Molecular and physiological characterization of AIP1, encoding the acetolactate synthase regulatory subunit in rice.

Flooding deprives plants of oxygen and thereby causes severe stress by interfering with energy production, leading to growth retardation. Enzymes and metabolites may help protect plants from waterlogging and hypoxic environmental conditions. Acetolactate synthase (ALS) is a key enzyme in the biosynthesis of branched-chain amino acids (BCAAs), providing the building blocks for proteins and various secondary metabolites. Additionally, under energy-poor conditions, free BCAAs can be used as an alternative energy source by mitochondria through a catabolic enzyme chain reaction. In this study, we characterized ALS-INTERACTING PROTEIN 1 (OsAIP1), which encodes the regulatory subunit of ALS in rice (Oryza sativa). This gene was expressed in all parts of the rice plant, and its expression level was significantly higher in submerged and low-oxygen environments. Rice transformants overexpressing OsAIP1 showed a higher survival rate under hypoxic stress than did non-transgenic control plants under the same conditions. The OsAIP1-overexpressing plants accumulated increased levels of BCAAs, demonstrating that OsAIP1 is an important factor in the hypoxia resistance mechanism. These results suggest that ALS proteins are part of a defense mechanism that improves the tolerance of plants to low-oxygen environments.

AIP1, Encoding the Small Subunit of Acetolactate Synthase, Is Partially Responsible for Resistance to Hypoxic Stress in Arabidopsis thaliana.

Flooding is a significant stress to land plants, depriving them of essential oxygen. Plants have evolved diverse strategies with variable success to survive flooding. Similar strategies have been described in organisms from other kingdoms. Several fungal species can successfully survive a low-oxygen environment by increasing their branched-chain amino acid (BCAA) contents. BCAAs may act as alternative electron acceptors in the respiratory chain under an oxygen-limited environment. The key and first enzyme for BCAA biosynthesis is acetolactate synthase (ALS). We identified two homologous genes encoding the small subunit of ALS in Arabidopsis (Arabidopsis thaliana). We determined that ALS INTERACTING PROTEIN1 (AIP1), which encodes the small subunit of ALS, is strongly expressed in all organs and highly expressed under submergence and low-oxygen stresses. We also showed that the overexpression of AIP1 confers tolerance to low-oxygen stress. These results indicate that ALS may play an essential role under prolonged flooding or oxygen deficiency in Arabidopsis.