Prospecção e caracterização funcional de genes associados a estresse abiótico em mamona (Ricinus communis L.)

Abstract

Castor (Ricinus communis L.) is an oilseed species of great economic and industrial importance, widely recognized for the high commercial value of the oil extracted from its seeds, which is used in the production of medicines, cosmetics, and biofuels. In Brazil, the crop holds significant socioeconomic relevance, being traditionally cultivated by smallholder farmers in the semi-arid region of the Northeast, where rudimentary agricultural systems and adverse environmental conditions prevail. In these regions, plants are frequently exposed to abiotic stresses such as drought, salinity, and high temperatures. To survive these challenges and maintain cellular homeostasis, castor bean exhibits complex defense mechanisms involving antioxidant enzymes, such as superoxide dismutase (SOD), and regulatory and protective proteins, such as heat shock transcription factors (HSFs) and heat shock proteins (HSPs). The objective of this thesis was to elucidate the molecular and biochemical mechanisms associated with castor bean adaptation to abiotic stress conditions, through the functional characterization of the RcFeSOD8 gene and the genomic analysis of the HSF, HSP70, and HSP100 gene families. In Chapter 1 (published article), the RcFeSOD8 gene was characterized, and its physiological, biochemical, and metabolic roles were investigated through overexpression in Arabidopsis thaliana. RcFeSOD8 was strongly induced under heat stress in R. communis during seed germination and seedling growth. Arabidopsis lines overexpressing RcFeSOD8 exhibited improved germination under drought, salinity, and heat conditions, accompanied by increased antioxidant enzyme activity, higher chlorophyll content, and enhanced thermotolerance. Metabolomic analysis revealed differential accumulation of key amino acids (threonine, valine, isoleucine, sarcosine, and glycine) and sucrose in transgenic lines under stress, suggesting that RcFeSOD8 participates in the modulation of carbon and nitrogen metabolism, contributing to redox homeostasis and protecting amino acid biosynthetic pathways from oxidative damage. In Chapter 2 (submitted article), 18 RcHSF genes were identified, distributed across seven chromosomes of R. communis. These genes were classified into three major classes and ten subclasses, containing cis-regulatory elements associated with abiotic stress responses (ABRE, MeJARE, GARE, and SARE). The results showed that RcHSFs respond to both high and low temperatures during seed germination and early seedling development, as demonstrated by microarray analysis at the stages of imbibition, radicle protrusion (germination per se), and post-germination (young seedlings), as well as by qPCR validation of selected genes. In particular, RcHSF08 (Class A) showed strong induction under heat stress during the early seedling stage, whereas RcHSF10 (Class C) was upregulated during the initial developmental stages under both control and stress conditions compared with dry seeds. In Chapter 3 (article 9 in preparation), 14 members of the RcHSP70 gene family and five RcHSP100 genes were identified and characterized in R. communis. Based on sequence divergence, members of these families from five plant species were systematically grouped according to their predicted subcellular localization. Genomic analyses revealed that these genes harbor regulatory elements related to growth, development, hormonal signaling, and abiotic stress tolerance. Expression profiling indicated differential expression patterns of HSP70 and HSP100 genes across tissues and under various stress conditions (drought, salinity, and heat), confirming their participation in the castor bean stress response and adaptation mechanisms. Altogether, the results of this thesis contribute to advancing the understanding of the molecular and physiological mechanisms underlying abiotic stress tolerance in R. communis and highlight the potential of genes such as RcFeSOD8 and members of the RcHSF, RcHSP70, and RcHSP100 families as promising candidates for genetic improvement programs aimed at enhancing castor bean vigor and resilience under adverse environmental conditions—ultimately supporting greater productivity, particularly for smallholder farmers in the Brazilian semi-arid region.