Sequence analysis of Ricinus communis small heat-shock protein (sHSP) subfamily and its role in abiotic stress responses

Small heat shock proteins (sHSPs) possess major roles in plant defense mechanisms towards abiotic stresses. sHSPs act as molecular chaperones providing the necessary tools to sustain cellular homeostasis under adverse conditions. sHSP genes display specific expression signatures, which depend on tissue-specificity, developmental stage and the nature of the abiotic stress. Despite the fact that Ricinus communis is an important oilseed crop with large socioeconomic impact on small family farmers in semi-arid regions worldwide, the characterization of RcsHSP genes and their possible contribution to plant survival under harsh environmental conditions has not been addressed. Hence, this study aimed at characterizing the R. communis sHSP subfamily, through phylogeny, gene structure, duplication, and expression profile analysis, as well as by characterizing Arabidopsis thaliana seeds overexpressing RcsHSP genes. We identified 41 RcsHSP genes with the a-crystallin domain and compatible molecular weight (< 43 kDa). The RcsHSP subfamily showed different homology levels with sHSP genes from other plant species, suggesting the occurrence of specific gene expansion and loss. The RcsHSP subfamily was classified according to the cellular locations of the genes, which included cytosolic, chloroplastic, mitochondrial, and endoplasmic reticulum groups. Ten putative motifs were found among RcsHSP genes, but only motifs 4, 6 and 8 were sHSP protein domains. The RcsHSP subfamily showed 19 genes produced by tandem duplication events, which might have been crucial for RcsHSP diversification and acquisition of tolerance in R. communis. Gene expression analysis showed that the RcsHSP subfamily possesses different regulatory mechanisms in response to various abiotic stresses. Additionally, overexpression of RcsHSP genes in A. thaliana was followed by enhanced SOD activity and higher content of osmoprotectants, which ultimately led to enhanced seed germination under a variety of abiotic stresses. Our results may contribute to breeding programs aiming at developing high tolerant R. communis plants, providing economic and social support for farmers in semiarid areas worldwide.