Arabidopsis HSP90 protein modulates RPP4-mediated temperature-dependent cell death and defense responses

Genome-wide identification of HSP90 gene family in Rosa chinensis and its response to salt and drought stresses

Heat shock protein 90 (HSP90) is important for many organisms, including plants. Based on the whole genome information, the gene number, gene structure, evolutionary relationship, protein structure, and active site of the HSP90 gene family in Rosa chinensis and Rubus idaeus were determined, and the expression of the HSP90 gene under salt, and drought stresses in two rose varieties Wangxifeng and Sweet Avalanche were analyzed. Six and eight HSP90 genes were identified from R. chinensis and Ru. idaeus, respectively. Phylogenetic analysis revealed that the analyzed genes were divided into two Groups and four subgroups (Classes 1a, 1b, 2a, and 2b). Although members within the same classes displayed highly similar gene structures, while the gene structures and conserved domains of Group 1 (Class 1a and 1b) and the Group 2 (Class 2a and 2b) are different. Tandem and segmental duplication genes were found in Ru. idaeus, but not in R. chinensis, perhaps explaining the difference in HSP90 gene quantity in the two analyzed species. Analysis of cis-acting elements revealed abundant abiotic stress, photolight-response, and hormone-response elements in R. chinensis HSP90s. qRT-PCR analysis suggested that RcHSP90-1-1, RcHSP90-5-1 and RcHSP90-6-1 in Sweet Avalanche and Wangxifeng varieties played important regulatory roles under salt and drought stress. The analysis of protein structure and active sites indicate that the potential different roles of RcHSP90-1-1, RcHSP90-5-1, and RcHSP90-6-1 in salt and drought stresses may come from the differences of corresponding protein structures and activation sites. These data will provide information for the breeding of rose varieties with high stress resistance.

Supplementary Information

The online version contains supplementary material available at 10.1007/s13205-024-04052-0.

Genome-wide characterization of LEA gene family reveals a positive role of BnaA.LEA6.a in freezing tolerance in rapeseed (Brassica napus L.)

BACKGROUND

Freezing stress is one of the major abiotic stresses that causes extensive damage to plants. LEA (Late embryogenesis abundant) proteins play a crucial role in plant growth, development, and abiotic stress. However, there is limited research on the function of LEA genes in low-temperature stress in Brassica napus (rapeseed).

RESULTS

Total 306 potential LEA genes were identified in B. rapa (79), B. oleracea (79) and B. napus (148) and divided into eight subgroups. LEA genes of the same subgroup had similar gene structures and predicted subcellular locations. Cis-regulatory elements analysis showed that the promoters of BnaLEA genes rich in cis-regulatory elements related to various abiotic stresses. Additionally, RNA-seq and real-time PCR results indicated that the majority of BnaLEA family members were highly expressed in senescent tissues of rapeseed, especially during late stages of seed maturation, and most BnaLEA genes can be induced by salt and osmotic stress. Interestingly, the BnaA.LEA6.a and BnaC.LEA6.a genes were highly expressed across different vegetative and reproductive organs during different development stages, and showed strong responses to salt, osmotic, and cold stress, particularly freezing stress. Further analysis showed that overexpression of BnaA.LEA6.a increased the freezing tolerance in rapeseed, as evidenced by lower relative electrical leakage and higher survival rates compared to the wild-type (WT) under freezing treatment.

CONCLUSION

This study is of great significance for understanding the functions of BnaLEA genes in freezing tolerance in rapeseed and offers an ideal candidate gene (BnaA.LEA6.a) for molecular breeding of freezing-tolerant rapeseed cultivars.

Unveiling HSP40/60/70/90/100 gene families and abiotic stress response in Jerusalem artichoke

Heat shock proteins (HSPs) are essential for plant growth, development, and stress adaptation. However, their roles in Jerusalem artichoke are largely unexplored. Using bioinformatics, we classified 143 HSP genes into distinct families: HSP40 (82 genes), HSP60 (22 genes), HSP70 (29 genes), HSP90 (6 genes), and HSP100 (4 genes). Our analysis covered their traits, evolution, and structures. Using RNA-seq data, we uncovered unique expression patterns of these HSP genes across growth stages and tissues. Notably, HSP40, HSP60, HSP70, HSP90, and HSP100 families each had specific roles. We also studied how these gene families responded to various stresses, from extreme temperatures to drought and salinity, revealing intricate expression dynamics. Remarkably, HSP40 showed remarkable flexibility, while HSP60, HSP70, HSP90, and HSP100 responded specifically to stress types. Moreover, our analysis unveiled significant correlations between gene pairs under stress, implying cooperative interactions. qRT-PCR validation underscored the significance of particular genes such as HtHSP60-7, HtHSP90-5, HtHSP100-2, and HtHSP100-3 in responding to stress. In summary, our study advances the understanding of how HSP gene families collectively manage stresses in Jerusalem artichoke. This provides insights into specific gene functions and broader plant stress responses.

Multiomics analysis reveals a link between Brassica-specific miR1885 and rapeseed tolerance to low temperature

Brassica crops include various edible vegetable and plant oil crops, and their production is limited by low temperature beyond their tolerant capability. The key regulators of low-temperature resistance in Brassica remain largely unexplored. To identify posttranscriptional regulators of plant response to low temperature, we performed small RNA profiling, and found that 16 known miRNAs responded to cold treatment in Brassica rapa. The cold response of seven of those miRNAs were further confirmed by qRT-PCR and/or northern blot analyses. In parallel, a genome-wide association study of 220 accessions of Brassica napus identified four candidate MIRNA genes, all of which were cold-responsive, at the loci associated with low-temperature resistance. Specifically, these large-scale data analyses revealed a link between miR1885 and the plant response to low temperature in both B. rapa and B. napus. Using 5' rapid amplification of cDNA ends approach, we validated that miR1885 can cleave its putative target gene transcripts, Bn.TIR.A09 and Bn.TNL.A03, in B. napus. Furthermore, overexpression of miR1885 in Semiwinter type B. napus decreased the mRNA abundance of Bn.TIR.A09 and Bn.TNL.A03 and resulted in increased sensitivity to low temperature. Knocking down of miR1885 in Spring type B. napus led to increased mRNA abundance of its targets and improved rapeseed tolerance to low temperature. Together, our results suggested that the loci of miR1885 and its targets could be potential candidates for the molecular breeding of low temperature-tolerant Spring type Brassica crops.

SlMYB41 positively regulates tomato thermotolerance by activating the expression of SlHSP90.3

High-temperature stress has become a major abiotic factor that dramatically limits plant growth and crop yield. Plants have evolved complex mechanisms to cope with high-temperature stress, but the factors that regulate plant thermotolerance remain to be discovered. Here, a high temperature-induced MYB transcription factor SlMYB41 was cloned from tomato (Solanum lycopersicum). Two individual SlMYB41-RNA interference (RNAi) lines (MR) and one CRISPR/Cas9 mediated myb41 mutant (MC) were obtained to investigate the function of SlMYB41 in tomato thermotolerance. Under high-temperature stress, we found that the MR and MC lines showed more wilting than the wild type (WT), with more ion leakage, more MDA accumulation, lower contents of osmotic adjustment substances, and more accumulation of reactive oxygen species (ROS) which was resulted from lower antioxidative enzyme activities. In addition, the photosynthetic capacity and complex of MR and MC lines were damaged more seriously than WT plants under high-temperature stress, mainly manifested in lower photosynthetic rate and Fv/Fm. Moreover, heat stress-related genes, such as SlHSP17.6, SlHSP17.7, and SlHSP90.3 were downregulated in MR and MC lines. Importantly, Y1H and LUC analysis indicated that SlMYB41 can directly activate the transcription of SlHSP90.3. Together, our study suggest that SlMYB41 positively regulates tomato thermotolerance by activating the expression of SlHSP90.3.

HSP90.2 modulates 2Q2-mediated wheat resistance against powdery mildew

Wheat (Triticum aestivum L.) is a critical food crop feeding the world, but pathogens threaten its production. Wheat Heat Shock Protein 90.2 (HSP90.2) is a pathogen-inducible molecular chaperone folding nascent preproteins. Here, we used wheat HSP90.2 to isolate clients regulated at the posttranslational level. Tetraploid wheat hsp90.2 knockout mutant was susceptible to powdery mildew, while the HSP90.2 overexpression line was resistant, suggesting that HSP90.2 was essential for wheat resistance against powdery mildew. We next isolated 1500 clients of HSP90.2, which contained a wide variety of clients with different biological classifications. We utilized 2Q2, a nucleotide-binding leucine repeat-rich protein, as a model to investigate the potential of HSP90.2 interactome in fungal resistance. The transgenic line co-suppressing 2Q2 was more susceptible to powdery mildew, suggesting 2Q2 as a novel Pm-resistant gene. The 2Q2 protein resided in chloroplasts, and HSP90.2 played a critical role in the accumulation of 2Q2 in thylakoids. Our data provided over 1500 HSP90.2 clients with a potential regulation at the protein folding process and contributed a nontypical approach to isolate pathogenesis-related proteins.

HSP90.2 promotes CO2 assimilation rate, grain weight and yield in wheat

Wheat fixes CO₂ by photosynthesis into kernels to nourish humankind. Improving the photosynthesis rate is a major driving force in assimilating atmospheric CO₂ and guaranteeing food supply for human beings. Strategies for achieving the above goal need to be improved. Here, we report the cloning and mechanism of CO₂ ASSIMILATION RATE AND KERNEL-ENHANCED 1 (CAKE1) from durum wheat (Triticum turgidum L. var. durum). The cake1 mutant displayed a lower photosynthesis rate with smaller grains. Genetic studies identified CAKE1 as HSP90.2-B, encoding cytosolic molecular chaperone folding nascent preproteins. The disturbance of HSP90.2 decreased leaf photosynthesis rate, kernel weight (KW) and yield. Nevertheless, HSP90.2 over-expression increased KW. HSP90.2 recruited and was essential for the chloroplast localization of nuclear-encoded photosynthesis units, for example PsbO. Actin microfilaments docked on the chloroplast surface interacted with HSP90.2 as a subcellular track towards chloroplasts. A natural variation in the hexaploid wheat HSP90.2-B promoter increased its transcription activity, enhanced photosynthesis rate and improved KW and yield. Our study illustrated an HSP90.2-Actin complex sorting client preproteins towards chloroplasts to promote CO₂ assimilation and crop production. The beneficial haplotype of Hsp90.2 is rare in modern varieties and could be an excellent molecular switch promoting photosynthesis rate to increase yield in future elite wheat varieties.

ANTHRACNOSE RESISTANCE GENE2 confers fungal resistance in sorghum

Sorghum is an important food and feed crop globally; its production is hampered by anthracnose disease caused by the fungal pathogen Colletotrichum sublineola (Cs). Here, we report identification and characterization of ANTHRACNOSE RESISTANCE GENE 2 (ARG2) encoding a nucleotide-binding leucine-rich repeat (NLR) protein that confers race-specific resistance to Cs strains. ARG2 is one of a cluster of several NLR genes initially identified in the sorghum differential line SC328C that is resistant to some Cs strains. This cluster shows structural and copy number variations in different sorghum genotypes. Different sorghum lines carrying independent ARG2 alleles provided the genetic validation for the identity of the ARG2 gene. ARG2 expression is induced by Cs, and chitin induces ARG2 expression in resistant but not in susceptible lines. ARG2-mediated resistance is accompanied by higher expression of defense and secondary metabolite genes at early stages of infection, and anthocyanin and zeatin metabolisms are upregulated in resistant plants. Interestingly, ARG2 localizes to the plasma membrane when transiently expressed in Nicotiana benthamiana. Importantly, ARG2 plants produced higher shoot dry matter than near-isogenic lines carrying the susceptible allele suggesting an absence of an ARG2 associated growth trade-off. Furthermore, ARG2-mediated resistance is stable at a wide range of temperatures. Our observations open avenues for resistance breeding and for dissecting mechanisms of resistance.

Advances in understanding the mitogenic, metabolic, and cell death signaling in teleost development: the case of greater amberjack (Seriola dumerili, Risso 1810)

Cell growth and differentiation signals of insulin-like growth factor-1 (IGF-1), a key regulator in embryonic and postnatal development, are mediated through the IGF-1 receptor (IGF-1R), which activates several downstream pathways. The present study aims to address crucial organogenesis and development pathways including Akt, MAPKs, heat shock response, apoptotic and autophagic machinery, and energy metabolism in relation to IGF-1R activation during five developmental stages of reared Seriola dumerili: 1 day prior to hatching fertilized eggs (D-1), hatching day (D0), 3 days post-hatching larvae (D3), 33 (D33) and 46 (D46) days post-hatching juveniles. During both the fertilized eggs stage and larval-to-juvenile transition, IGF-1R/Akt pathway activation may mediate the hypertrophic signaling, while p44/42 MAPK phosphorylation was apparent at S. dumerili post-hatching processes and juvenile organs completion. On the contrary, apoptosis was induced during embryogenesis and autophagy at hatching day indicating a potential involvement in morphogenetic rearrangements and yolk-sac reserves depletion. Larvae morphogenesis was accompanied by a metabolic turnover with increased substantial energy consumption. The findings of the present study demonstrate the developmental stages-specific shift in critical signaling pathways during the ontogeny of reared S. dumerili.

The Disease Progression and Molecular Defense Response in Chenopodium Quinoa Infected with Peronospora Variabilis, the Causal Agent of Quinoa Downy Mildew

Downy mildew disease, caused by the biotrophic oomycete Peronospora variabilis, is the largest threat to the cultivation of quinoa (Chenopodium quinoa Willd.) in the Andean highlands, and occurs worldwide. However, so far, no molecular study of the quinoa-Peronospora interaction has been reported. Here, we developed tools to study downy mildew disease in quinoa at the gene expression level. P. variabilis was isolated and maintained, allowing the study of downy mildew disease progression in two quinoa cultivars under controlled conditions. Quinoa gene expression changes induced by P. variabilis were analyzed by qRT-PCR, for quinoa homologues of A. thaliana pathogen-associated genes. Overall, we observed a slower disease progression and higher tolerance in the quinoa cultivar Kurmi than in the cultivar Maniqueña Real. The quinoa orthologs of putative defense genes such as the catalase CqCAT2 and the endochitinase CqEP3 showed no changes in gene expression. In contrast, quinoa orthologs of other defense response genes such as the transcription factor CqWRKY33 and the chaperone CqHSP90 were significantly induced in plants infected with P. variabilis. These genes could be used as defense response markers to select quinoa cultivars that are more tolerant to P. variabilis infection.

HSP90 Contributes to chs3-2D-Mediated Autoimmunity

Plants employ multi-layered immune system to fight against pathogen infections. Different receptors are able to detect the invasion activities of pathogens, transduce signals to downstream components, and activate defense responses. Among those receptors, nucleotide-binding domain leucine-rich repeat containing proteins (NLRs) are the major intracellular ones. CHILLING SENSITIVE 3 (CHS3) is an Arabidopsis NLR with an additional Lin-11, Isl-1 and Mec-3 (LIM) domain at its C terminus. The gain-of-function mutant, chs3-2D, exhibiting severe dwarfism and constitutively activated defense responses, was selected as a genetic background in this study for a forward genetic screen. A mutant allele of hsp90.2 was isolated as a partial suppressor of chs3-2D, suggesting that HSP90 is required for CHS3-mediated defense signaling. In addition, HSP90 is also required for the autoimmunity of the Dominant Negative (DN)-SNIPER1 and gain-of-function ADR1-L2 D484V transgenic lines, suggesting a broad role for HSP90 in NLR-mediated defense. Overall, our work indicates a larger contribution of HSP90 not only at the sensor, but also the helper NLR levels.

The tomato OST1-VOZ1 module regulates drought-mediated flowering

Flowering is a critical agricultural trait that substantially affects tomato fruit yield. Although drought stress influences flowering time, the molecular mechanism underlying drought-regulated flowering in tomato remains elusive. In this study, we demonstrated that loss of function of tomato OPEN STOMATA 1 (SlOST1), a protein kinase essential for abscisic acid (ABA) signaling and abiotic stress responses, lowers the tolerance of tomato plants to drought stress. slost1 mutants also exhibited a late flowering phenotype under both normal and drought stress conditions. We also established that SlOST1 directly interacts with and phosphorylates the NAC (NAM, ATAF and CUC)-type transcription factor VASCULAR PLANT ONE-ZINC FINGER 1 (SlVOZ1), at residue serine 67, thereby enhancing its stability and nuclear translocation in an ABA-dependent manner. Moreover, we uncovered several SlVOZ1 binding motifs from DNA affinity purification sequencing analyses and revealed that SlVOZ1 can directly bind to the promoter of the major flowering-integrator gene SINGLE FLOWER TRUSS to promote tomato flowering transition in response to drought. Collectively, our data uncover the essential role of the SlOST1-SlVOZ1 module in regulating flowering in response to drought stress in tomato and offer insights into a novel strategy to balance drought stress response and flowering.

Genome Structures and Evolution Analysis of Hsp90 Gene Family in Brassica napus Reveal the Possible Roles of Members in Response to Salt Stress and the Infection of Sclerotinia sclerotiorum

Heat shock proteins 90 (Hsp90s) are conserved proteins participating in the responses to heat stress and are found to be involved in different kinds of abiotic and biotic stresses. Brassica napus (B. napus) is an important heteropolyploid crop, producing edible oil. Salt stress is one of the most important hazards to the growth of rape in the world, while Sclerotinia stem rot is one of the most serious diseases, caused by Sclerotinia sclerotiorum (S. sclerotiorum). In this study, the evolution of Hsp90 genes and their responses to these two stresses were elucidated. Bioinformatic analysis through the whole genome of B. napus identified 35 Hsp90 gene family members. Five groups were obtained via phylogenetic analysis with the 35 Hsp genes, Hsps from its two ancestor species Brassica rapa, Brassica oleracea, and AtHsps. Gene structure and conservative motif analysis of these 35 Hsps indicated that the Hsps were relatively conservative in each group. Strong collinearity was also detected between the genomes of Brassica rapa, Brassica oleracea and B. napus, along with identifying syntenic gene pairs of Hsps among the three genomes. In addition, whole genome duplication was discovered as the main reason for the generation of BnHsp gene family. The analysis of cis-acting elements indicated that BnHsp90 might be involved in a variety of abiotic and biotic stress responses. Analysis of the expression pattern indicated that BnHsp90 participates in the responses of B. napus to salt stress and the infection of S. sclerotiorum. Fourteen and nine BnHsp90s were validated to be involved in the defense responses of B. napus against salt stress and S. sclerotiorum, respectively. Our results provide new insights for the roles of BnHsp90s in the responses of B. napus to salt stress and S. sclerotiorum.

Global identification and integrated analysis of heat-responsive long non-coding RNAs in contrasting rice cultivars

Identified 2743 rice lncRNAs LncRNAs in response to heat stress Function prediction of HRLs Network among HRLs, genes and miRNAs co-localization of HRLs with QTLs Significant motifs in HRL sequences Long non-coding RNAs (lncRNAs) play vital roles in plant responses to environmental challenges. A better understanding of the gene regulation mediated by lncRNAs and their systematic identification would provide great benefits for modern agriculture. In this study, we performed strand-specific RNA sequencing for two rice varieties, heat-tolerant ZS97B and heat-susceptible SYD2 under heat stress. In total, 2743 putative lncRNAs were identified, and their expression profiles in response to heat treatments were established. We identified 231 differentially expressed lncRNAs (DELs) under heat stress, including 31 DELs common to both varieties and 103 and 97 specific to ZS97B and SYD2, respectively, all defined as heat-responsive lncRNAs (HRLs). The target-coding genes of HRLs were predicted, and GO and KEGG annotations of HRL targets revealed functions in which HRLs might be involved. The interaction network between HRLs, target genes and relevant miRNAs was constructed. The HRLs and their targets were compared with publicly available QTLs for rice seedling growth under heat stimulus. Ten HRLs and twelve target genes were linked with five heat stress-relevant QTLs. Sequence analysis revealed several motifs significantly enriched within the 231 HRL sequences. Our findings provide a valuable resource for further characterization of lncRNAs in terms of heat response and plant heat tolerance improvement.

Fine-Tuning Cold Stress Response Through Regulated Cellular Abundance and Mechanistic Actions of Transcription Factors

Inflictions caused by cold stress can result in disastrous effects on the productivity and survival of plants. Cold stress response in plants requires crosstalk between multiple signaling pathways including cold, heat, and reactive oxygen species (ROS) signaling networks. CBF, MYB, bHLH, and WRKY families are among the TFs that function as key players in the regulation of cold stress response at the molecular level. This review discusses some of the latest understanding on the regulation of expression and the mechanistic actions of plant TFs to address cold stress response. It was shown that the plant response consists of early and late responses as well as memory reprogramming for long-term protection against cold stress. The regulatory network can be differentiated into CBF-dependent and independent pathways involving different sets of TFs. Post-transcriptional regulation by miRNAs, control during ribosomal translation process, and post-translational regulation involving 26S proteosomic degradation are processes that affect the cellular abundance of key regulatory TFs, which is an important aspect of the regulation for cold acclimation. Therefore, fine-tuning of the regulation by TFs for adjusting to the cold stress condition involving the dynamic action of protein kinases, membrane ion channels, adapters, and modifiers is emphasized in this review.

Identification of Micro Ribonucleic Acids and Their Targets in Response to Plasmodiophora brassicae Infection in Brassica napus

Clubroot disease, which is caused by the soil-borne pathogen Plasmodiophora brassicae War (P. brassicae), is one of the oldest and most destructive diseases of Brassica and cruciferous crops in the world. Plant microRNAs [micro ribonucleic acids (miRNAs)] play important regulatory roles in several developmental processes. Although the role of plant miRNAs in plant-microbe interaction has been extensively studied, there are only few reports on the specific functions of miRNAs in response to P. brassicae. This study investigated the roles of miRNAs and their targets during P. brassicae infection in a pair of Brassica napus near-isogenic lines (NILs), namely clubroot-resistant line 409R and clubroot-susceptible line 409S. Small RNA sequencing (sRNA-seq) and degradome-seq were performed on root samples of 409R and 409S with or without P. brassicae inoculation. sRNA-seq identified a total of 48 conserved and 72 novel miRNAs, among which 18 had a significant differential expression in the root of 409R, while only one miRNA was differentially expressed in the root of 409S after P. brassicae inoculation. The degradome-seq analysis identified 938 miRNA target transcripts, which are transcription factors, enzymes, and proteins involved in multiple biological processes and most significantly enriched in the plant hormone signal transduction pathway. Between 409R and 409S, we found eight different degradation pathways in response to P. brassicae infection, such as those related to fatty acids. By combining published transcriptome data, we identified a total of six antagonistic miRNA-target pairs in 409R that are responsive to P. brassicae infection and involved in pathways associated with root development, hypersensitive cell death, and chloroplast metabolic synthesis. Our results reveal that P. brassicae infection leads to great changes in miRNA pool and target transcripts. More interestingly, these changes are different between 409R and 409S. Clarification of the crosstalk between miRNAs and their targets may shed new light on the possible mechanisms underlying the pathogen resistance against P. brassicae.

NBS-LRR Gene TaRPS2 is Positively Associated with the High-Temperature Seedling Plant Resistance of Wheat Against Puccinia striiformis f. sp. tritici

Xiaoyan6 (XY6) is a wheat (Triticum aestivum) cultivar possessing nonrace-specific high-temperature seedling plant (HTSP) resistance against stripe rust, caused by Puccinia striiformis f. sp. tritici. Previously, we identified one particular gene, TaRPS2, for its involvement in the HTSP resistance. To elucidate the role of TaRPS2 in the HTSP resistance, we cloned the full length of TaRPS2 from XY6. The transcriptional expression of TaRPS2 was rapidly upregulated (19.11-fold) under the normal-high-normal temperature treatment that induces the HTSP resistance. The expression level of TaRPS2 in leaves was higher than that in the stems and roots. Quantification of the endogenous hormones in wheat leaves after P. striiformis f. sp. tritici inoculation showed that 1-aminocyclopropane-1-carboxylic acid, salicylic acid (SA), and jasmonic acid were involved in the HTSP resistance. In addition, detection of hydrogen peroxide (H₂O₂) accumulation indicated that reactive oxygen species burst was also associated with the HTSP resistance. Two hours after exogenous H₂O₂ treatment or 0.5 h after SA treatment, the expression level of TaRPS2 was increased by 2.66 and 2.35 times, respectively. The subcellular localization of enhanced green fluorescent protein-TaRPS2 fusion protein was in the nuclei and plasma membranes. Virus-induced gene silencing of TaRPS2 reduced the level of HTSP resistance in XY6. Compared with the nonsilenced leaves, the TaRPS2-silenced leaves had the reduction of necrotic cells but a greater number of uredinia. These results indicated that TaRPS2 positively regulates the HTSP resistance of XY6 against P. striiformis f. sp. tritici and is related to the SA and H₂O₂ signaling pathways.

Autophagy-related genes serve as heat shock protein 90 co-chaperones in disease resistance against cassava bacterial blight

Heat shock protein 90 (HSP90) is involved in plant growth and various stress responses via regulating protein homeostasis. Autophagy keeps cellular homeostasis by recycling the components of cellular cytoplasmic constituents. Although they have similar effects on cellular protein homeostasis, the direct association between HSP90 and autophagy signaling remains unclear in plants, especially in tropical crops. In this study, the correlation between HSP90 and autophagy signaling was systematically analyzed by protein-protein interaction in cassava, one of the most important economy fruit in tropic. In addition, their effects on plant disease response and underlying mechanisms in cassava were investigated by functional genomics and genetic phenotype assay. The potential MeHSP90.9-MeSGT1-MeRAR1 chaperone complex interacts with MeATGs and subsequently triggers autophagy signaling, conferring improved disease resistance to cassava bacterial blight (CBB). On the contrary, HSP90 inhibitor and autophagy inhibitor decreased disease resistance against CBB in cassava, and autophagy may be involved in the potential MeHSP90.9-MeSGT1-MeRAR1 chaperone complex-mediated multiple immune responses. This study highlights the precise modulation of autophagy signaling by potential MeHSP90.9-MeSGT1-MeRAR1 chaperone complex in autophagy-mediated disease resistance to CBB.

The novel pathogen-responsive glycosyltransferase UGT73C7 mediates the redirection of phenylpropanoid metabolism and promotes SNC1-dependent Arabidopsis immunity

Recent studies have shown that global metabolic reprogramming is a common event in plant innate immunity; however, the relevant molecular mechanisms remain largely unknown. Here, we identified a pathogen-induced glycosyltransferase, UGT73C7, that plays a critical role in Arabidopsis disease resistance through mediating redirection of the phenylpropanoid pathway. Loss of UGT73C7 function resulted in significantly decreased resistance to Pseudomonas syringae pv. tomato DC3000, whereas constitutive overexpression of UGT73C7 led to an enhanced defense response. UGT73C7-activated immunity was demonstrated to be dependent on the upregulated expression of SNC1, a Toll/interleukin 1 receptor-type NLR gene. Furthermore, in vitro and in vivo assays indicated that UGT73C7 could glycosylate p-coumaric acid and ferulic acid, the upstream metabolites in the phenylpropanoid pathway. Mutations that lead to the loss of UGT73C7 enzyme activities resulted in the failure to induce SNC1 expression. Moreover, glycosylation activity of UGT73C7 resulted in the redirection of phenylpropanoid metabolic flux to biosynthesis of hydroxycinnamic acids and coumarins. The disruption of the phenylpropanoid pathway suppressed UGT73C7-promoted SNC1 expression and the immune response. This study not only identified UGT73C7 as an important regulator that adjusts phenylpropanoid metabolism upon pathogen challenge, but also provided a link between phenylpropanoid metabolism and an NLR gene.

Comparative transcriptome analyses revealed different heat stress responses in pigeonpea (Cajanus cajan) and its crop wild relatives

Comparative transcriptome analyses accompanied by biochemical assays revealed high variability in heat stress response in Cajanus species. Among the studied species, C. scarabaeoides was the most thermotolerant followed by C. cajanifolius, C. cajan, and C. acutifolius. Pigeonpea is one of the climate-resilient grain legumes. Though the optimum temperature for cultivated pigeonpea is ~ 25-35 °C, its wild relatives grow in temperatures ranging between 18 and 45 °C. To gain insight into molecular mechanisms responsible for the heat stress tolerance in pigeonpea, we conducted time-series transcriptome analysis of one pigeonpea cultivar (Cajanus cajan) and two wild relatives, Cajanus acutifolius, and Cajanus scarabaeoides subjected to heat stress at 42 ± 2 ºC for 30 min and 3 h. A total of 9521, 12,447, and 5282 identified transcripts were differentially expressed in C. cajan, C. acutifolius, and C. scarabaeoides, respectively. In this study, we observed that a significant number of genes undergo alternative splicing in a species-specific pattern during heat stress. Gene expression profiling analysis, histochemical assay, chlorophyll content, and electrolyte leakage assay showed that C. scarabaeoides has adaptive features for heat stress tolerance. The gene set enrichment analyses of differentially expressed genes in these Cajanus species during heat stress revealed that oxidoreductase activity, transcription factor activity, oxygen-evolving complex, photosystem-II, thylakoid, phenylpropanoid biosynthetic process, secondary metabolic process, and flavonoid biosynthetic process were highly affected. The histochemical assay showed more lipid peroxidation in C. acutifolius compared to other Cajanus species inferring the presence of higher quantities of polyunsaturated fatty acids in the plasma membrane which might have led to severe damage of membrane-bound organelles like chloroplast, and high electrolyte leakage during heat stress. This study paves the way for the identification of candidate genes, which can be useful for the development of thermo-tolerant pigeonpea cultivars.

Genome-Wide Characterization of HSP90 Gene Family in Cucumber and Their Potential Roles in Response to Abiotic and Biotic Stresses

Heat shock protein 90 (HSP90) possesses critical functions in plant developmental control and defense reactions. The HSP90 gene family has been studied in various plant species. However, the HSP90 gene family in cucumber has not been characterized in detail. In this study, a total of six HSP90 genes were identified from the cucumber genome, which were distributed to five chromosomes. Phylogenetic analysis divided the cucumber HSP90 genes into two groups. The structural characteristics of cucumber HSP90 members in the same group were similar but varied among different groups. Synteny analysis showed that only one cucumber HSP90 gene, Csa1G569290, was conservative, which was not collinear with any HSP90 gene in Arabidopsis and rice. The other five cucumber HSP90 genes were collinear with five Arabidopsis HSP90 genes and six rice HSP90 genes. Only one pair of paralogous genes in the cucumber HSP90 gene family, namely one pair of tandem duplication genes (Csa1G569270/Csa1G569290), was detected. The promoter analysis showed that the promoters of cucumber HSP90 genes contained hormone, stress, and development-related cis-elements. Tissue-specific expression analysis revealed that only one cucumber HSP90 gene Csa3G183950 was highly expressed in tendril but low or not expressed in other tissues, while the other five HSP90 genes were expressed in all tissues. Furthermore, the expression levels of cucumber HSP90 genes were differentially induced by temperature and photoperiod, gibberellin (GA), downy mildew, and powdery mildew stimuli. Two cucumber HSP90 genes, Csa1G569270 and Csa1G569290, were both differentially expressed in response to abiotic and biotic stresses, which means that these two HSP90 genes play important roles in the process of cucumber growth and development. These findings improve our understanding of cucumber HSP90 family genes and provide preliminary information for further studies of cucumber HSP90 gene functions in plant growth and development.

Heat shock protein 90 co-chaperone modules fine-tune the antagonistic interaction between salicylic acid and auxin biosynthesis in cassava

Heat shock protein 90 (HSP90) is an important molecular chaperone in plants. However, HSP90-mediated plant immune response remains elusive in cassava. In this study, cassava bacterial blight (CBB) induces the expression of MeHsf8, which directly targets MeHSP90.9 to activate its expression and immune response. Further identification of SHI-related sequence 1 (MeSRS1) and MeWRKY20 as MeHSP90.9 co-chaperones revealed the underlying mechanism of MeHSP90.9-mediated immune response. MeHSP90.9 interacts with MeSRS1 and MeWRKY20 to promote their transcriptional activation of salicylic acid (SA) biosynthetic gene avrPphB Susceptible 3 (MePBS3) and tryptophan metabolic gene N-acetylserotonin O-methyltransferase 2 (MeASMT2), respectively, so as to activate SA biosynthesis but inhibit tryptophan-derived auxin biosynthesis. Notably, genetic experiments confirmed that overexpressing MePBS3 and MeASMT2 could rescue the effects of silencing MeHsf8-MeHSP90.9 on disease resistance. This study highlights the dual regulation of SA and auxin biosynthesis by MeHSP90.9, providing the mechanistic understanding of MeHSP90.9 client partners in plant immunity.

The Arabidopsis AGC kinases NDR2/4/5 interact with MOB1A/1B and play important roles in pollen development and germination

Pollen formation and pollen tube growth are essential for the delivery of male gametes into the female embryo sac for double fertilization. Little is known about the mechanisms that regulate the late developmental process of pollen formation and pollen germination. In this study, we characterized a group of Arabidopsis AGC kinase proteins, NDR2/4/5, involved in pollen development and pollen germination. The NDR2/4/5 genes are mainly expressed in pollen grains at the late developmental stages and in pollen tubes. They function redundantly in pollen formation and pollen germination. At the tricellular stages, the ndr2 ndr4 ndr5 mutant pollen grains exhibit an abnormal accumulation of callose, precocious germination and burst in anthers, leading to a drastic reduction in fertilization and a reduced seed set. NDR2/4/5 proteins can interact with another group of proteins (MOB1A/1B) homologous to the MOB proteins from the Hippo signaling pathway in yeast and animals. The Arabidopsis mob1a mob1b mutant pollen grains also have a phenotype similar to that of ndr2 ndr4 ndr5 pollen grains. These results provide new evidence demonstrating that the Hippo signaling components are conserved in plants and play important roles in sexual plant reproduction.

SUSA2 is an F-box protein required for autoimmunity mediated by paired NLRs SOC3-CHS1 and SOC3-TN2

Both higher plants and mammals rely on nucleotide-binding leucine-rich repeat (NLR) immune receptors to detect pathogens and initiate immunity. Upon effector recognition, plant NLRs oligomerize for defense activation, the mechanism of which is poorly understood. We previously showed that disruption of the E3 ligase, Senescence-Associated E3 Ubiquitin Ligase 1 (SAUL1) leads to the activation of the NLR SOC3. Here, we report the identification of suppressor of saul1 2 (susa2) and susa3 from the saul1-1 suppressor screen. Pairwise interaction analysis suggests that both SUSA proteins interact with components of an SCF^SUSA2 E3 ligase complex as well as CHS1 or TN2, truncated NLRs that pair with SOC3. susa2-2 only suppresses the autoimmunity mediated by either CHS1 or TN2, suggesting its specific involvement in SOC3-mediated immunity. In summary, our study indicates links between plant NLRs and an SCF complex that may enable ubiquitination and degradation of unknown downstream components to activate defense.

Plant E3 ligases SNIPER1 and SNIPER2 broadly regulate the homeostasis of sensor NLR immune receptors

In both plants and animals, nucleotide-binding leucine-rich repeat (NLR) immune receptors perceive pathogen-derived molecules to trigger immunity. Global NLR homeostasis must be tightly controlled to ensure sufficient and timely immune output while avoiding aberrant activation, the mechanisms of which are largely unclear. In a previous reverse genetic screen, we identified two novel E3 ligases, SNIPER1 and its homolog SNIPER2, both of which broadly control the levels of NLR immune receptors in Arabidopsis. Protein levels of sensor NLRs (sNLRs) are inversely correlated with SNIPER1 amount and the interactions between SNIPER1 and sNLRs seem to be through the common nucleotide-binding (NB) domains of sNLRs. In support, SNIPER1 can ubiquitinate the NB domains of multiple sNLRs in vitro. Our study thus reveals a novel process of global turnover of sNLRs by two master E3 ligases for immediate attenuation of immune output to effectively avoid autoimmunity. Such unique mechanism can be utilized in the future for engineering broad-spectrum resistance in crops to fend off pathogens that damage our food supply.

The chaperone MeHSP90 recruits MeWRKY20 and MeCatalase1 to regulate drought stress resistance in cassava

The 90 kDa heat shock protein (HSP90) is widely involved in various developmental processes and stress responses in plants. However, the molecular chaperone HSP90-constructed protein complex and its function in cassava remain elusive. In this study, we report that HSP90 is essential for drought stress resistance in cassava by regulating abscisic acid (ABA) and hydrogen peroxide (H₂ O₂ ) using two specific protein inhibitors of HSP90 (geldanamycin (GDA) and radicicol (RAD)). Among 10 MeHSP90s, the transcript of MeHSP90.9 is largely induced during drought stress. Further investigation identifies MeWRKY20 and MeCatalase1 as MeHSP90.9-interacting proteins. MeHSP90.9-, MeWRKY20-, or MeCatalase1-silenced plants through virus-induced gene silencing display drought sensitivity in cassava, indicating that they are important to drought stress response. MeHSP90.9 can promote the direct transcriptional activation of MeWRKY20 on the W-box element of MeNCED5 promoter, encoding a key enzyme in ABA biosynthesis. Moreover, MeHSP90.9 positively regulates the activity of MeCatalase1, and MeHSP90.9-silenced cassava leaves accumulate more H₂ O₂ under drought stress. Taken together, we demonstrate that the MeHSP90.9 chaperone complex is a regulator of drought stress resistance in cassava.

Genetically Modified Heat Shock Protein90s and Polyamine Oxidases in Arabidopsis Reveal Their Interaction under Heat Stress Affecting Polyamine Acetylation, Oxidation and Homeostasis of Reactive Oxygen Species

One Sentence Summary

Heat shock proteins90 (HSP90s) induce acetylation of polyamines (PAs) and interact with polyamine oxidases (PAOs) affecting oxidation of PAs and hydrogen peroxide (H₂O₂) homeostasis in Arabidopsis thaliana.

Abstract

The chaperones, heat shock proteins (HSPs), stabilize proteins to minimize proteotoxic stress, especially during heat stress (HS) and polyamine (PA) oxidases (PAOs) participate in the modulation of the cellular homeostasis of PAs and reactive oxygen species (ROS). An interesting interaction of HSP90s and PAOs was revealed in Arabidopsis thaliana by using the pLFY:HSP90RNAi line against the four AtHSP90 genes encoding cytosolic proteins, the T-DNA Athsp90-1 and Athsp90-4 insertional mutants, the Atpao3 mutant and pharmacological inhibitors of HSP90s and PAOs. Silencing of all cytosolic HSP90 genes resulted in several-fold higher levels of soluble spermidine (S-Spd), acetylated Spd (N⁸-acetyl-Spd) and acetylated spermine (N¹-acetyl-Spm) in the transgenic Arabidopsis thaliana leaves. Heat shock induced increase of soluble-PAs (S-PAs) and soluble hydrolyzed-PAs (SH-PAs), especially of SH-Spm, and more importantly of acetylated Spd and Spm. The silencing of HSP90 genes or pharmacological inhibition of the HSP90 proteins by the specific inhibitor radicicol, under HS stimulatory conditions, resulted in a further increase of PA titers, N⁸-acetyl-Spd and N¹-acetyl-Spm, and also stimulated the expression of PAO genes. The increased PA titers and PAO enzymatic activity resulted in a profound increase of PAO-derived hydrogen peroxide (H₂O₂) levels, which was terminated by the addition of the PAO-specific inhibitor guazatine. Interestingly, the loss-of-function Atpao3 mutant exhibited increased mRNA levels of selected AtHSP90 genes. Taken together, the results herein reveal a novel function of HSP90 and suggest that HSP90s and PAOs cross-talk to orchestrate PA acetylation, oxidation, and PA/H₂O₂ homeostasis.

The plant hypersensitive response: concepts, control and consequences

The hypersensitive defence response is found in all higher plants and is characterized by a rapid cell death at the point of pathogen ingress. It is usually associated with pathogen resistance, though, in specific situations, it may have other consequences such as pathogen susceptibility, growth retardation and, over evolutionary timescales, speciation. Due to the potentially severe costs of inappropriate activation, plants employ multiple mechanisms to suppress inappropriate activation of HR and to constrain it after activation. The ubiquity of this response among higher plants despite its costs suggests that it is an extremely effective component of the plant immune system.

Advances and challenges in uncovering cold tolerance regulatory mechanisms in plants

Contents Summary I. Introduction II. Cold stress and physiological responses in plants III. Sensing of cold signals in plants IV. Messenger molecules involved in cold signal transduction V. Cold signal transduction in plants VI. Conclusions and perspectives Acknowledgements References SUMMARY: Cold stress is a major environmental factor that seriously affects plant growth and development, and influences crop productivity. Plants have evolved a series of mechanisms that allow them to adapt to cold stress at both the physiological and molecular levels. Over the past two decades, much progress has been made in identifying crucial components involved in cold-stress tolerance and dissecting their regulatory mechanisms. In this review, we summarize recent major advances in our understanding of cold signalling and put forward open questions in the field of plant cold-stress responses. Answering these questions should help elucidate the molecular mechanisms underlying plant tolerance to cold stress.

Xylopia aethiopica Seeds from Two Countries in West Africa Exhibit Differences in Their Proteomes, Mineral Content and Bioactive Phytochemical Composition

Aside from its multiple medicinal uses, the fruit of Xylopia aethiopica is widely used in Africa as food. Herein, we characterize the protein profiles, mineral content and bioactive phytochemical composition of the seeds of this plant sourced in Ghana and Nigeria. Using label-free proteomics, a total of 677 proteins were identified, with 260 found in the Ghana-sourced samples while 608 proteins were detected in the samples from Nigeria. However, 114 proteins were common between the samples from the two countries, among which 48 were significantly changed. Bioinformatics and functional analyses revealed that the differential levels of the proteins were mainly linked to pathways involved amino acids metabolism and biosynthesis. The significantly changed proteins related mainly to catalytic activity and carbon metabolism. The samples from Nigeria also exhibited superior qualities in terms of their antioxidant effects, and total phenolic and flavonoid content. Finally, only the content of Na varied to a statistically significant level. This study lends support to its culinary use and hints towards the impact of location of cultivation on the quality of the seeds. There is however need for further mechanistic investigations to unravel the underlying reasons for the observed differences.

Solanum lycopersicum microRNA1916 targets multiple target genes and negatively regulates the immune response in tomato

MicroRNA1916 (miR1916) is one of the nonconserved miRNAs that respond to various stresses in plants, but little has been known at present about its mechanisms in biotic stresses. In this study, the expression of Solanum lycopersicum (sly)-miR1916 in tomato was found to be down-regulated after infection with Phytophthora infestans or Botrytis cinerea. Tomato plants that overexpressed sly-miR1916 displayed significant enhancement in susceptibility to P. infestans and B. cinerea infection, as well as increased tendency to produce reactive oxygen species. Silencing of sly-miR1916 by short tandem target mimic and artificial microRNA strategies caused the tomato plants to become more tolerant to adverse conditions. In addition, lower sly-miR1916 expression could up-regulate the expression of strictosidine synthase (STR-2), UDP-glycosyltransferases (UGTs), late blight resistance protein homolog R1B-16, disease resistance protein RPP13-like, and MYB transcription factor (MYB12), which ultimately resulted in the accumulation of α-tomatine and anthocyanins via STR-2, UGT, and MYB12. Furthermore, ectopic expression of sly-miR1916/STR-2 significantly changed the tolerance of tobacco to B. cinerea. Taken together, the results demonstrated that sly-miR1916 might regulate the expression of STR-2, UGT, and MYB12 in tomato plant, conferring sensitivity to biotic stress via modulating α-tomatine and anthocyanins.

Expansion of PmBEAT genes in the Prunus mume genome induces characteristic floral scent production

Prunus mume is the only plant in the genus Prunus of the Rosaceae family with a characteristic floral scent, and the main component of this scent is benzyl acetate. By contrast, benzyl acetate is not synthesized in Prunus persica flowers. Here, we searched for benzyl alcohol acetyltransferase (BEAT) genes based on genomic data from P. mume and P. persica and found 44 unique PmBEATs in P. mume. These genes, which were mainly detected in clusters on chromosomes, originated from gene duplication events during the species evolution of P. mume, and retroduplication and tandem duplication were the two dominant duplication patterns. The genes PmBEAT34, PmBEAT36 and PmBEAT37, which were generated by tandem duplication, were highly expressed in flowers, and their highest levels were detected during the blooming stage. In vitro, PmBEAT34, PmBEAT3, and PmBEAT37 all had benzyl alcohol acetyltransferase activity that was localized in the cytoplasm. Overexpression of the PmBEAT36 or PmBEAT37 genes increased benzyl acetate production in the petal protoplasts of P. mume, and interference in the expression of these genes slightly decreased the benzyl acetate content. In addition, light and temperature regulated the expression of the PmBEAT34, PmBEAT36 and PmBEAT37 genes. According to these results, we hypothesize that the expansion of the PmBEAT genes in the genome induce the characteristic floral scent of P. mume.

A downy mildew effector evades recognition by polymorphism of expression and subcellular localization

Pathogen co-evolution with plants involves selection for evasion of host surveillance systems. The oomycete Hyaloperonospora arabidopsidis (Hpa) causes downy mildew on Arabidopsis, and race-specific interactions between Arabidopsis accessions and Hpa isolates fit the gene-for-gene model in which host resistance or susceptibility are determined by matching pairs of plant Resistance (R) genes and pathogen Avirulence (AVR) genes. Arabidopsis Col-0 carries R gene RPP4 that confers resistance to Hpa isolates Emoy2 and Emwa1, but its cognate recognized effector(s) were unknown. We report here the identification of the Emoy2 AVR effector gene recognized by RPP4 and show resistance-breaking isolates of Hpa on RPP4-containing Arabidopsis carry the alleles that either are not expressed, or show cytoplasmic instead of nuclear subcellular localization.

Plant pathogens have evolved to evade detection by their hosts. Here, Asai et al. show that virulent isolates of the oomycete Hyaloperonospora arabidopsidis can break resistance conferred by the Arabidopsis RPP4 resistance gene via variation in effector expression or subcellular localization.

Extremophiles as a Model of a Natural Ecosystem: Transcriptional Coordination of Genes Reveals Distinct Selective Responses of Plants Under Climate Change Scenarios

The goal of this research was to generate networks of co-expressed genes to explore the genomic responses of Rhizophora mangle L. populations to contrasting environments and to use gene network analysis to investigate their capacity for adaptation in the face of historical and future perturbations and climatic changes. RNA sequencing data were generated for R. mangle samples collected under field conditions from contrasting climate zones in the equatorial and subtropical regions of Brazil. A gene co-expression network was constructed using Pearson's correlation coefficient, showing correlations among 78,364 transcriptionally coordinated genes. Each region exhibited two distinct network profiles; genes correlated with the oxidative stress response showed higher relative expression levels in subtropical samples than in equatorial samples, whereas genes correlated with the hyperosmotic salinity response, heat response and UV response had higher expression levels in the equatorial samples than in the subtropical samples. In total, 992 clusters had enriched ontology terms, which suggests that R. mangle is under higher stress in the equatorial region than in the subtropical region. Increased heat may thus pose a substantial risk to species diversity at the center of its distribution range in the Americas. This study, which was performed using trees in natural field conditions, allowed us to associate the specific responses of genes previously described in controlled environments with their responses to the local habitat where the species occurs. The study reveals the effects of contrasting environments on gene expression in R. mangle, shedding light on the different abiotic variables that may contribute to the genetic divergence previously described for the species through the use of simple sequence repeats (SSRs). These effects may result from two fundamental processes in evolution, namely, phenotypic plasticity and natural selection.

Unraveling Field Crops Sensitivity to Heat Stress：Mechanisms, Approaches, and Future Prospects

The astonishing increase in temperature presents an alarming threat to crop production worldwide. As evident by huge yield decline in various crops, the escalating drastic impacts of heat stress (HS) are putting global food production as well as nutritional security at high risk. HS is a major abiotic stress that influences plant morphology, physiology, reproduction, and productivity worldwide. The physiological and molecular responses to HS are dynamic research areas, and molecular techniques are being adopted for producing heat tolerant crop plants. In this article, we reviewed recent findings, impacts, adoption, and tolerance at the cellular, organellar, and whole plant level and reported several approaches that are used to improve HS tolerance in crop plants. Omics approaches unravel various mechanisms underlying thermotolerance, which is imperative to understand the processes of molecular responses toward HS. Our review about physiological and molecular mechanisms may enlighten ways to develop thermo-tolerant cultivars and to produce crop plants that are agriculturally important in adverse climatic conditions.

The Antagonistic Action of Abscisic Acid and Cytokinin Signaling Mediates Drought Stress Response in Arabidopsis

As sessile organisms, plants encounter a variety of environmental stresses and must optimize their growth for survival. Abscisic acid (ABA) and cytokinin antagonistically regulate many developmental processes and environmental stress responses in plants. However, the molecular mechanism underlying this antagonism remains poorly defined. In this study, we demonstrated that Sucrose nonfermenting1-related kinases SnRK2.2, SnRK2.3, and SnRK2.6, the key kinases of the ABA signaling pathway, directly interact with and phosphorylate type-A response regulator 5 (ARR5), a negative regulator of cytokinin signaling. The phosphorylation of ARR5 Ser residues by SnRK2s enhanced ARR5 protein stability. Accordingly, plants overexpressing ARR5 showed ABA hypersensitivity and drought tolerance, and these phenotypes could not be recapitulated by overexpressing a non-phosphorylated ARR5 mimic. Moreover, the type-B ARRs, ARR1, ARR11 and ARR12, physically interacted with SnRK2s and repressed the kinase activity of SnRK2.6. The arr1,11,12 triple mutant exhibited hypersensitivity to ABA. Genetic analysis demonstrated that SnRK2s act upstream of ARR5 but downstream of ARR1, ARR11 and ARR12 in mediating ABA response and drought tolerance. Taken together, this study unravels the antagonistic actions of several molecular components of the ABA and cytokinin signaling pathways in mediates drought stress response, providing significant insights into how plants coordinate growth and drought stress response by integrating multiple hormone pathways.

The heat-shock protein/chaperone network and multiple stress resistance

Crop yield has been greatly enhanced during the last century. However, most elite cultivars are adapted to temperate climates and are not well suited to more stressful conditions. In the context of climate change, stress resistance is a major concern. To overcome these difficulties, scientists may help breeders by providing genetic markers associated with stress resistance. However, multistress resistance cannot be obtained from the simple addition of single stress resistance traits. In the field, stresses are unpredictable and several may occur at once. Consequently, the use of single stress resistance traits is often inadequate. Although it has been historically linked with the heat stress response, the heat-shock protein (HSP)/chaperone network is a major component of multiple stress responses. Among the HSP/chaperone 'client proteins', many are primary metabolism enzymes and signal transduction components with essential roles for the proper functioning of a cell. HSPs/chaperones are controlled by the action of diverse heat-shock factors, which are recruited under stress conditions. In this review, we give an overview of the regulation of the HSP/chaperone network with a focus on Arabidopsis thaliana. We illustrate the role of HSPs/chaperones in regulating diverse signalling pathways and discuss several basic principles that should be considered for engineering multiple stress resistance in crops through the HSP/chaperone network.

Plasma Membrane CRPK1-Mediated Phosphorylation of 14-3-3 Proteins Induces Their Nuclear Import to Fine-Tune CBF Signaling during Cold Response

In plant cells, changes in fluidity of the plasma membrane may serve as the primary sensor of cold stress; however, the precise mechanism and how the cell transduces and fine-tunes cold signals remain elusive. Here we show that the cold-activated plasma membrane protein cold-responsive protein kinase 1 (CRPK1) phosphorylates 14-3-3 proteins. The phosphorylated 14-3-3 proteins shuttle from the cytosol to the nucleus, where they interact with and destabilize the key cold-responsive C-repeat-binding factor (CBF) proteins. Consistent with this, the crpk1 and 14-3-3κλ mutants show enhanced freezing tolerance, and transgenic plants overexpressing 14-3-3λ show reduced freezing tolerance. Further study shows that CRPK1 is essential for the nuclear translocation of 14-3-3 proteins and for 14-3-3 function in freezing tolerance. Thus, our study reveals that the CRPK1-14-3-3 module transduces the cold signal from the plasma membrane to the nucleus to modulate CBF stability, which ensures a faithfully adjusted response to cold stress of plants.

Identification, transcriptional and functional analysis of heat-shock protein 90s in banana (Musa acuminata L.) highlight their novel role in melatonin-mediated plant response to Fusarium wilt

As one popular fresh fruit, banana (Musa acuminata) is cultivated in the world's subtropical and tropical areas. In recent years, pathogen Fusarium oxysporum f. sp. cubense (Foc) has been widely and rapidly spread to banana cultivated areas, causing substantial yield loss. However, the molecular mechanism of banana response to Foc remains unclear, and functional identification of disease-related genes is also very limited. In this study, nine 90 kDa heat-shock proteins (HSP90s) were genomewide identified. Moreover, the expression profile of them in different organs, developmental stages, and in response to abiotic and fungal pathogen Foc were systematically analyzed. Notably, we found that the transcripts of 9 MaHSP90s were commonly regulated by melatonin (N-acetyl-5-methoxytryptamine) and Foc infection. Further studies showed that exogenous application of melatonin improved banana resistance to Fusarium wilt, but the effect was lost when cotreated with HSP90 inhibitor (geldanamycin, GDA). Moreover, melatonin and GDA had opposite effect on auxin level in response to Foc4, while melatonin and GDA cotreated plants had no significant effect, suggesting the involvement of MaHSP90s in the cross talk of melatonin and auxin in response to fungal infection. Taken together, this study demonstrated that MaHSP90s are essential for melatonin-mediated plant response to Fusarium wilt, which extends our understanding the putative roles of MaHSP90s as well as melatonin in the biological control of banana Fusarium wilt.

Heat Shock Proteins: A Review of the Molecular Chaperones for Plant Immunity

As sessile organisms, plants are exposed to persistently changing stresses and have to be able to interpret and respond to them. The stresses, drought, salinity, chemicals, cold and hot temperatures, and various pathogen attacks have interconnected effects on plants, resulting in the disruption of protein homeostasis. Maintenance of proteins in their functional native conformations and preventing aggregation of non-native proteins are important for cell survival under stress. Heat shock proteins (HSPs) functioning as molecular chaperones are the key components responsible for protein folding, assembly, translocation, and degradation under stress conditions and in many normal cellular processes. Plants respond to pathogen invasion using two different innate immune responses mediated by pattern recognition receptors (PRRs) or resistance (R) proteins. HSPs play an indispensable role as molecular chaperones in the quality control of plasma membrane-resident PRRs and intracellular R proteins against potential invaders. Here, we specifically discuss the functional involvement of cytosolic and endoplasmic reticulum (ER) HSPs/chaperones in plant immunity to obtain an integrated understanding of the immune responses in plant cells.

IBR5 Modulates Temperature-Dependent, R Protein CHS3-Mediated Defense Responses in Arabidopsis

Plant responses to low temperature are tightly associated with defense responses. We previously characterized the chilling-sensitive mutant chs3-1 resulting from the activation of the Toll and interleukin 1 receptor-nucleotide binding-leucine-rich repeat (TIR-NB-LRR)-type resistance (R) protein harboring a C-terminal LIM (Lin-11, Isl-1 and Mec-3 domains) domain. Here we report the identification of a suppressor of chs3, ibr5-7 (indole-3-butyric acid response 5), which largely suppresses chilling-activated defense responses. IBR5 encodes a putative dual-specificity protein phosphatase. The accumulation of CHS3 protein at chilling temperatures is inhibited by the IBR5 mutation. Moreover, chs3-conferred defense phenotypes were synergistically suppressed by mutations in HSP90 and IBR5. Further analysis showed that IBR5, with holdase activity, physically associates with CHS3, HSP90 and SGT1b (Suppressor of the G2 allele of skp1) to form a complex that protects CHS3. In addition to the positive role of IBR5 in regulating CHS3, IBR5 is also involved in defense responses mediated by R genes, including SNC1 (Suppressor of npr1-1, Constitutive 1), RPS4 (Resistance to P. syringae 4) and RPM1 (Resistance to Pseudomonas syringae pv. maculicola 1). Thus, the results of the present study reveal a role for IBR5 in the regulation of multiple R protein-mediated defense responses.

Resistance (R) genes play central roles in recognizing pathogens and triggering plant defense responses. CHS3 encodes a TIR-NB-LRR-type R protein harboring a C-terminal LIM domain. A point mutation in CHS3 activates the defense response under chilling stress. Here we identified and characterized ibr5-7, a mutant that suppresses the chilling-induced defense responses of chs3-1. We observed that the enhanced defense responses and cell death in the chs3-1 mutant are synergistically dependent on IBR5 and HSP90. IBR5 physically interacts with CHS3, forming a complex with SGT1b/ HSP90. Moreover, IBR5 is also involved in the R-gene resistance mediated by SNC1, RPS4 and RPM1. Thus, IBR5 plays key roles in regulating defense responses mediated by multiple R proteins.

HSP90s are required for NLR immune receptor accumulation in Arabidopsis

Heat shock proteins (HSPs) serve as molecular chaperones for diverse client proteins in many biological processes. In plant immunity, cytosolic HSP90s participate in the assembly, stability control and/or activation of immune receptor complexes. In this paper we report that in addition to the well-established positive roles that HSP90 isoforms play in plant immunity, they are also involved in the negative regulation of immune receptor accumulation. Point mutations in two HSP90 genes, HSP90.2 and HSP90.3, were identified from a forward genetic screen designed to isolate mutants with enhanced disease resistance. We found that specific mutations in HSP90.2 and HSP90.3 lead to heightened accumulation of immune receptors, including SNC1, RPS2 and RPS4. HSP90s may assist SGT1 in the formation of SCF E3 ubiquitin ligase complexes that target immune receptors for degradation. Such regulation is critical for maintaining appropriate levels of immune receptor proteins to avoid autoimmunity.