Salt stress inhibits germination of Stylosanthes humilis seeds through abscisic acid accumulation and associated changes in ethylene production

In Stylosanthes humilis, salt stress tolerance is associated with ethylene production by the seeds, however, how salt stress controls seed germination and ethylene production is poorly understood. Here, we studied the hormonal and metabolic changes triggered by salt stress on germination of S. humilis seeds. Salt stress led to decreased seed germination and ethylene production, concomitantly with higher abscisic acid (ABA) production by seeds. Treatment with NaCl and ABA promoted distinct changes in energy metabolism, allowing seeds to adapt to salt stress conditions. Treatment with the ABA biosynthesis inhibitor fluridone or ethylene precursor 1-aminocyclopropane-1-carboxylic acid (ACC) reversed the effects of salt stress on seed germination and ethylene production. Moreover, ethylene concentration was decreased by increasing the pH of the salt solution. High pH, however, did not influence concentration of ABA in seeds under salt stress. We conclude that biosynthesis of ABA and ethylene in response to salt stress constitutes a point of convergence that provides flexibility to regulate energy metabolism and embryo growth potential of S. humilis seeds within a given pH condition.