New insights into the mechanism underlying Ricinus communis L. tolerance to drought stress during germination

Ricinus communis L is an important oilseed crop species which is mostly cultivated in Brazil by family farmers in the northeastern semiarid region. This species is renowned by the quality and properties of the oil extracted from its seeds, which can be used for a diverse range of industrial applications. Furthermore, this species is considered to be tolerant to abiotic stresses such as drought stress. The use of post-harvest and pre-germination treatments on mature seeds can enhance their viability and vigor. In this context, the objective of this study was to investigate the effect of osmoconditioning on R. communis L. seed germinability parameters and events related to the reactivation of cell cycle, i.e. tubulin accumulation and microtubular cytoskeleton configurations. Osmoconditioning was applied to both fresh-harvested (FH) and dry seeds (DS), originating fresh-harvested osmoconditioned (FHO) and dry osmoconditioned (DO) seeds. In general, osmoconditioning promoted seed priming by enhancing seed germinability of FHO and DO seeds by means of better germination speed and uniformity, except for a decrease of uniformity in FHO seeds. Immunocytochemical and immunohistochemical analysis revealed an increasing accumulation of tubulin in FHO and DO seeds, followed by the accumulation of tubulin granules in DO seeds. Mitotic microtubules were only visible in FHO and DO seeds after radicle protrusion and, therefore, seed germination occurred solely by cell elongation, whereas FH and DS seeds required cell cycle reactivation and cell proliferation during imbibition, i.e. prior to radicle protrusion or germination per se. The prompt activation of microtubular cytoskeleton synthesis in seeds that were osmoconditioned might be a possible strategy of the species for faster and more uniform and vigorous seed germination and seedling establishment and, therefore, better survival during periods of drought in harsh environments. These findings provide important insights into the mechanism underlying plant adaptation to harsh environmental conditions, which would be very helpful to develop germination and seedling production protocols to obtain faster and more vigorous and uniform germination seedling stand for the sustainable exploitation of R. communis by small family farmers under the typical harsh conditions of the semiarid regions worldwide. (C) 2017 Elsevier B.V. All rights reserved.