Nitrogen (N) and phosphorus (P) are essential macronutrients required for plant growth and development. Under N and inorganic phosphate (Pi) deficiencies, plants undergo adaptation at physiological, morphological, and transcriptional levels via modulation of endogenous signals, such as phytohormones, in an attempt to increase nutrient acquisition from the environment. Biosynthesis of brassinosteroid (BR), a class of plant hormones, has been shown to be crucial in nutrient deficiency responses in the plant model Arabidopsis. In this study, the responses of rice (Oryza sativa L.) to N and Pi deficiencies were investigated using rice seedlings grown in the presence and absence of a BR biosynthesis inhibitor, propiconazole (PPZ). Transcript levels of BR biosynthesis genes were induced by N and Pi deficiencies. PPZ-treated plants showed retarded growth in both sufficient and deficient conditions. Besides, gene expression of N-and Pi-deficiency-responsive genes was also attenuated by PPZ treatments. These results suggest that inhibition of BR biosynthesis by PPZ could restrain plant growth and adaptation in response to both N and Pi deficiencies.

Root growth depends on cell proliferation and cell elongation at the root meristem, which are controlled by plant hormones and nutrient availability. As a foraging strategy, rice (Oryza sativa L.) grows longer roots when nitrogen (N) is scarce. However, how the plant steroid hormone brassinosteroid (BR) regulates rice root meristem development and responses to N deficiency remains unclear. Here, we show that BR has a negative effect on meristem size and a dose-dependent effect on cell elongation in roots of rice seedlings treated with exogenous BR (24-epicastasterone, ECS) and the BR biosynthesis inhibitor propiconazole (PPZ). A genome-wide transcriptome analysis identified 4110 and 3076 differentially expressed genes in response to ECS and PPZ treatments, respectively. The gene ontology (GO) analysis shows that terms related to cell proliferation and cell elongation were enriched among the ECS-repressed genes. Furthermore, microscopic analysis of ECS- and PPZ-treated roots grown under N-sufficient and N-deficient conditions demonstrates that exogenous BR or PPZ application could not enhance N deficiency-mediated root elongation promotion as the treatments could not promote root meristem size and cell elongation simultaneously. Our study demonstrates that optimal levels of BR in the rice root meristem are crucial for optimal root growth and the foraging response to N deficiency.