Ethylene coordinates seed germination behavior in response to low soil pH in Stylosanthes humilis

Aims Stylosanthes humilis Stylosanthes humilis is known to exhibit high persistence in acid soils, however, how low soil pH controls seed germination as well as root and hypocotyl growth remains unknown. This study was carried out to evaluate the hormonal and metabolic alterations induced by low soil pH on seed germination behavior of S. humilis. Methods Seeds of S. humilis were sown in acid soil samples or sand soaked in buffer solution with pH ranging from 4.0 to 7.0. Concentrations of indole-3-acetic acid, ethylene, 1-aminocyclopropane-1-carboxylic acid (ACC), primary metabolite profile and final seed germination were evaluated after four days. Results Low soil pH led to increased final seed germination, concomitantly with higher root penetration into the soil as well as higher ACC and ethylene production by seedlings. Treatment with the ethylene biosynthesis inhibitor L-alpha-(2-aminoethoxyvinyl)-glycine (AVG) greatly reduced final seed germination under acidic conditions. Final seed germination of seeds treated with AVG was increased by exogenous ethylene application in a dose-dependent manner. Furthermore, low soil pH promoted distinct changes in IAA concentrations, and in carbon and nitrogen metabolism in hypocotyl and roots. Conclusions Low soil pH increases the final germination of S. humilis seeds through alterations in ethylene metabolism, allowing root penetration into the soil.