Quantitative Proteome and PTMome Analysis of Arabidopsis thaliana Root Responses to Persistent Osmotic and Salinity Stress

Light changes promote distinct responses of plastid protein acetylation marks

Protein acetylation is a key co- and post-translational modification. However, how different types of acetylation respond to environmental stress is still unknown. To address this, we investigated the role of a member of the newly discovered family of plastid acetyltransferases (GNAT2), which features both lysine- and N-terminal acetyltransferase activities. Our study aimed to provide a holistic multi-omics acetylation-dependent view of plant acclimation to short-term light changes. We found that both the yield and coverage of the N-terminal acetylome remained unchanged in wild-type and gnat2-knockout backgrounds after two hours of exposure to high light or darkness. Similarly, no differences in transcriptome or adenylate energy charge were observed between the genotypes under the tested light conditions. In contrast, the lysine acetylome proved to be sensitive to the changes in light conditions, especially in the gnat2 background. This suggests unique strategies of plant acclimation for quick responses to environmental changes involving lysine, but not N-terminal, GNAT2-mediated acetylation activity.

Sexually dimorphic acetyl-CoA biosynthesis and utilization in response to drought and exogenous acetic acid

Female willows exhibit greater drought tolerance and benefit more from exogenous acetic acid (AA)-improved drought tolerance than males. However, the potential mechanisms driving these sex-specific responses remain unclear. To comprehensively investigate the sexually dimorphic responsive mechanisms of willows to drought and exogenous AA, here, we performed physiological, proteomic, Lys-acetylproteomic, and transgenic analyses in female and male Salix myrtillacea exposed to drought and AA-applicated drought treatments, focusing on protein abundance and lysine acetylation (LysAc) changes. Drought-tolerant females suffered less drought-induced photosynthetic and oxidative damage, did not activate AA and acetyl-CoA biosynthesis, TCA cycle, fatty acid metabolism, and jasmonic acid signaling as strongly as drought-sensitive males. Exogenous AA caused overaccumulation of endogenous AA and inhibition of acetyl-CoA biosynthesis and utilization in males. However, exogenous AA greatly enhanced acetyl-CoA biosynthesis and utilization and further enhanced drought performance of females, possibly determining that AA improved drought tolerance more in females than in males. Interestingly, overexpression of acetyl-CoA synthetase (ACS) could reprogram fatty acids, increase LysAc levels, and improve drought tolerance, highlighting the involvement of ACS-derived acetyl-CoA in drought responses. In addition, drought and exogenous AA induced sexually dimorphic LysAc associated with histones, transcription factors, and metabolic enzymes in willows. Especially, exogenous AA may greatly improve the photosynthetic capacity of S. myrtillacea males by decreasing LysAc levels and increasing the abundances of photosynthetic proteins. While hyperacetylation in glycolysis, TCA cycle, and fatty acid biosynthesis potentially possibly serve as negative feedback to acclimate acetyl-CoA biosynthesis and utilization in drought-stressed males and AA-applicated females. Thus, acetyl-CoA biosynthesis and utilization determine the sexually dimorphic responses of S. myrtillacea to drought and exogenous AA.

The proteome landscape of the root cap reveals a role for the jacalin-associated lectin JAL10 in the salt-induced endoplasmic reticulum stress pathway

Rapid climate change has led to enhanced soil salinity, one of the major determinants of land degradation, resulting in low agricultural productivity. This has a strong negative impact on food security and environmental sustainability. Plants display various physiological, developmental, and cellular responses to deal with salt stress. Recent studies have highlighted the root cap as the primary stress sensor and revealed its crucial role in halotropism. The root cap covers the primary root meristem and is the first cell type to sense and respond to soil salinity, relaying the signal to neighboring cell types. However, it remains unclear how root-cap cells perceive salt stress and contribute to the salt-stress response. Here, we performed a root-cap cell-specific proteomics study to identify changes in the proteome caused by salt stress. The study revealed a very specific salt-stress response pattern in root-cap cells compared with non-root-cap cells and identified several novel proteins unique to the root cap. Root-cap-specific protein-protein interaction (PPI) networks derived by superimposing proteomics data onto known global PPI networks revealed that the endoplasmic reticulum (ER) stress pathway is specifically activated in root-cap cells upon salt stress. Importantly, we identified root-cap-specific jacalin-associated lectins (JALs) expressed in response to salt stress. A JAL10-GFP fusion protein was shown to be localized to the ER. Analysis of jal10 mutants indicated a role for JAL10 in regulating the ER stress pathway in response to salt. Taken together, our findings highlight the participation of specific root-cap proteins in salt-stress response pathways. Furthermore, root-cap-specific JAL proteins and their role in the salt-mediated ER stress pathway open a new avenue for exploring tolerance mechanisms and devising better strategies to increase plant salinity tolerance and enhance agricultural productivity.

Multifaceted functions of histone deacetylases in stress response

Histone deacetylases (HDACs) are important chromatin regulators essential for plant tolerance to adverse environments. In addition to histone deacetylation and epigenetic regulation, HDACs deacetylate non-histone proteins and thereby regulate multiple pathways. Like other post-translational modifications (PTMs), acetylation/deacetylation is a reversible switch regulating different cellular processes in plants. Here, by focusing on results obtained in arabidopsis (Arabidopsis thaliana) and rice plants, we analyze the different aspects of HDAC functions and the underlying regulatory mechanisms in modulating plant responses to stress. We hypothesize that, in addition to epigenetic regulation of gene expression, HDACs can also control plant tolerance to stress by regulating transcription, translation, and metabolic activities and possibly assembly-disassembly of stress granules (SGs) through lysine deacetylation of non-histone proteins.

Quantitative Time-Course Analysis of Osmotic and Salt Stress in Arabidopsis thaliana Using Short Gradient Multi-CV FAIMSpro BoxCar DIA

A major limitation when undertaking quantitative proteomic time-course experimentation is the tradeoff between depth-of-analysis and speed-of-analysis. In high complexity and high dynamic range sample types, such as plant extracts, balance between resolution and time is especially apparent. To address this, we evaluate multiple compensation voltage (CV) high field asymmetric waveform ion mobility spectrometry (FAIMSpro) settings using the latest label-free single-shot Orbitrap-based DIA acquisition workflows for their ability to deeply quantify the Arabidopsis thaliana seedling proteome. Using a BoxCarDIA acquisition workflow with a -30 -50 -70 CV FAIMSpro setting, we were able to consistently quantify >5000 Arabidopsis seedling proteins over a 21-min gradient, facilitating the analysis of ∼42 samples per day. Utilizing this acquisition approach, we then quantified proteome-level changes occurring in Arabidopsis seedling shoots and roots over 24 h of salt and osmotic stress, to identify early and late stress response proteins and reveal stress response overlaps. Here, we successfully quantify >6400 shoot and >8500 root protein groups, respectively, quantifying nearly ∼9700 unique protein groups in total across the study. Collectively, we pioneer a short gradient, multi-CV FAIMSpro BoxCarDIA acquisition workflow that represents an exciting new analysis approach for undertaking quantitative proteomic time-course experimentation in plants.

The many faces of lysine acylation in proteins: Phytohormones as unexplored substrates

Protein post-translational modification (PTM) is a ubiquitous process that occurs in most proteins. Lysine residues containing an ε-amino group are recognized as hotspots for the addition of different chemical groups. Lysine acetylation, extensively studied in histones, serves as an epigenetic hallmark capable of promoting changes in chromatin structure and availability. Acyl groups derived from molecules involved in carbohydrate and lipid metabolisms, such as lactate, succinate and hydroxybutyrate, were identified as lysine modifications of histones and other proteins. Lysine-acyltransferases do not exhibit significant substrate specificity concerning acyl donors. Furthermore, plant hormones harboring acyl groups often form conjugates with free amino acids to regulate their activity and function during plant physiological processes and responses, a process mediated by GH3 enzymes. Besides forming low-molecular weight conjugates, auxins have been shown to covalently modify proteins in bean seeds. Aside from auxins, other phytohormones with acyl groups are unexplored potential substrates for post-translational acylation of proteins. Using MS data searches, we revealed various proteins with lysine residues linked to auxin, abscisic acid, gibberellic acid, jasmonic acid, and salicylic acid. These findings raise compelling questions about the ability of plant hormones harboring carboxyl groups to serve as new candidates for protein acylation and acting in protein PTM and modulation.

Plant phase extraction: A method for enhanced discovery of the RNA-binding proteome and its dynamics in plants

RNA-binding proteins (RBPs) play critical roles in posttranscriptional gene regulation. Current methods of systematically profiling RBPs in plants have been predominantly limited to proteins interacting with polyadenylated (poly(A)) RNAs. We developed a method called plant phase extraction (PPE), which yielded a highly comprehensive RNA-binding proteome (RBPome), uncovering 2,517 RBPs from Arabidopsis (Arabidopsis thaliana) leaf and root samples with a highly diverse array of RNA-binding domains. We identified traditional RBPs that participate in various aspects of RNA metabolism and a plethora of nonclassical proteins moonlighting as RBPs. We uncovered constitutive and tissue-specific RBPs essential for normal development and, more importantly, revealed RBPs crucial for salinity stress responses from a RBP-RNA dynamics perspective. Remarkably, 40% of the RBPs are non-poly(A) RBPs that were not previously annotated as RBPs, signifying the advantage of PPE in unbiasedly retrieving RBPs. We propose that intrinsically disordered regions contribute to their nonclassical binding and provide evidence that enzymatic domains from metabolic enzymes have additional roles in RNA binding. Taken together, our findings demonstrate that PPE is an impactful approach for identifying RBPs from complex plant tissues and pave the way for investigating RBP functions under different physiological and stress conditions at the posttranscriptional level.

Proteome-wide lysine acetylation profiling to investigate the involvement of histone deacetylase HDA5 in the salt stress response of Arabidopsis leaves

Post-translational modifications (PTMs) of proteins play important roles in the acclimation of plants to environmental stress. Lysine acetylation is a dynamic and reversible PTM, which can be removed by histone deacetylases. Here we investigated the role of lysine acetylation in the response of Arabidopsis leaves to 1 week of salt stress. A quantitative mass spectrometry analysis revealed an increase in lysine acetylation of several proteins from cytosol and plastids, which was accompanied by altered histone deacetylase activities in the salt-treated leaves. While activities of HDA14 and HDA15 were decreased upon salt stress, HDA5 showed a mild and HDA19 a strong increase in activity. Since HDA5 is a cytosolic-nuclear enzyme from the class II histone deacetylase family with yet unknown protein substrates, we performed a lysine acetylome analysis on hda5 mutants and characterized its substrate proteins. Next to histone H2B, the salt stress-responsive transcription factor GT2L and the dehydration-related protein ERD7 were identified as HDA5 substrates. In addition, in protein-protein interaction studies, HDA18 was discovered, among other interacting proteins, to work in a complex together with HDA5. Altogether, this study revealed the substrate proteins of HDA5 and identified new lysine acetylation sites which are hyperacetylated upon salt stress. The identification of specific histone deacetylase substrate proteins, apart from histones, will be important to unravel the acclimation response of Arabidopsis to salt stress and their role in plant physiology.

BREVIPEDICELLUS Positively Regulates Salt-Stress Tolerance in Arabidopsis thaliana

Salt stress is one of the major environmental threats to plant growth and development. However, the mechanisms of plants responding to salt stress are not fully understood. Through genetic screening, we identified and characterized a salt-sensitive mutant, ses5 (sensitive to salt 5), in Arabidopsis thaliana. Positional cloning revealed that the decreased salt-tolerance of ses5 was caused by a mutation in the transcription factor BP (BREVIPEDICELLUS). BP regulates various developmental processes in plants. However, the biological function of BP in abiotic stress-signaling and tolerance are still not clear. Compared with wild-type plants, the bp mutant exhibited a much shorter primary-root and lower survival rate under salt treatment, while the BP overexpressors were more tolerant. Further analysis showed that BP could directly bind to the promoter of XTH7 (xyloglucan endotransglucosylase/hydrolase 7) and activate its expression. Resembling the bp mutant, the disruption of XTH7 gave rise to salt sensitivity. These results uncovered novel roles of BP in positively modulating salt-stress tolerance, and illustrated a putative working mechanism.

Growth-limiting drought stress induces time-of-day-dependent transcriptome and physiological responses in hybrid poplar

Drought stress negatively impacts the health of long-lived trees. Understanding the genetic mechanisms that underpin response to drought stress is requisite for selecting or enhancing climate change resilience. We aimed to determine how hybrid poplars respond to prolonged and uniform exposure to drought; how responses to moderate and more severe growth-limiting drought stresses differed; and how drought responses change throughout the day. We established hybrid poplar trees (Populus × 'Okanese') from unrooted stem cutting with abundant soil moisture for 6 weeks. We then withheld water to establish well-watered, moderate and severe growth-limiting drought conditions. These conditions were maintained for 3 weeks during which growth was monitored. We then measured photosynthetic rates and transcriptomes of leaves that had developed during the drought treatments at two times of day. The moderate and severe drought treatments elicited distinct changes in growth and development, photosynthetic rates and global transcriptome profiles. Notably, the time of day of sampling produced the strongest effect in the transcriptome data. The moderate drought treatment elicited global transcriptome changes that were intermediate to the severe and well-watered treatments in the early evening but did not elicit a strong drought response in the morning. Stable drought conditions that are sufficient to limit plant growth elicit distinct transcriptional profiles depending on the degree of water limitation and on the time of day at which they are measured. There appears to be a limited number of genes and functional gene categories that are responsive to all of the tested drought conditions in this study emphasizing the complex nature of drought regulation in long-lived trees.

Multi-omic analysis shows REVEILLE clock genes are involved in carbohydrate metabolism and proteasome function

Plants are able to sense changes in their light environments, such as the onset of day and night, as well as anticipate these changes in order to adapt and survive. Central to this ability is the plant circadian clock, a molecular circuit that precisely orchestrates plant cell processes over the course of a day. REVEILLE (RVE) proteins are recently discovered members of the plant circadian circuitry that activate the evening complex and PSEUDO-RESPONSE REGULATOR genes to maintain regular circadian oscillation. The RVE8 protein and its two homologs, RVE 4 and 6 in Arabidopsis (Arabidopsis thaliana), have been shown to limit the length of the circadian period, with rve 4 6 8 triple-knockout plants possessing an elongated period along with increased leaf surface area, biomass, cell size, and delayed flowering relative to wild-type Col-0 plants. Here, using a multi-omics approach consisting of phenomics, transcriptomics, proteomics, and metabolomics we draw new connections between RVE8-like proteins and a number of core plant cell processes. In particular, we reveal that loss of RVE8-like proteins results in altered carbohydrate, organic acid, and lipid metabolism, including a starch excess phenotype at dawn. We further demonstrate that rve 4 6 8 plants have lower levels of 20S proteasome subunits and possess significantly reduced proteasome activity, potentially explaining the increase in cell-size observed in RVE8-like mutants. Overall, this robust, multi-omic dataset provides substantial insight into the far-reaching impact RVE8-like proteins have on the diel plant cell environment.

Salt stress proteins in plants: An overview

Salinity stress is considered the most devastating abiotic stress for crop productivity. Accumulating different types of soluble proteins has evolved as a vital strategy that plays a central regulatory role in the growth and development of plants subjected to salt stress. In the last two decades, efforts have been undertaken to critically examine the genome structure and functions of the transcriptome in plants subjected to salinity stress. Although genomics and transcriptomics studies indicate physiological and biochemical alterations in plants, it do not reflect changes in the amount and type of proteins corresponding to gene expression at the transcriptome level. In addition, proteins are a more reliable determinant of salt tolerance than simple gene expression as they play major roles in shaping physiological traits in salt-tolerant phenotypes. However, little information is available on salt stress-responsive proteins and their possible modes of action in conferring salinity stress tolerance. In addition, a complete proteome profile under normal or stress conditions has not been established yet for any model plant species. Similarly, a complete set of low abundant and key stress regulatory proteins in plants has not been identified. Furthermore, insufficient information on post-translational modifications in salt stress regulatory proteins is available. Therefore, in recent past, studies focused on exploring changes in protein expression under salt stress, which will complement genomic, transcriptomic, and physiological studies in understanding mechanism of salt tolerance in plants. This review focused on recent studies on proteome profiling in plants subjected to salinity stress, and provide synthesis of updated literature about how salinity regulates various salt stress proteins involved in the plant salt tolerance mechanism. This review also highlights the recent reports on regulation of salt stress proteins using transgenic approaches with enhanced salt stress tolerance in crops.

qPTMplants: an integrative database of quantitative post-translational modifications in plants

As a crucial molecular mechanism, post-translational modifications (PTMs) play critical roles in a wide range of biological processes in plants. Recent advances in mass spectrometry-based proteomic technologies have greatly accelerated the profiling and quantification of plant PTM events. Although several databases have been constructed to store plant PTM data, a resource including more plant species and more PTM types with quantitative dynamics still remains to be developed. In this paper, we present an integrative database of quantitative PTMs in plants named qPTMplants (http://qptmplants.omicsbio.info), which hosts 1 242 365 experimentally identified PTM events for 429 821 nonredundant sites on 123 551 proteins under 583 conditions for 23 PTM types in 43 plant species from 293 published studies, with 620 509 quantification events for 136 700 PTM sites on 55 361 proteins under 354 conditions. Moreover, the experimental details, such as conditions, samples, instruments and methods, were manually curated, while a variety of annotations, including the sequence and structural characteristics, were integrated into qPTMplants. Then, various search and browse functions were implemented to access the qPTMplants data in a user-friendly manner. Overall, we anticipate that the qPTMplants database will be a valuable resource for further research on PTMs in plants.