Nodulin 26-like Intrinsic Protein NIP2;1 and NIP7;1: Characterization of Transport Functions and Roles in Developmental and Stress Responses in Arabidopsis

Nodulin-intrinsic proteins (NIPs) are plant-specific, water and solute transporters with homology to soybean nodulin 26. In this study, it is shown that Arabidopsis NIP2;1 (AtNIP2;1) expression is acutely stimulated upon waterlogging (70-fold in whole seedlings within 1 hr) and hypoxia (> 1000-fold in roots within 2 hr). Subcellular localization of a AtNIP2;1::YFP fusion protein shows localization to the plasma membrane. Analysis of AtNIP2;1 protein in Xenopus oocytes shows that it is a transporter of lactic acid, a fermentation end product.

Experiments with T-DNA insertional mutants in the AtNIP2;1 promoter showed that reduced AtNIP2;1 expression induced higher lactic acid accumulation in roots compared to wild type, both under normoxic and hypoxic conditions. Under normal growth conditions, atnip2;1 mutants grew normally but showed subtle changes in root morphology with increased numbers of lateral roots as well as increased primary root length and mass. Surprisingly, these T-DNA insertional mutants showed enhanced survival after severe hypoxia compared to wild type plants. Microarray analysis of a mutant (atnip2;1-1) and wild type roots showed that over 1300 transcripts were significantly upregulated in response to oxygen deprivation. Some genes were uniquely upregulated both under normoxia (54 genes) and hypoxia (14 genes) only in atnip2;1-1 roots.

Overall, the data suggest that AtNIP2;1 is anaerobic-induced gene that encodes a lactic acid transporter, and may play a role in adaptation to lactic fermentation under anaerobic stress. Experiments with a second Arabidopsis NIP, AtNIP7;1 revealed specific expression in flowers,especially in developing pollen grains. A T-DNA insertional mutant (atnip7;1-1) showed no apparent defects in flower development under normal growth conditions. However, the mutant showed defects in pollen tube growth in the absence of boric acid, a known transport substrate for NIPs. Overall, these observations suggest that AtNIP7;1 might be involved in boric acid uptake necessary for pollen development in Arabidopsis.