Histone deacetylases (HDACs) are important regulators of gene transcription thus controlling multiple cellular processes. Despite its essential role in plants, HDA6 is yet to be validated in common bean. In this study, we show that HDA6 is involved in plant development and stress response. Differential expression of HDA6 was determined in various tissues and the expression was seen to be upregulated with plant age (seedling < flowering < maturity). Higher expression was observed in flowers and pods than in stem, leaf, and root. Upregulation of HDA6 gene during cold stress implies its prominent role in abiotic stress. Furthermore, the HDA6 gene was isolated from three common bean genotypes and sequence analyses revealed homology with functionally characterized homologs in model species. The 53 kDa translated product was detected using an HDA6 specific antibody and recombinant protein overexpressed in Escherichia coli showed HDAC activity in vitro. To our knowledge, this is the first report in the agriculturally important crop common bean describing the functional characterization and biological role of HDA6.

We conducted a genome-wide transcriptomic analysis of three drought tolerant and sensitive genotypes of common bean to examine their transcriptional responses to terminal drought stress. We then conducted pairwise comparisons between the root and leaf transcriptomes from the resulting tissue based on combined transcriptomic data from the tolerant and sensitive genotypes. Our transcriptomic data revealed that 491 (6.4%) DEGs (differentially expressed genes) were upregulated in tolerant genotypes, whereas they were downregulated in sensitive genotypes; likewise, 396 (5.1%) DEGs upregulated in sensitive genotypes were downregulated in tolerant genotypes. Several transcription factors, heat shock proteins, and chaperones were identified in the study. Several DEGs in drought DB (data Base) overlapped between genotypes. The GO (gene ontology) terms for biological processes showed upregulation of DEGs in tolerant genotypes for sulfate and drug transmembrane transport when compared to sensitive genotypes. A GO term for cellular components enriched with upregulated DEGs for the apoplast in tolerant genotypes. These results substantiated the temporal pattern of root growth (elongation and initiation of root growth), and ABA-mediated drought response in tolerant genotypes. KEGG (kyoto encyclopedia of genes and genomes) analysis revealed an upregulation of MAPK (mitogen activated protein kinase) signaling pathways and plant hormone signaling pathways in tolerant genotypes. As a result of this study, it will be possible to uncover the molecular mechanisms of drought tolerance in response to terminal drought stress in the field. Further, genome-wide transcriptomic analysis of both tolerant and sensitive genotypes will assist us in identifying potential genes that may contribute to improving drought tolerance in the common bean.

Many environmental stresses can affect the accumulation of metabolites in plants, including drought. In the present study, we found a great deal of variability in the seed metabolic profiles of the tolerant (Matterhorn, SB-DT2 and SB-DT3) common bean genotypes in comparison to the sensitive genotypes (Sawtooth, Merlot and Stampede) using ultrahigh performance liquid chromatography−tandem mass spectrometry (UPLC-MS). The genotypes were grown in the field and subjected to drought stress after flowering (terminal drought stress). We aimed to investigate the accumulation of genotype-specific metabolites and related pathways under terminal drought stress by comparing tolerant and sensitive genotypes within a race. A total of 26 potential metabolites were identified across genotype comparisons. Significant metabolic pathways, including monobactam biosynthesis, flavone and flavonol biosynthesis, pentose phosphate pathway, C5-branched dibasic acid metabolism, cysteine and methionine metabolism, vitamin B6 metabolism and flavonoid biosynthesis, were derived from the enriched metabolites. Many of these metabolic pathways were specific and varied with genotype comparisons. SB-DT2 vs. stampede revealed more significant metabolites and metabolic pathways compared to Matterhorn vs. Sawtooth and SB-DT3 vs. Merlot under terminal drought stress. Our study provides useful information regarding the metabolite profiles of seeds and their related pathways in comparisons of tolerant and sensitive common bean genotypes under terminal drought conditions. Further research, including transcriptomic and proteomic analyses, may contribute to a better understanding of molecular mechanisms and nutritional differences among seeds of common bean genotypes grown under terminal drought conditions.

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