Involvement of brassinosteroids in mediating phosphorus acquisition and utilization of rice

Brassinosteroids (BRs) are a novel class of plant hormones that play important roles in regulating plant growth and development, as well as in responding to biotic and abiotic stresses. However, little is known whether and how BRs mediate phosphorus (P) acquisition and utilization in rice. This study investigated the question. Both hydroponics and field experiments were conducted in 2019-2024 by using rice varieties either with strong tolerance to low P (SVs) or with weak tolerance to low P (WVs). The results showed that the SVs had higher levels of BRs including 24-epibrassinolide (24-EBL) and 28-homobrassinolide (28-HBL) in both roots and leaves than WVs at each growth stage and under a low P (LP) condition. Levels of 24-EBL and 28-HBL were very significantly and positively correlated with the parameters reflecting P acquisition and utilization, such as P content, activities of acid phosphatase and proton-pumping adenosine triphosphatase, and P remobilization, leading to more P accumulation and higher P harvest index, internal P use efficiency, and grain yield for SVs. In contrast, levels of other phytohormones including cytokinins (zeatin+zeatin riboside), indole-3-acetic acid, gibberellic acids (GA1+GA4), abscisic acid, jasmonic acid, and ethylene were neither markedly different between SVs and WVs nor significantly correlated with the parameters reflecting P acquisition and utilization under LP. Applying 24-EBL prominently increased BRs levels in plants, improved the parameters reflecting P acquisition and utilization, up-regulated expression of the genes involved in P uptake and transport, and increased P remobilization, internal P use efficiency, and grain yield, whereas applying brassinazole, an inhibitor of BRs synthesis, exhibited opposite effects. These findings shed light on the role and mechanism of BRs in mediating P acquisition and utilization, and provide a strategy for synergistically improving grain yield and P use efficiency through increasing BRs levels in the plants in rice breeding and crop management.