Identification of MAPK Substrates Using Quantitative Phosphoproteomics

Enhancing rice panicle branching and grain yield through tissue-specific brassinosteroid inhibition

Crop yield potential is constrained by the inherent trade-offs among traits such as between grain size and number. Brassinosteroids (BRs) promote grain size, yet their role in regulating grain number is unclear. By deciphering the clustered-spikelet rice germplasm, we show that activation of the BR catabolic gene BRASSINOSTEROID-DEFICIENT DWARF3 (BRD3) markedly increases grain number. We establish a molecular pathway in which the BR signaling inhibitor GSK3/SHAGGY-LIKE KINASE2 phosphorylates and stabilizes OsMADS1 transcriptional factor, which targets TERMINAL FLOWER1-like gene RICE CENTRORADIALIS2. The tissue-specific activation of BRD3 in the secondary branch meristems enhances panicle branching, minimizing negative effects on grain size, and improves grain yield. Our study showcases the power of tissue-specific hormonal manipulation in dismantling the trade-offs among various traits and thus unleashing crop yield potential in rice.

Glucose-driven TOR-FIE-PRC2 signalling controls plant development

Nutrients and energy have emerged as central modulators of developmental programmes in plants and animals^1-3. The evolutionarily conserved target of rapamycin (TOR) kinase is a master integrator of nutrient and energy signalling that controls growth. Despite its key regulatory roles in translation, proliferation, metabolism and autophagy^2-5, little is known about how TOR shapes developmental transitions and differentiation. Here we show that glucose-activated TOR kinase controls genome-wide histone H3 trimethylation at K27 (H3K27me3) in Arabidopsis thaliana, which regulates cell fate and development^6-10. We identify FERTILIZATION-INDEPENDENT ENDOSPERM (FIE), an indispensable component of Polycomb repressive complex 2 (PRC2), which catalyses H3K27me3 (refs. ^6-8,10-12), as a TOR target. Direct phosphorylation by TOR promotes the dynamic translocation of FIE from the cytoplasm to the nucleus. Mutation of the phosphorylation site on FIE abrogates the global H3K27me3 landscape, reprogrammes the transcriptome and disrupts organogenesis in plants. Moreover, glucose-TOR-FIE-PRC2 signalling modulates vernalization-induced floral transition. We propose that this signalling axis serves as a nutritional checkpoint leading to epigenetic silencing of key transcription factor genes that specify stem cell destiny in shoot and root meristems and control leaf, flower and silique patterning, branching and vegetative-to-reproduction transition. Our findings reveal a fundamental mechanism of nutrient signalling in direct epigenome reprogramming, with broad relevance for the developmental control of multicellular organisms.

Proteomics and phosphoproteomics revealed molecular networks of stomatal immune responses

Dynamic protein and phosphoprotein profiles uncovered the overall regulation of stomata movement against pathogen invasion and phosphorylation states of proteins involved in ABA, SA, calcium and ROS signaling, which may modulate the stomatal immune response. Stomatal openings represent a major route of pathogen entry into the plant, and plants have evolved mechanisms to regulate stomatal aperture as innate immune response against bacterial invasion. However, the mechanisms underlying stomatal immunity are not fully understood. Taking advantage of high-throughput liquid chromatography mass spectrometry (LC-MS), we performed label-free proteomic and phosphoproteomic analyses of enriched guard cells in response to a bacterial pathogen Pseudomonas syringae pv. tomato (Pst) DC3000. In total, 495 proteins and 1229 phosphoproteins were identified as differentially regulated. These proteins are involved in a variety of signaling pathways, including abscisic acid and salicylic acid hormone signaling, calcium and reactive oxygen species signaling. We also showed that dynamic changes of phosphoprotein WRKY transcription factors may play a crucial role in regulating stomata movement in plant immunity. The identified proteins/phosphoproteins and the pathways form interactive molecular networks to regulate stomatal immunity. This study has provided new insights into the multifaceted mechanisms of stomatal immunity. The differential proteins and phosphoproteins are potential targets for engineering or breeding of crops for enhanced pathogen defense.

Proteomics data of SNF1-related protein kinase 2.4 interacting proteins revealed by immunoprecipitation-mass spectrometry

Identification of kinase substrates is a prerequisite for elucidating the mechanism by which a kinase transduces internal or external stimuli to cellular responses. Conventional methods to profile this type of protein-protein interaction typically deal with one kinase-substrate pair at a time. Mass spectrometry-based proteomics, on the other hand, can determine putative kinase-substrate pairs at a large-scale in an unbiased manner. In this study, we identified the interacting partners of SNF1-related protein kinase 2.4 (SnRK2.4) via immunoprecipitation coupled with mass spectrometry. Proteins from stable transgenic Arabidopsis plants overexpressing a FLAG-tagged SnRK2.4 (cloned from Brassica napus) were pulled down using an anti-FLAG antibody. The protein components were then identified by mass spectrometry. In parallel, proteins from wild type plants were also analyzed, providing a list of nonspecific binding proteins that were further removed from the candidate SnRK2.4-interacting protein list. Our data identified over 30 putative SnRK2.4 interacting partners, which included many key players in stress responses, transport, and cellular metabolic processes.

Proteomic characterization of MPK4 signaling network and putative substrates

KEY MESSAGE

Combining genetic engineering of MPK4 activity and quantitative proteomics, we established an in planta system that enables rapid study of MPK4 signaling networks and potential substrate proteins. Mitogen activated protein kinase 4 (MPK4) is a multifunctional kinase that regulates various signaling events in plant defense, growth, light response and cytokinesis. The question of how a single protein modulates many distinct processes has spurred extensive research into the physiological outcomes resulting from genetic perturbation of MPK4. However, the mechanism by which MPK4 functions is still poorly understood due to limited data on the MPK4 networks including substrate proteins and downstream pathways. Here we introduce an experimental system that combines genetic engineering of kinase activity and quantitative proteomics to rapidly study the signaling networks of MPK4. First, we transiently expressed a constitutively active (MPK4CA) and an inactive (MPK4IN) version of a Brassica napus MPK4 (BnMPK4) in Nicotiana benthamiana leaves. Proteomics analysis revealed that BnMPK4 activation affects multiple pathways (e.g., metabolism, redox regulation, jasmonic acid biosynthesis and stress responses). Furthermore, BnMPK4 activation also increased protein phosphorylation in the phosphoproteome, from which putative MPK4 substrates were identified. Using protein kinase assay, we validated that a transcription factor TCP8-like (TCP8) and a PP2A regulatory subunit TAP46-like (TAP46) were indeed phosphorylated by BnMPK4. Taken together, we demonstrated the utility of proteomics and phosphoproteomics in elucidating kinase signaling networks and in identification of downstream substrates.

MPK4 Phosphorylation Dynamics and Interacting Proteins in Plant Immunity

Arabidopsis MAP kinase 4 (MPK4) has been proposed to be a negative player in plant immunity, and it is also activated by pathogen-associated molecular patterns (PAMPs), such as flg22. The molecular mechanisms by which MPK4 is activated and regulates plant defense remain elusive. In this study, we investigated Arabidopsis defense against a bacterial pathogen Pseudomonas syringae pv tomato ( Pst) DC3000 when Brassica napus MPK4 ( BnMPK4) is overexpressed. We showed an increase in pathogen resistance and suppression of jasmonic acid (JA) signaling in the BnMPK4 overexpressing (OE) plants. We also showed that the OE plants have increased sensitivity to flg22-triggered reactive oxygen species (ROS) burst in guard cells, which resulted in enhanced stomatal closure compared to wild-type (WT). During flg22 activation, dynamic phosphorylation events within and outside of the conserved TEY activation loop were observed. To elucidate how BnMPK4 functions during the defense response, we used immunoprecipitation coupled with mass spectrometry (IP-MS) to identify BnMPK4 interacting proteins in the absence and presence of flg22. Quantitative proteomic analysis revealed a shift in the MPK4-associated protein network, providing insight into the molecular functions of MPK4 at the systems level.

Metabolomics of Early Stage Plant Cell-Microbe Interaction Using Stable Isotope Labeling

Metabolomics has been used in unraveling metabolites that play essential roles in plant-microbe (including pathogen) interactions. However, the problem of profiling a plant metabolome with potential contaminating metabolites from the coexisting microbes has been largely ignored. To address this problem, we implemented an effective stable isotope labeling approach, where the metabolome of a plant bacterial pathogen Pseudomonas syringae pv. tomato (Pst) DC3000 was labeled with heavy isotopes. The labeled bacterial cells were incubated with Arabidopsis thaliana epidermal peels (EPs) with guard cells, and excessive bacterial cells were subsequently removed from the plant tissues by washing. The plant metabolites were characterized by liquid chromatography mass spectrometry using multiple reactions monitoring, which can differentiate plant and bacterial metabolites. Targeted metabolomic analysis suggested that Pst DC3000 infection may modulate stomatal movement by reprograming plant signaling and primary metabolic pathways. This proof-of-concept study demonstrates the utility of this strategy in differentiation of the plant and microbe metabolomes, and it has broad applications in studying metabolic interactions between microbes and other organisms.

Post-translational regulation of plant immunity

Plants have evolved multi-layered molecular defense strategies to protect against pathogens. Plant immune signaling largely relies on post-translational modifications (PTMs) to induce rapid alterations of signaling pathways to achieve a response that is appropriate to the type of pathogen and infection pressure. In host cells, dynamic PTMs have emerged as powerful regulatory mechanisms that cells use to adjust their immune response. PTM is also a virulence strategy used by pathogens to subvert host immunity through the activities of effector proteins secreted into the host cell. Recent studies focusing on deciphering post-translational mechanisms underlying plant immunity have offered an in-depth view of how PTMs facilitate efficient immune responses and have provided a more dynamic and holistic view of plant immunity.