Abstract
Under anaerobic stress, Arabidopsis thaliana induces the expression of a collection of core hypoxia genes that encode proteins for an adaptive response. Among these genes is NIP2;1, which encodes a member of the "Nodulin 26-like Intrinsic Protein" (NIP) subgroup of the aquaporin superfamily of membrane channel proteins. NIP2;1 expression is limited to the "anoxia core" region of the root stele under normal growth conditions, but shows substantial induction (up to 1,000-fold by 2-4 h of hypoxia) by low oxygen stress, and accumulation in all root tissues. During hypoxia, NIP2;1-GFP accumulates predominantly on the plasma membrane by 2 h, is distributed between the plasma and internal membranes during sustained hypoxia, and remains elevated in root tissues through 4 h of reoxygenation recovery. In response to hypoxia challenge, T-DNA insertion mutant nip2;1 plants exhibit elevated lactic acid within root tissues, reduced efflux of lactic acid, and reduced acidification of the external medium compared to wild-type plants. Previous biochemical evidence demonstrates that NIP2;1 has lactic acid channel activity, and our work supports the hypothesis that NIP2;1 prevents lactic acid toxicity by facilitating release of cellular lactic acid from the cytosol to the apoplast, supporting eventual efflux to the rhizosphere. In evidence, nip2;1 plants demonstrate poorer survival during argon-induced hypoxia stress. Expressions of the ethanolic fermentation transcript Alcohol Dehydrogenase1 and the core hypoxia-induced transcript Alanine Aminotransferase1 are elevated in nip2;1, and expression of the Glycolate Oxidase3 transcript is reduced, suggesting NIP2;1 lactic acid efflux regulates other pyruvate and lactate metabolism pathways.