

Brassinosteroids (BRs) play important regulatory roles in plant growth and development, with functional BR receptors being crucial for BR recognition or signaling. Although functional BR receptors have been extensively studied in herbaceous plants, they remain largely under-studied in forest tree species. In this study, nine BR receptors were identified in three representative oak species, of which BRI1s and BRL1s were functional BR receptors. Dispersed duplications were a driving force for oak BR receptor expansion, among which the Brassinosteroid-Insensitive-1 (BRI1)-type genes diverged evolutionarily from most rosids. In oak BRI1s, we identified that methionine in the conserved Asn-Gly-Ser-Met (NGSM) motif was replaced by isoleucine and that the amino acid mutation occurred after the divergence of Quercus and Fagus. Compared with QmBRL1, QmBRI1 was relatively highly expressed during BR-induced xylem differentiation and in young leaves, shoots, and the phloem and xylem of young stems of Quercus mongolica. Based on Arabidopsis complementation experiments, we proved the important role of QmBRI1 in oak growth and development, especially in vascular patterning and xylem differentiation. These findings serve as an important supplement to the findings of the structural, functional and evolutionary studies on functional BR receptors in woody plants and provide the first example of natural mutation occurring in the conserved BR-binding region (NGSM motif) of angiosperm BRI1s.

In silico and in vitro approaches allow the identification of the Prosystemin molecular network

Tomato Prosystemin (ProSys), the precursor of Systemin, a small peptidic hormone, is produced at very low concentration in unchallenged plants, while its expression greatly increases in response to several different stressors triggering an array of defence responses. The molecular mechanisms that underpin such a wide array of defence barriers are not fully understood and are likely correlated with the intrinsically disordered (ID) structure of the protein. ID proteins interact with different protein partners forming complexes involved in the modulation of different biological mechanisms. Here we describe the ProSys-protein network that shed light on the molecular mechanisms underpinning ProSys associated defence responses. Three different approaches were used. In silico prediction resulted in 98 direct interactors, most clustering in phytohormone biosynthesis, transcription factors and signal transduction gene classes. The network shows the central role of ProSys during defence responses, that reflects its role as central hub. In vitro ProSys interactors, identified by Affinity Purification-Mass Spectrometry (AP-MS), revealed over three hundred protein partners, while Bimolecular Fluorescent Complementation (BiFC) experiments validated in vivo some interactors predicted in silico and in vitro. Our results demonstrate that ProSys interacts with several proteins and reveal new key molecular events in the ProSys-dependent defence response of tomato plant.

Regulation of pattern-triggered immunity and growth by phytocytokines

Endogenous signalling peptides play diverse roles during plant growth, development and stress responses. Research in recent years has unravelled peptides with previously known growth-regulatory function as immune-modulatory agents that fine-tune pattern-triggered immunity (PTI). Moreover, peptides that are long known as endogenous danger signals were recently implicated in growth and development. In analogy to metazoan systems these peptides are referred to as phytocytokines. In this review we will highlight recent progress made on our understanding of phytocytokines simultaneously regulating growth and PTI which shows the complex interplay of peptide signalling pathways regulating multiple aspects of a plant's life.

SlCESTA Is a Brassinosteroid-Regulated bHLH Transcription Factor of Tomato That Promotes Chilling Tolerance and Fruit Growth When Over-Expressed

Brassinosteroids (BRs) are required for various aspects of plant growth and development, but also participate in stress responses. The hormones convey their activity through transcriptional regulation and posttranslational modification of transcription factors and one class are basic helix-loop-helix (bHLH) proteins of the BR Enhanced Expression (BEE) subfamily, which in Arabidopsis thaliana include BEE1-3 and CESTA (CES). CES and the BEEs promote the expression of different BR-responsive genes, including genes encoding gibberellin (GA) biosynthetic and catabolizing enzymes, as well as cold-responsive genes. Interestingly, in terms of an application, CES could promote both fruit growth and cold stress tolerance when over-expressed in A. thaliana and here it was investigated, if this function is conserved in the fruit crop Solanum lycopersicum (cultivated tomato). Based on amino acid sequence similarity and the presence of regulatory motifs, a CES orthologue of S. lycopersicum, SlCES, was identified and the effects of its over-expression were analysed in tomato. This showed that SlCES, like AtCES, was re-localized to nuclear bodies in response to BR signaling activation and that it effected GA homeostasis, with related phenotypes, when over-expressed. In addition, over-expression lines showed an increased chilling tolerance and had altered fruit characteristics. The possibilities and potential limitations of a gain of SlCES function as a breeding strategy for tomato are discussed.

Conjunctive Analyses of BSA-Seq and BSR-Seq Unveil the Msβ-GAL and MsJMT as Key Candidate Genes for Cytoplasmic Male Sterility in Alfalfa (Medicago sativa L.)

Knowing the molecular mechanism of male sterility in alfalfa is important to utilize the heterosis more effectively. However, the molecular mechanisms of male sterility in alfalfa are still unclear. In this study, the bulked segregant analysis (BSA) and bulked segregant RNA-seq (BSR) were performed with F2 separation progeny to study the molecular mechanism of male sterility in alfalfa. The BSA-seq analysis was located in a candidate region on chromosome 5 containing 626 candidate genes which were associated with male sterility in alfalfa, while the BSR-seq analysis filtered seven candidate DEGs related to male sterility, and these candidate genes including EF-Tu, β-GAL, CESA, PHGDH, and JMT. The conjunctive analyses of BSR and BSA methods revealed that the genes of Msβ-GAL and MsJMT are the common detected candidate genes involved in male sterility in alfalfa. Our research provides a theory basis for further study of the molecular mechanism of male sterility in alfalfa and significant information for the genetic breeding of Medicago sativa.

Evolutionary Analysis and Functional Identification of Ancient Brassinosteroid Receptors in Ceratopteris richardii

Phytohormones play an important role in the adaptive evolution of terrestrial plants. Brassinosteroids (BRs) are essential hormones that regulate multiple aspects of plant growth and development in angiosperms, but the presence of BR signaling in non-seed plants such as ferns remains unknown. Here, we found that BR promotes the growth of Ceratopteris richardii, while the synthetic inhibitor PCZ inhibits the growth. Using full-length transcriptome sequencing, we identified four BRI1-like receptors. By constructing chimeric receptors, we found that the kinase domains of these four receptors could trigger BR downstream signaling. Further, the extracellular domains of two receptors were functionally interchangeable with that of BRI1. In addition, we identified a co-receptor, CtSERK1, that could phosphorylate with CtBRL2s in vitro. Together, these proved the presence of a receptor complex in Ceratopteris richardii that might perceive BR and activate downstream hormone signaling. Our results shed light on the biological and molecular mechanisms of BR signaling in ferns and the role of BR hormone signaling in the adaptive evolution of terrestrial plants.

Brassinosteroids and metalloids: Regulation of plant biology

Metalloid contamination in the environment is one of the serious concerns posing threat to our ecosystems. Excess of metalloid concentrations (including antimony, arsenic, boron, selenium etc.) in soil results in their over accumulation in plant tissues, which ultimately causes phytotoxicity and their bio-magnification. So, it is very important to find some ecofriendly approaches to counter negative impacts of above mentioned metalloids on plant system. Brassinosteroids (BRs) belong to family of plant steroidal hormones, and are considered as one of the ecofriendly way to counter metalloid phytotoxicity. This phytohormone regulates the plant biology in presence of metalloids by modulating various key biological processes like cell signaling, primary and secondary metabolism, bio-molecule crosstalk and redox homeostasis. The present review explains the in-depth mechanisms of BR regulated plant responses in presence of metalloids, and provides some biotechnological aspects towards ecofriendly management of metalloid contamination.

Characterization of Endogenous Levels of Brassinosteroids and Related Genes in Grapevines

Agronomic breeding practices for grapevines (Vitis vinifera L.) include the application of growth regulators in the field. Brassinosteroids (BRs) are a family of sterol-derived plant hormones that regulate several physiological processes and responses to biotic and abiotic stress. In grapevine berries, the production of biologically active BRs, castasterone and 6-deoxocastasterone, has been reported. In this work, key BR genes were identified, and their expression profiles were determined in grapevine. Bioinformatic homology analyses of the Arabidopsis genome found 14 genes associated with biosynthetic, perception and signaling pathways, suggesting a partial conservation of these pathways between the two species. The tissue- and development-specific expression profiles of these genes were determined by qRT-PCR in nine different grapevine tissues. Using UHPLC-MS/MS, 10 different BR compounds were pinpointed and quantified in 20 different tissues, each presenting specific accumulation patterns. Although, in general, the expression profile of the biosynthesis pathway genes of BRs did not directly correlate with the accumulation of metabolites, this could reflect the complexity of the BR biosynthesis pathway and its regulation. The development of this work thus generates a contribution to our knowledge about the presence, and diversity of BRs in grapevines.

Brassinosteroids synthesised by CYP85A/A1 but not CYP85A2 function via a BRI1-like receptor but not via BRI1 in Picea abies

Brassinosteroids (BRs) are essential plant hormones. In angiosperms, brassinolide and castasterone, the first and second most active BRs, respectively, are synthesised by CYP85A2 and CYP85A/A1, respectively. BRs in angiosperms function through an essential receptor, BR Insensitive 1 (BRI1). In addition, some angiosperms also have non-essential BRI1-like 1/3 (BRL1/3). In conifers, BRs promote seed germination under drought stress; however, how BRs function in gymnosperms is unknown. In this study, we performed functional complementation of BR biosynthesis and receptor genes from Picea abies with respective Arabidopsis mutants. We found that P. abies possessed functional PaCYP85A and PaBRL1 but not PaCYP85A2 or PaBRI1, and this results in weak BR signaling, and both PaCYP85A and PaBRL1 were abundantly expressed. However, neither BR treatment of P. abies seedlings nor expression of PaBRL1 in the Arabidopsis Atbri1 mutant promoted plant height, despite the fact that BR-responsive genes were activated. Importantly, chimeric AtBRI1 replaced with the BR-binding domain of PaBRL1 complemented the Atbri1 phenotypes. Furthermore, PaBRL1 had less kinase activity than BRI1 in vitro. Overall, P. abies had weak but still active BR signaling, explaining aspects of its slow growth and high stress tolerance. Our study sheds light on the functional and evolutionary significance of distinct BR signaling that is independent of BRI1 and brassinolide.

The conserved brassinosteroid-related transcription factor BIM1a negatively regulates fruit growth in tomato

Brassinosteroids (BRs) are steroid hormones that play key roles in plant development and defense. Our goal is to harness the extensive knowledge of the Arabidopsis BR signaling network to improve productivity in crop species. This first requires identifying components of the conserved network and their function in the target species. Here, we investigated the function of SlBIM1a, the closest tomato homolog of AtBIM1, which is highly expressed in fruit. SlBIM1a-overexpressing lines displayed severe plant and fruit dwarfism, and histological characterization of different transgenic lines revealed that SlBIM1a expression negatively correlated with fruit pericarp cell size, resulting in fruit size modifications. These growth phenotypes were in contrast to those found in Arabidopsis, and this was confirmed by the reciprocal ectopic expression of SlBIM1a/b in Arabidopsis and of AtBIM1 in tomato. These results determined that BIM1 function depends more on the recipient species than on its primary sequence. Yeast two-hybrid interaction studies and transcriptomic analyses of SlBIM1a-overexpressing fruit further suggested that SlBIM1a acts through its interaction with SlBZH1 to govern the transcriptional regulation of growth-related BR target genes. Together, these results suggest that SlBIM1a is a negative regulator of pericarp cell expansion, possibly at the crossroads with auxin and light signaling.

Tomato BZR/BES transcription factor SlBZR1 positively regulates BR signaling and salt stress tolerance in tomato and Arabidopsis

Brassinosteroids (BRs) play critical roles in plant growth and development, as well as in responses to abiotic stresses. The BRASSINAZOLE RESISTANT 1 (BZR1) and BRI1-EMS-SUPPRESSOR 1 (BES1) families of transcription factors have been elucidated largely in Arabidopsis and rice but not in other plant species. Here, we studied the functional characterization of a tomato (Solanum lycopersicum) BZR homolog gene, SlBZR1, in BR-regulated plant growth and tolerance to salt stress. SlBZR1 was highly expressed in the flowers and developing fruits of tomato. Both SlBZR1 and SlBZR1D (proline to leucine mutation at the 239th amino acid of SlBZR1) were transcriptional repressors and localized in the nucleus. SlBZR1 or SlBZR1D could interact with SlMYB30, SlMYBL2, SlPIF4, SlHAT1, SlIWS1 and SlREF6 in tomato. Overexpression of SlBZR1D enhanced the BR response and improved tolerance to salt stress in Arabidopsis, consistent with the phenotype of the Arabidopsis bes1-D mutant. Moreover, SlBZR1D-overexpressing tomato lines showed a short plant height, smaller and curly leaves, and delayed flowering. Additionally, SlBZR1D positively regulated salt tolerance in tomato and upregulated the expression of multiple stress-related genes. Our study provides new insights for understanding the function and mechanism of BZR transcription factors in BR-regulated plant growth and abiotic stress responses.

Systemin-mediated long-distance systemic defense responses

Systemin, a peptide plant hormone of 18 amino acids, coordinates local and systemic immune responses. The activation of the canonical systemin-mediated systemic signaling pathway involves systemin release from its precursor prosystemin, systemin binding to its membrane receptor SYSTEMIN RECEPTOR1 (SYR1), and the transport of long-distance signaling molecules, including jasmonic acid, the prosystemin mRNA, volatile organic compounds and possibly systemin itself. Here, we review emerging evidence that the disordered structure and unconventional processing and secretion of systemin contribute to the regulation of systemin-mediated signaling during plant defense. We highlight recent advances in systemin research, which elucidated how cells integrate multiple long-distance signals into the systemic defense response. In addition, we discuss the perception of systemin by SYR1 and its mediation of downstream defense responses.

Perception of Damaged Self in Plants

Plants use specific receptor proteins on the cell surface to detect host-derived danger signals released in response to attacks by pathogens or herbivores and activate immune responses against them.

Brassinosteroids: Multidimensional Regulators of Plant Growth, Development, and Stress Responses

Brassinosteroids (BRs) are a group of polyhydroxylated plant steroid hormones that are crucial for many aspects of a plant's life. BRs were originally characterized for their function in cell elongation, but it is becoming clear that they play major roles in plant growth, development, and responses to several stresses such as extreme temperatures and drought. A BR signaling pathway from cell surface receptors to central transcription factors has been well characterized. Here, we summarize recent progress toward understanding the BR pathway, including BR perception and the molecular mechanisms of BR signaling. Next, we discuss the roles of BRs in development and stress responses. Finally, we show how knowledge of the BR pathway is being applied to manipulate the growth and stress responses of crops. These studies highlight the complex regulation of BR signaling, multiple points of crosstalk between BRs and other hormones or stress responses, and the finely tuned spatiotemporal regulation of BR signaling.

Tomato Plants Treated with Systemin Peptide Show Enhanced Levels of Direct and Indirect Defense Associated with Increased Expression of Defense-Related Genes

Plant defense peptides represent an important class of compounds active against pathogens and insects. These molecules controlling immune barriers can potentially be used as novel tools for plant protection, which mimic natural defense mechanisms against invaders. The constitutive expression in tomato plants of the precursor of the defense peptide systemin was previously demonstrated to increase tolerance against moth larvae and aphids and to hamper the colonization by phytopathogenic fungi, through the expression of a wealth of defense-related genes. In this work we studied the impact of the exogenous supply of systemin to tomato plants on pests to evaluate the use of the peptide as a tool for crop protection in non-transgenic approaches. By combining gene expression studies and bioassays with different pests we demonstrate that the exogenous supply of systemin to tomato plants enhances both direct and indirect defense barriers. Experimental plants, exposed to this peptide by foliar spotting or root uptake through hydroponic culture, impaired larval growth and development of the noctuid moth Spodoptera littoralis, even across generations, reduced the leaf colonization by the fungal pathogen Botrytis cinerea and were more attractive towards natural herbivore antagonists. The induction of these defense responses was found to be associated with molecular and biochemical changes under control of the systemin signalling cascade. Our results indicate that the direct delivery of systemin, likely characterized by a null effect on non-target organisms, represents an interesting tool for the sustainable protection of tomato plants.

Identification of Potential Auxin-Responsive Small Signaling Peptides through a Peptidomics Approach in Arabidopsis thaliana

Small signaling peptides (SSPs) are a class of short peptides playing critical roles in plant growth and development. SSPs are also involved in the phytohormone signaling pathway. However, identification of mature SSPs is still a technical challenge because of their extremely low concentrations in plant tissue and complicated interference by many other metabolites. Here, we report an optimized protocol to extract SSPs based on protoplast extraction and to analyze SSPs based on tandem mass spectrometry peptidomics. Using plant protoplasts as the material, soluble peptides were directly extracted into phosphate buffer. The interference of non-signaling peptides was significantly decreased. Moreover, we applied the protocol to identify potential SSPs in auxin treated wild type and auxin biosynthesis defective mutant yuc2yuc6. Over 100 potential SSPs showed a response to auxin in Arabidopsis thaliana.

Modification of Threonine-1050 of SlBRI1 regulates BR Signalling and increases fruit yield of tomato

BACKGROUND

Appropriate brassinosteroid (BR) signal strength caused by exogenous application or endogenous regulation of BR-related genes can increase crop yield. However, precise control of BR signals is difficult and can cause unstable effects and failure to reach full potential. Phosphorylated BRASSINOSTEROID INSENSITIVE1 (BRI1), the rate-limiting receptor in BR signalling, transduces BR signals, and we recently demonstrated that modifying BRI1 phosphorylation sites alters BR signal strength and botanical characteristics in Arabidopsis. However, the functions of such phosphorylation sites in agronomic characteristics of crops remain unclear.

RESULTS

In this work, we investigated the roles of tomato SlBRI1 threonine-1050 (Thr-1050). SlBRI1 mutant cu3^-abs1 plants expressing SlBRI1 with a non-phosphorylatable Thr-1050 (T1050A), with a wild-type SlBRI1 transformant used as a control, were examined. The results showed enhanced autophosphorylation of SlBRI1 and BR signal strength for cu3^-abs1 harbouring T1050A, which promoted yield through increased plant expansion, leaf area, fruit weight and fruit number per cluster but reduced nutrient contents, including ascorbic acid and soluble sugar levels. Moreover, plant height, stem diameter, and internodal distance were similar between the transgenic plants.

CONCLUSION

Our results reveal the biological role of Thr-1050 in tomato and provide a molecular basis for establishing high-yield crops by precisely controlling BR signal strength via phosphorylation site modification.

Look Closely, the Beautiful May Be Small: Precursor-Derived Peptides in Plants

During the past decade, a flurry of research focusing on the role of peptides as short- and long-distance signaling molecules in plant cell communication has been undertaken. Here, we focus on peptides derived from nonfunctional precursors, and we address several key questions regarding peptide signaling. We provide an overview of the regulatory steps involved in producing a biologically active peptide ligand that can bind its corresponding receptor(s) and discuss how this binding and subsequent activation lead to specific cellular outputs. We discuss different experimental approaches that can be used to match peptide ligands with their receptors. Lastly, we explore how peptides evolved from basic signaling units regulating essential processes in plants to more complex signaling systems as new adaptive traits developed and how nonplant organisms exploit this signaling machinery by producing peptide mimics.

Regulation of plant peptide hormones and growth factors by post-translational modification

The number, diversity and significance of peptides as regulators of cellular differentiation, growth, development and defence of plants has long been underestimated. Peptides have now emerged as an important class of signals for cell-to-cell communication over short distances, and also for long-range signalling. We refer to these signalling molecules as peptide growth factors and peptide hormones, respectively. As compared to remarkable progress with respect to the mechanisms of peptide perception and signal transduction, the biogenesis of signalling peptides is still in its infancy. This review focuses on the biogenesis and activity of small post-translationally modified peptides. These peptides are derived from inactive pre-pro-peptides of approximately 70-120 amino acids. Multiple post-translational modifications (PTMs) may be required for peptide maturation and activation, including proteolytic processing, tyrosine sulfation, proline hydroxylation and hydroxyproline glycosylation. While many of the enzymes responsible for these modifications have been identified, their impact on peptide activity and signalling is not fully understood. These PTMs may or may not be required for bioactivity, they may inactivate the peptide or modify its signalling specificity, they may affect peptide stability or targeting, or its binding affinity with the receptor. In the present review, we will first introduce the peptides that undergo PTMs and for which these PTMs were shown to be functionally relevant. We will then discuss the different types of PTMs and the impact they have on peptide activity and plant growth and development. We conclude with an outlook on the open questions that need to be addressed in future research.

Brassinosteroid Signaling in Plant⁻Microbe Interactions

As sessile organisms, plants are frequently exposed to different stress conditions caused by either biotic or abiotic factors. Understanding the mechanisms that underlie plant interaction with the biotic and abiotic environments is fundamental to both plant biotechnology and sustainable agriculture. Brassinosteroids (BRs) are a group of plant-specific steroidal compounds essential for normal growth and development. Recent research evidence indicates that BRs are also actively involved in plant–environment interactions and play important roles in shaping plant fitness and the growth–defense trade-offs. In this minireview, we focus our attention on recent advances in the understanding of BR functions in modulating plant interactions with different pathogenic microbes, with particular focus on how BR signaling primes the plant innate immunity pathways and achieves a trade-off between growth and immunity.

Tomato PEPR1 ORTHOLOG RECEPTOR-LIKE KINASE1 Regulates Responses to Systemin, Necrotrophic Fungi, and Insect Herbivory

Endogenous peptides regulate plant immunity and growth. Systemin, a peptide specific to the Solanaceae, is known for its functions in plant responses to insect herbivory and pathogen infections. Here, we describe the identification of the tomato (Solanum lycopersicum) PEPR1/2 ORTHOLOG RECEPTOR-LIKE KINASE1 (PORK1) as the TOMATO PROTEIN KINASE1b (TPK1b) interacting protein and demonstrate its biological functions in systemin signaling and tomato immune responses. Tomato PORK1 RNA interference (RNAi) plants with significantly reduced PORK1 expression showed increased susceptibility to tobacco hornworm (Manduca sexta), reduced seedling growth sensitivity to the systemin peptide, and compromised systemin-mediated resistance to Botrytis cinerea. Systemin-induced expression of Proteinase Inhibitor II (PI-II), a classical marker for systemin signaling, was abrogated in PORK1 RNAi plants. Similarly, in response to systemin and wounding, the expression of jasmonate pathway genes was attenuated in PORK1 RNAi plants. TPK1b, a key regulator of tomato defense against B. cinerea and M. sexta, was phosphorylated by PORK1. Interestingly, wounding- and systemin-induced phosphorylation of TPK1b was attenuated when PORK1 expression was suppressed. Our data suggest that resistance to B. cinerea and M. sexta is dependent on PORK1-mediated responses to systemin and subsequent phosphorylation of TPK1b. Altogether, PORK1 regulates tomato systemin, wounding, and immune responses.

Exploration of ABA Responsive miRNAs Reveals a New Hormone Signaling Crosstalk Pathway Regulating Root Growth of Populus euphratica

Abscisic acid (ABA) plays an important role in the regulation of plant adaptation, seed germination, and root development in plants. However, the mechanism of ABA regulation of root development is still poorly understood, especially through the miRNA-mediated pathway. Here, small RNA (sRNA)-seq and degradome-seq were used to analyze the miRNAs’ responsive to ABA in the stems and roots of P. euphratica, a model tree species for abiotic stress-resistance research. In total, 255 unique mature sequences, containing 154 known miRNAs and 101 novel miRNAs were identified, among which 33 miRNAs and 54 miRNAs were responsive to ABA in the roots and stems, respectively. Furthermore, the analysis of these miRNAs and their targets revealed a new hormone signaling crosstalk model of ABA regulation of root growth through miRNA-mediated pathways, such as peu-miR-n68 mediation of the crosstalk between ABA and the brassinosteroid (BR) signaling pathway and peu-miR477b mediation of the crosstalk between ABA and Gibberellic acid (GA) signaling. Taken together, our genome-wide analysis of the miRNAs provides a new insight into the mechanism of ABA regulation of root growth in Populus.

The systemin receptor SYR1 enhances resistance of tomato against herbivorous insects

The discovery in tomato of systemin, the first plant peptide hormone^1,2, was a fundamental change for the concept of plant hormones. Numerous other peptides have since been shown to play regulatory roles in many aspects of the plant life, including growth, development, fertilization and interactions with symbiotic organisms^3-6. Systemin, an 18 amino acid peptide derived from a larger precursor protein ⁷ , was proposed to act as the spreading signal that triggers systemic defence responses observed in plants after wounding or attack by herbivores^1,7,8. Further work culminated in the identification of a leucine-rich repeat receptor kinase (LRR-RK) as the systemin receptor 160 (SR160)^9,10. SR160 is a tomato homologue of Brassinosteroid Insensitive 1 (BRI1), which mediates the regulation of growth and development in response to the steroid hormone brassinolide^11-13. However, a role of SR160/BRI1 as systemin receptor could not be corroborated by others^14-16. Here, we demonstrate that perception of systemin depends on a pair of distinct LRR-RKs termed SYR1 and SYR2. SYR1 acts as a genuine systemin receptor that binds systemin with high affinity and specificity. Further, we show that presence of SYR1, although not decisive for local and systemic wound responses, is important for defence against insect herbivory.

MdWRKY9 overexpression confers intensive dwarfing in the M26 rootstock of apple by directly inhibiting brassinosteroid synthetase MdDWF4 expression

Dwarfing rootstocks enable high-density planting and are therefore highly desirable in modern apple (Malus domestica) production. M26 is a semi-dwarfing rootstock that is used worldwide, but identifying intensive dwarfing rootstock is a major goal of apple breeding programs. Herein, we show that MdWRKY9 mediates dwarfing by directly inhibiting the transcription of the brassinosteroid (BR) rate-limiting synthetase MdDWF4 and reducing BR production. We found that the transcriptional factor MdWRKY9 is highly expressed in all tested dwarfing rootstocks. Transgenic lines of M26 rootstock overexpressing MdWRKY9 exhibit further dwarfing, which resulted from the reduced BR levels and was reversed via exogenous brassinolide treatment. Both an in vivo chromatin immunoprecipitation (ChIP) analysis and an in vitro electrophoretic mobility shift assay (EMSA) indicated that MdWRKY9 binds to the promoter of MdDWF4. Furthermore, MdWRKY9 repressed MdDWF4 expression in stable transgenic apple plants as determined by quantitative PCR. In addition, RNA-interfered expression of MdWRKY9 in transiently transformed apple calli led to a significant increase of MdDWF4, suggesting MdWRKY9 plays a critical role in regulating the expression of MdDWF4. We report a novel dwarfing mechanism in perennial woody plants that involves WRKY-controlled BR production, and present a new dwarfing M26 rootstock for potential applications in apple production.

Expression of tomato prosystemin gene in Arabidopsis reveals systemic translocation of its mRNA and confers necrotrophic fungal resistance

Systemin (SYS), an octadecapeptide hormone processed from a 200-amino-acid precursor (prosystemin, PS), plays a central role in the systemic activation of defense genes in tomato in response to herbivore and pathogen attacks. However, whether PS mRNA is transferable and its role in systemic defense responses remain unknown. We created the transgenic tomato PS gene tagged with the green fluorescent protein (PS-GFP) using a shoot- or root-specific promoter, and the constitutive 35S promoter in Arabidopsis. Subcellular localization of PS-/SYS-GFP was observed using confocal laser scanning microscopy and gene transcripts were determined using quantitative real-time PCR. In Arabidopsis, PS protein can be processed and SYS is secreted. Shoot-/root-specific expression of PS-GFP in Arabidopsis, and grafting experiments, revealed that the PS mRNA moves in a bi-directional manner. We also found that ectopic expression of PS improves Arabidopsis resistance to the necrotrophic fungus Botrytis cinerea, consistent with substantial upregulation of the transcript levels of specific pathogen-responsive genes. Our results provide novel insights into the multifaceted mechanism of SYS signaling transport and its potential application in genetic engineering for increasing pathogen resistance across diverse plant families.

SlBIR3 Negatively Regulates PAMP Responses and Cell Death in Tomato

Bri1-associated kinase 1 (BAK1)-interacting receptor-like kinase (BIR) proteins have been shown to play important roles in regulating growth and development, pathogen associated molecular pattern (PAMP)-triggered immunity (PTI) responses, and cell death in the model plant, Arabidopsis thaliana. We identified four BIR family members in tomato (Solanum lycopersicum), including SlBIR3, an ortholog of AtBIR3 from A. thaliana. SlBIR3 is predicted to encode a membrane localized non-arginine-aspartate (non-RD) kinase that, based on protein sequence, does not have autophosphorylation activity but that can be phosphorylated in vivo. We established that SlBIR3 interacts with SlBAK1 and AtBAK1 using yeast two-hybrid assays and co-immunoprecipitation and maltose-binding protein pull down assays. We observed that SlBIR3 overexpression in tomato (cv. micro-tom) and A. thaliana has weak effect on growth and development through brassinosteroid (BR) signaling. SlBIR3 overexpression in A. thaliana suppressed flg22-induced defense responses, but did not affect infection with the bacterial pathogen Pseudomonas syringae (PstDC3000). This result was confirmed using virus-induced gene silencing (VIGS) in tomato in conjunction with PstDC3000 infection. Overexpression of SlBIR3 in tomato (cv. micro-tom) and A. thaliana resulted in enhanced susceptibility to the necrotrophic fungus Botrytis cinerea. In addition, co-silencing SlBIR3 with SlSERK3A or SlSERK3B using VIGS and the tobacco rattle virus (TRV)-RNA2 vector containing fragments of both the SlSERK3 and SlBIR3 genes induced spontaneous cell death, indicating a cooperation between the two proteins in this process. In conclusion, our study revealed that SlBIR3 is the ortholog of AtBIR3 and that it participates in BR, PTI, and cell death signaling pathways.

Enhancing Brassinosteroid Signaling via Overexpression of Tomato (Solanum lycopersicum) SlBRI1 Improves Major Agronomic Traits

Brassinosteroids (BRs) play important roles in plant growth, development, and stress responses through the receptor, Brassinosteroid-insensitive 1 (BRI1), which perceives BRs and initiates BR signaling. There is considerable potential agricultural value in regulating BR signaling in crops. In this study, we investigated the effects of overexpressing the tomato (Solanum lycopersicum) BRI1 gene, SlBRI1, on major agronomic traits, such as seed germination, vegetative growth, fruit ethylene production, carotenoid accumulation, yield, and quality attributes. SlBRI1 overexpression enhanced the endogenous BR signaling intensity thereby increasing the seed germination rate, lateral root number, hypocotyl length, CO₂ assimilation, plant height, and flower size. The transgenic plants also showed an increase in fruit yield and fruit number per plant, although the mean weight of individual fruit was reduced, compared with wild type. SlBRI1 overexpression also promoted fruit ripening and ethylene production, and caused an increase in levels of carotenoids, ascorbic acid, soluble solids, and soluble sugars during fruit ripening. An increased BR signaling intensity mediated by SlBRI1 overexpression was therefore positively correlated with carotenoid accumulation and fruit nutritional quality. Our results indicate that enhancing BR signaling by overexpression of SlBRI1 in tomato has the potential to improve multiple major agronomic traits.

DAMPs, MAMPs, and NAMPs in plant innate immunity

BACKGROUND

Multicellular organisms have evolved systems/mechanisms to detect various forms of danger, including attack by microbial pathogens and a variety of pests, as well as tissue and cellular damage. Detection via cell-surface receptors activates an ancient and evolutionarily conserved innate immune system.

RESULT

Potentially harmful microorganisms are recognized by the presence of molecules or parts of molecules that have structures or chemical patterns unique to microbes and thus are perceived as non-self/foreign. They are referred to as Microbe-Associated Molecular Patterns (MAMPs). Recently, a class of small molecules that is made only by nematodes, and that functions as pheromones in these organisms, was shown to be recognized by a wide range of plants. In the presence of these molecules, termed Nematode-Associated Molecular Patterns (NAMPs), plants activate innate immune responses and display enhanced resistance to a broad spectrum of microbial and nematode pathogens. In addition to pathogen attack, the relocation of various endogenous molecules or parts of molecules, generally to the extracellular milieu, as a result of tissue or cellular damage is perceived as a danger signal, and it leads to the induction of innate immune responses. These relocated endogenous inducers are called Damage-Associated Molecular Patterns (DAMPs).

CONCLUSIONS

This mini-review is focused on plant DAMPs, including the recently discovered Arabidopsis HMGB3, which is the counterpart of the prototypic animal DAMP HMGB1. The plant DAMPs will be presented in the context of plant MAMPs and NAMPs, as well as animal DAMPs.

DAMPs, MAMPs, and NAMPs in plant innate immunity

BACKGROUND

Multicellular organisms have evolved systems/mechanisms to detect various forms of danger, including attack by microbial pathogens and a variety of pests, as well as tissue and cellular damage. Detection via cell-surface receptors activates an ancient and evolutionarily conserved innate immune system.

RESULT

Potentially harmful microorganisms are recognized by the presence of molecules or parts of molecules that have structures or chemical patterns unique to microbes and thus are perceived as non-self/foreign. They are referred to as Microbe-Associated Molecular Patterns (MAMPs). Recently, a class of small molecules that is made only by nematodes, and that functions as pheromones in these organisms, was shown to be recognized by a wide range of plants. In the presence of these molecules, termed Nematode-Associated Molecular Patterns (NAMPs), plants activate innate immune responses and display enhanced resistance to a broad spectrum of microbial and nematode pathogens. In addition to pathogen attack, the relocation of various endogenous molecules or parts of molecules, generally to the extracellular milieu, as a result of tissue or cellular damage is perceived as a danger signal, and it leads to the induction of innate immune responses. These relocated endogenous inducers are called Damage-Associated Molecular Patterns (DAMPs).

CONCLUSIONS

This mini-review is focused on plant DAMPs, including the recently discovered Arabidopsis HMGB3, which is the counterpart of the prototypic animal DAMP HMGB1. The plant DAMPs will be presented in the context of plant MAMPs and NAMPs, as well as animal DAMPs.

Molecular characterization of barley 3H semi-dwarf genes

The barley chromosome 3H accommodates many semi-dwarfing genes. To characterize these genes, the two-rowed semi-dwarf Chinese barley landrace ‘TX9425’ was crossed with the Australian barley variety ‘Franklin’ to generate a doubled haploid (DH) population, and major QTLs controlling plant height have been identified in our previous study. The major QTL derived from ‘TX9425’ was targeted to investigate the allelism of the semi-dwarf gene uzu in barley. Twelve sets of near-isogenic lines and a large NILF₂ fine mapping population segregating only for the dwarfing gene from ‘TX9425’ were developed. The semi-dwarfing gene in ‘TX9425’ was located within a 2.8 cM region close to the centromere on chromosome 3H by fine mapping. Molecular cloning and sequence analyses showed that the ‘TX9425’-derived allele contained a single nucleotide substitution from A to G at position 2612 of the HvBRI1 gene. This was apparently the same mutation as that reported in six-rowed uzu barley. Markers co-segregating with the QTL were developed from the sequence of the HvBRI1 gene and were validated in the ‘TX9425’/‘Franklin’ DH population. The other major dwarfing QTL derived from the Franklin variety was distally located on chromosome 3HL and co-segregated with the sdw1 diagnostic marker hv20ox2. A third dwarfing gene, expressed only in winter-sown trials, was identified and located on chromosome 3HS. The effects and interactions of these dwarfing genes under different growing conditions are discussed. These results improve our understanding of the genetic mechanisms controlling semi-dwarf stature in barley and provide diagnostic markers for the selection of semi-dwarfness in barley breeding programs.

Surveying the potential of secreted antimicrobial peptides to enhance plant disease resistance

Antimicrobial peptides (AMPs) are natural products found across diverse taxa as part of the innate immune system against pathogen attacks. Some AMPs are synthesized through the canonical gene expression machinery and are called ribosomal AMPs. Other AMPs are assembled by modular enzymes generating nonribosomal AMPs and harbor unusual structural diversity. Plants synthesize an array of AMPs, yet are still subject to many pathogen invasions. Crop breeding programs struggle to release new cultivars in which complete disease resistance is achieved, and usually such resistance becomes quickly overcome by the targeted pathogens which have a shorter generation time. AMPs could offer a solution by exploring not only plant-derived AMPs, related or unrelated to the crop of interest, but also non-plant AMPs produced by bacteria, fungi, oomycetes or animals. This review highlights some promising candidates within the plant kingdom and elsewhere, and offers some perspectives on how to identify and validate their bioactivities. Technological advances, particularly in mass spectrometry (MS) and nuclear magnetic resonance (NMR), have been instrumental in identifying and elucidating the structure of novel AMPs, especially nonribosomal peptides which cannot be identified through genomics approaches. The majority of non-plant AMPs showing potential for plant disease immunity are often tested using in vitro assays. The greatest challenge remains the functional validation of candidate AMPs in plants through transgenic experiments, particularly introducing nonribosomal AMPs into crops.

Kinase domain-targeted isolation of defense-related receptor-like kinases (RLK/Pelle) in Platanus×acerifolia: phylogenetic and structural analysis

BACKGROUND

Plant receptor-like kinase (RLK/Pelle) family regulates growth and developmental processes and interaction with pathogens and symbionts.Platanaceae is one of the earliest branches of Eudicots temporally located before the split which gave rise to Rosids and Asterids. Thus investigations into the RLK family in Platanus can provide information on the evolution of this gene family in the land plants.Moreover RLKs are good candidates for finding genes that are able to confer resistance to Platanus pathogens.

RESULTS

Degenerate oligonucleotide primers targeting the kinase domain of stress-related RLKs were used to isolate for the first time 111 RLK gene fragments in Platanus×acerifolia. Sequences were classified as candidates of the following subfamilies: CrRLK1L, LRR XII, WAK-like, and LRR X-BRI1 group. All the structural features typical of the RLK kinase domain were identified, including the non-RD motif which marks potential pathogen recognition receptors (PRRs). The LRR XII candidates, whose counterpart in Arabidopsis and rice comprises non-RD PRRs, were mostly non-RD kinases, suggesting a group of PRRs. Region-specific signatures of a relaxed purifying selection in the LRR XII candidates were also found, which is novel for plant RLK kinase domain and further supports the role of LRR XII candidates as PRRs. As we obtained CrRLK1L candidates using primers designed on Pto of tomato, we analysed the phylogenetic relationship between CrRLK1L and Pto-like of plant species. We thus classified all non-solanaceous Pto-like genes as CrRLK1L and highlighted for the first time the close phylogenetic vicinity between CrRLK1L and Pto group. The origins of Pto from CrRLK1L is proposed as an evolutionary mechanism.

CONCLUSIONS

The signatures of relaxed purifying selection highlight that a group of RLKs might have been involved in the expression of phenotypic plasticity and is thus a good candidate for investigations into pathogen resistance.Search of Pto-like genes in Platanus highlighted the close relationship between CrRLK1L and Pto group. It will be exciting to verify if sensu strictu Pto are present in taxonomic groups other than Solanaceae, in order to further clarify the evolutionary link with CrRLK1L.We obtained a first valuable resource useful for an in-depth study on stress perception systems.

On the origin and evolution of plant brassinosteroid receptor kinases

Brassinosteroid (BR) signaling pathway is so far the best-understood receptor-kinase signaling pathway in plants. In Arabidopsis, the activation of this pathway requires binding of BRs to the receptor kinase BRASSINOSTEROID-INSENSITIVE I (AtBRI1). Although the function of AtBRI1 has been extensively studied, it is not known when the binding function emerged and how this important component of BR signaling pathway and related genes (the BRI1-BRL gene family) have evolved in plants. We define BRI1-BRL genes in sequenced plant genomes, construct profiles for critical protein domains, scan them against all accessible plant gene/EST resources, and reveal the evolution of domain configuration of this family. We also investigate its evolutionary pattern through phylogenetic analysis. The complete BR receptor domain configuration originates through two domain gain events in the ancestral receptor-like kinase: first juxtamembrane domain gained during the early diversification of land plants, and then island domain (ID) acquired in the common ancestor of angiosperms and gymnosperms after its divergence from spike moss. The 70 amino acid ID has characteristic sequences of BRI1-BRL family and this family keeps relative stable copy numbers during the history of angiosperms and the majority of duplications and losses have occurred in terminal taxa in current taxon sampling. This study reveals important events shaping structural and functional characteristics of plant BR receptors. It answers the question of how and when BR receptors originates, which provide insights into the origin and evolution of the BR signaling pathway.

Solution NMR studies of the plant peptide hormone CEP inform function

The C-terminally Encoded Peptide (CEP) family of regulatory peptides controls root development in vascular plants. Here, we present the first NMR structures of CEP. We show that root-knot nematode (RKN: Meloidogyne spp.) also encodes CEP, presumably to mimic plant CEP as part of their stereotypic, parasitic interaction with vascular plants. Molecular dynamics simulations of plant- and nematode-encoded CEP displaying known posttranslational modifications (PTM) provided insight into the structural effects of PTM and the conformational plasticity and rigidity of CEP. Potential mechanisms of action are discussed with respect to the structure and sampling of conformational space.

Identification and functional analysis of tomato BRI1 and BAK1 receptor kinase phosphorylation sites

Brassinosteroids (BRs) are plant hormones that are perceived at the cell surface by a membrane-bound receptor kinase, BRASSINOSTEROID INSENSITIVE1 (BRI1). BRI1 interacts with BRI1-ASSOCIATED RECEPTOR KINASE1 (BAK1) to initiate a signal transduction pathway in which autophosphorylation and transphosphorylation of BRI1 and BAK1, as well as phosphorylation of multiple downstream substrates, play critical roles. Detailed mechanisms of BR signaling have been examined in Arabidopsis (Arabidopsis thaliana), but the role of BRI1 and BAK1 phosphorylation in crop plants is unknown. As a foundation for understanding the mechanism of BR signaling in tomato (Solanum lycopersicum), we used liquid chromatography-tandem mass spectrometry to identify multiple in vitro phosphorylation sites of the tomato BRI1 and BAK1 cytoplasmic domains. Kinase assays showed that both tomato BRI1 and BAK1 are active in autophosphorylation as well as transphosphorylation of each other and specific peptide substrates with a defined sequence motif. Site-directed mutagenesis revealed that the highly conserved kinase domain activation loop residue threonine-1054 was essential for tomato BRI1 autophosphorylation and peptide substrate phosphorylation in vitro. Furthermore, analysis of transgenic lines expressing full-length tomato BRI1-Flag constructs in the weak tomato bri1 allele, curl3(-abs1), demonstrated that threonine-1054 is also essential for normal BRI1 signaling and tomato growth in planta. Finally, we cloned the tomato ortholog of TGF-β Receptor Interacting Protein (TRIP1), which was previously shown to be a BRI1-interacting protein and kinase domain substrate in Arabidopsis, and found that tomato TRIP1 is a substrate of both tomato BRI1 and BAK1 kinases in vitro.

Silencing brassinosteroid receptor BRI1 impairs herbivory-elicited accumulation of jasmonic acid-isoleucine and diterpene glycosides, but not jasmonic acid and trypsin proteinase inhibitors in Nicotiana attenuata

The brassinosteroid (BR) receptor, BR insensitive 1 (BRI1), plays a critical role in plant development, but whether BRI1-mediated BR signaling is involved in plant defense responses to herbivores was largely unknown. Here, we examined the function of BRI1 in the resistance of Nicotiana attenuata (Solanaceae) to its specialist insect herbivore Manduca sexta. Jasmonic acid (JA) and JA-isoleucine conjugate (JA-Ile) are important hormones that mediate resistance to herbivores and we found that after wounding or simulated herbivory NaBRI1 had little effect on JA levels, but was important for the induction of JA-Ile. Further experiments revealed that decreased JAR (the enzyme for JA-Ile production) activity and availability of Ile in NaBRI1-silenced plants were likely responsible for the low JA-Ile levels. Consistently, M. sexta larvae gained more weight on NaBRI1-silenced plants than on the control plants. Quantification of insect feeding-induced secondary metabolites revealed that silencing NaBRI1 resulted in decreased levels of carbon-rich defensive secondary metabolites (hydroxygeranyllinalool diterpene glycosides, chlorogenic acid, and rutin), but had little effect on the nitrogen-rich ones (nicotine and trypsin proteinase inhibitors). Thus, NaBRI1-mediated BR signaling is likely involved in plant defense responses to M. sexta, including maintaining JA-Ile levels and the accumulation of several carbon-rich defensive secondary metabolites.

Heavy traffic in the fast lane: long-distance signalling by macromolecules

The two major vascular conduits in plants, the xylem and phloem, theoretically provide opportunities for the long-distance translocation of almost any type of water-borne molecule. This review focuses on the signalling functions conveyed by the movement of macromolecules. Here, a signal is defined as the communication of information from source to destination, where it modifies development, physiology or defence through altered gene expression or by direct influences on other cellular processes. Xylem and phloem sap both contain diverse classes of proteins; in addition, phloem contains many full-length and small RNA species. Only a few of these mobile molecules have proven functions in signalling. The transduction of signals typically depends on connection to appropriate signalling pathways. Incoming protein signals require specific detection systems, generally via receptors. Mobile RNAs require either the translation or presence of a homologous target. Given that phloem sieve elements are enucleate and lack translation machinery, RNA function requires subsequent unloading at least into adjacent companion cells. The binding of RNA by proteins in ribonucleoprotein complexes enables the translocation of some signals, with evidence for both sequence-specific and size-specific binding. Several examples of long-distance macromolecular signalling are highlighted, including the FT protein signal which regulates flowering time and other developmental switches.

MAMP (microbe-associated molecular pattern) triggered immunity in plants

Plants are sessile organisms that are under constant attack from microbes. They rely on both preformed defenses, and their innate immune system to ward of the microbial pathogens. Preformed defences include for example the cell wall and cuticle, which act as physical barriers to microbial colonization. The plant immune system is composed of surveillance systems that perceive several general microbe elicitors, which allow plants to switch from growth and development into a defense mode, rejecting most potentially harmful microbes. The elicitors are essential structures for pathogen survival and are conserved among pathogens. The conserved microbe-specific molecules, referred to as microbe- or pathogen-associated molecular patterns (MAMPs or PAMPs), are recognized by the plant innate immune systems pattern recognition receptors (PRRs). General elicitors like flagellin (Flg), elongation factor Tu (EF-Tu), peptidoglycan (PGN), lipopolysaccharides (LPS), Ax21 (Activator of XA21-mediated immunity in rice), fungal chitin, and β-glucans from oomycetes are recognized by plant surface localized PRRs. Several of the MAMPs and their corresponding PRRs have, in recent years, been identified. This review focuses on the current knowledge regarding important MAMPs from bacteria, fungi, and oomycetes, their structure, the plant PRRs that recognizes them, and how they induce MAMP-triggered immunity (MTI) in plants.

Brassinosteroids regulate organ boundary formation in the shoot apical meristem of Arabidopsis

Spatiotemporal control of the formation of organ primordia and organ boundaries from the stem cell niche in the shoot apical meristem (SAM) determines the patterning and architecture of plants, but the underlying signaling mechanisms remain poorly understood. Here we show that brassinosteroids (BRs) play a key role in organ boundary formation by repressing organ boundary identity genes. BR-hypersensitive mutants display organ-fusion phenotypes, whereas BR-insensitive mutants show enhanced organ boundaries. The BR-activated transcription factor BZR1 directly represses the cup-shaped cotyledon (CUC) family of organ boundary identity genes. In WT plants, BZR1 accumulates at high levels in the nuclei of central meristem and organ primordia but at a low level in organ boundary cells to allow CUC gene expression. Activation of BR signaling represses CUC gene expression and causes organ fusion phenotypes. This study uncovers a role for BR in the spatiotemporal control of organ boundary formation and morphogenesis in the SAM.

The shoot apical meristem regulatory peptide CLV3 does not activate innate immunity

The Arabidopsis thaliana leucine-rich repeat receptor kinase FLAGELLIN SENSING2 (FLS2) is required for the recognition of bacterial flagellin in innate immunity. Recently, FLS2 was proposed to act as a multispecific receptor recognizing unrelated exogenous and endogenous peptide ligands, including CLAVATA3 (CLV3), a key regulator of shoot meristem stem cell production. Here, we report experimental evidence demonstrating that FLS2 does not recognize CLV3 and that the shoot apical meristem is immune to bacteria independently of CLV3 perception.

Brassinosteroids

Brassinosteroids (BRs) are endogenous plant hormones essential for the proper regulation of multiple physiological processes required for normal plant growth and development. Since their discovery more than 30 years ago, extensive research on the mechanisms of BR action using biochemistry, mutant studies, proteomics and genome-wide transcriptome analyses, has helped refine the BR biosynthetic pathway, identify the basic molecular components required to relay the BR signal from perception to gene regulation, and expand the known physiological responses influenced by BRs. These mechanistic advances have helped answer the intriguing question of how BRs can have such dramatic pleiotropic effects on a broad range of diverse developmental pathways and have further pointed to BR interactions with other plant hormones and environmental cues. This chapter briefly reviews historical aspects of BR research and then summarizes the current state of knowledge on BR biosynthesis, metabolism and signal transduction. Recent studies uncovering novel phosphorelays and gene regulatory networks through which BR influences both vegetative and reproductive development are examined and placed in the context of known BR physiological responses including cell elongation and division, vascular differentiation, flowering, pollen development and photomorphogenesis.

Engineering plant resistance by constructing chimeric receptors that recognize damage-associated molecular patterns (DAMPs)

An efficient sensing of danger and a rapid activation of the immune system are crucial for the survival of plants. Conserved pathogen/microbe-associated molecular patterns (PAMPs/MAMPs) and endogenous molecular patterns, which are present only when the tissue is infected or damaged (damage-associated molecular patterns or DAMPs), can act as danger signals and activate the plant immune response. These molecules are recognized by surface receptors that are indicated as pattern recognition receptors (PRRs). In this paper we summarize recent information on oligogalacturonides (OGs), a class of DAMPs that is released from the extracellular matrix of the plant cell during pathogen attack or wounding. We also describe the characteristics of the Arabidopsis Wall-Associated Kinase 1 (WAK1), a PRR recently identified as a receptor of OGs and discuss the use of WAK1, PRRs and chimeric receptors to engineer resistance in crop plants.