Rice is one of the most susceptible plant to iron (Fe) deficiency under neutral and alkaline conditions. Alkaline stress induces H2O2 production and increases the deposition of Fe on roots surface, which causes leaf chlorosis and Fe deficiency in rice. Gene chip and qRT-PCR analysis indicated that the expression of nitrate reductase (NR) genes were down-regulated by alkaline treatment, which resulted in significantly decreased nitrate activities and nitric oxide (NO) production in epidermis and stele, where the H2O2 was accumulated. In contrast, treatment with sodium nitroprusside (SNP), a NO donor, strongly alleviates alkaline-induced Fe deficiency by limiting Fe plaque formation. Increasing the NO signal significantly reduces the accumulation of H2O2 and lignin barrier, but enhances phenolic acid secretion in root epidermis and stele under alkaline stress. The secreted phenolic acid effectively mobilized the apoplast Fe and increased Fe uptake in root, thus which alleviate the Fe deficiency response and down-regulate expression of Fe uptake genes under alkaline condition. In conclusion, alkaline stress inhibits the NR activity and NO production in roots of rice, which plays a vital role in mobilizing the apoplast Fe by regulation of H2O2 and phenolic acids concentrations.

The concentration and homeostasis of intracellular phosphate (Pi) are crucial for sustaining cell metabolism and growth. During short-term Pi starvation, intracellular Pi is maintained relatively constant at the expense of vacuolar Pi. After the vacuolar stored Pi is exhausted, the plant cells induce the synthesis of intracellular acid phosphatase (APase) to recycle Pi from expendable organic phosphate (Po). In this study, the expression, enzymatic activity and subcellular localization of ACID PHOSPHATASE 1 (OsACP1) were determined. OsACP1 expression is specifically induced in almost all cell types of leaves and roots under Pi stress conditions. OsACP1 encodes an acid phosphatase with broad Po substrates and localizes in the endoplasmic reticulum (ER) and Golgi apparatus (GA). Phylogenic analysis demonstrates that OsACP1 has a similar structure with human acid phosphatase PHOSPHO1. Overexpression or mutation of OsACP1 affected Po degradation and utilization, which further influenced plant growth and productivity under both Pi-sufficient and Pi-deficient conditions. Moreover, overexpression of OsACP1 significantly affected intracellular Pi homeostasis and Pi starvation signalling. We concluded that OsACP1 is an active acid phosphatase that regulates rice growth under Pi stress conditions by recycling Pi from Po in the ER and GA.