Effect of Exogenous Hormones on the Growth of Alfalfa Seedlings under Phosphate Starvation Stress

Background: Alfalfa (Medicago sativa) is a high-quality legume forage. Phosphorus (P) is essential for plant productivity. Approximately half of global arable soils exhibit available-P deficiency, constraining plants growth and yield. This study aimed to explore how hormones mediate the responses of alfalfa to phosphate starvation. Methods: We studied the morphological and physiological changes alfalfa (Zhongmu No.3) seedlings treated with auxin, ethylene and gibberellin under conditions of phosphate deficiency. 20-day-old alfalfa seedlings were exposed to varying concentrations of hormones in Hoagland solution. Changes in root morphology, photosynthesis, physiological characters and the levels of phosphate starvation induced (PSI) genes expression were monitored. Result: IAA stimulated lateral-root growth (1.16-fold vs. control), up-regulated the organic phosphate transporter (PHT1-4) and purple acid phosphatase (PAP23) and inhibited primary-root elongation. Ethylene promoted the growth of root hairs and 10 μM aminocyclopropane-1-carboxylic acid (ACC) increased the elongation of roots and their surface area.1 μM ACC boosted shoot and root acid phosphatase activity and up-regulated 1-aminocyclopropane-1-carboxylate oxidase (ACO), whereas 5 μM gibberellin A3 (GA3) promoted stem and leaf growth, increasing stem diameter by 16.10% and leaf area by 36.90%, while elevating sulfoquinovosyl diacylglycerol (SQD2) content and DELLA expression. There was a strong negative correlation (-0.79) between the plant height and root-to-shoot ratio and a positive correlation (0.62) between the number of lateral roots and the activity of acid phosphatase (ACP) in the roots. The most effective concentrations of hormones was 1 μM ACC, which enhanced the plant height, stem diameter, lateral root formation, chlorophyll biosynthesis and nitrogen content and induced the levels of expression of PSI and key genes in hormone signaling.