Involvement of auxin in the regulation of ammonium tolerance in rice (Oryza sativa L.)

Abstract

Ammonium (NH4+) is an important nitrogen source and is widely used as a fertilizer in agricultural systems. However, excess NH4+ inhibits root growth, and, subsequently, vegetative shoot growth and yield. This study examines whether auxin is involved in differential NH4+ tolerance in rice (Oryza sativa L.), and how auxin is regulated under high-NH4+ conditions in rice. An NH4+-sensitive (Kasalath, Kas) and an NH4+-insensitive (Koshihikari, Kos) rive cultivar were cultured hydroponically with or without exogenous indole-3-acetic acid (IAA) and auxin biosynthesis inhibitors. Root growth, root area, tissue IAA content, and transcription of genes involved in auxin biosynthesis, conjugation and degradation were determined. pDR5::GUS staining and auxin measurement show that high NH4+ can decrease free IAA content in roots. In addition, quantitative RT-PCR, pharmacology, and genetics analysis suggest that Kos possesses a higher capacity for auxin biosynthesis and a weaker capacity for auxin metabolism compared to Kas under high-NH4+ stress. We conclude that the NH4+-tolerant cultivar possesses a higher capacity to maintain auxin homeostasis under high-NH4+ stress, and that this advantage is incurred by promotion of auxin biosynthesis and a suppression of auxin metabolism.