Under anaerobic stress Arabidopsis thaliana induces the expression of a collection of core hypoxia genes that encode proteins associated with an adaptive response. Included in these core hypoxia genes is NIP2;1, which encodes a member of the “Nodulin-like Intrinsic Protein” (NIP) subgroup of the aquaporin superfamily of membrane channel proteins. Under normal growth, NIP2;1 expression is limited to the “anoxia core” region of the root stele, but shows substantial induction in response to low oxygen stress (as high as 1000-fold by 2-4 hr of hypoxia challenge), and accumulates in all root tissues. During hypoxia, NIP2;1-GFP, accumulates on the cell surface by 2 hr and then is distributed between the cell surface and internal membranes during sustained hypoxia, and remains elevated in root tissues through 4 hrs of reoxygenation recovery. T-DNA insertional mutant nip2;1 plants show elevation of lactic acid within root tissues, and a reduced efflux of lactic acid and acidification of the external medium. Together with previous biochemical evidence demonstrating that NIP2;1 has lactic acid permease activity, the present work supports the hypothesis that the protein facilitates the release of cellular lactate to the rhizosphere to prevent lactic acid toxicity. In support of this, nip2;1 plants show poorer survival to argon-induced hypoxia stress. Nip2;1 mutant plants also show elevated expression of ethanolic fermentation transcripts, as well as reduced expression the lactate metabolic enzyme GOX3, suggesting that the altered efflux of lactate through NIP2;1 regulates other pyruvate and lactate metabolism pathways.
