Examination of aquaporin (AQP) membrane channels in extremophile plants may increase our understanding of plant tolerance to high salt, drought or other conditions. Here, we cloned a tonoplast AQP gene (TsTIP1;2) from the halophyte Thellungiella salsuginea and characterized its biological functions. TsTIP1;2 transcripts accumulate to high levels in several organs, increasing in response to multiple external stimuli. Ectopic overexpression of TsTIP1;2 in Arabidopsis significantly increased plant tolerance to drought, salt and oxidative stresses. TsTIP1;2 had water channel activity when expressed in Xenopus oocytes. TsTIP1;2 was also able to conduct H₂O₂ molecules into yeast cells in response to oxidative stress. TsTIP1;2 was not permeable to Na⁺ in Xenopus oocytes, but it could facilitate the entry of Na⁺ ions into plant cell vacuoles by an indirect process under high-salinity conditions. Collectively, these data showed that TsTIP1;2 could mediate the conduction of both H₂O and H₂O₂ across membranes, and may act as a multifunctional contributor to survival of T. salsuginea in highly stressful habitats.

SLC26 proteins function as anion exchangers, channels, and sensors. Previous cellular studies have shown that Slc26a3 and Slc26a6 interact with the R-region of the cystic fibrosis transmembrane conductance regulator (CFTR), (R)CFTR, via the Slc26-STAS (sulfate transporter anti-sigma) domain, resulting in mutual transport activation. We recently showed that Slc26a9 has both nCl⁻-HCO₃⁻ exchanger and Cl⁻ channel function. In this study, we show that the purified STAS domain of Slc26a9 (a9STAS) binds purified (R)CFTR. When Slc26a9 and (R)CFTR fragments are co-expressed in Xenopus oocytes, both Slc26a9-mediated nCl⁻-HCO₃⁻ exchange and Cl⁻ currents are almost fully inhibited. Deletion of the Slc26a9 STAS domain (a9-ΔSTAS) virtually eliminated the Cl⁻ currents with only a modest affect on nCl⁻-HCO₃⁻ exchange activity. Co-expression of a9-ΔSTAS and the (R)CFTR fragment did not alter the residual a9-ΔSTAS function. Replacing the Slc26a9 STAS domain with the Slc26a6 STAS domain (a6-a9-a6) does not change Slc26a9 function and is no longer inhibited by (R)CFTR. These data indicate that the Slc26a9-STAS domain, like other Slc26-STAS domains, binds CFTR in the R-region. However, unlike previously reported data, this binding interaction inhibits Slc26a9 ion transport activity. These results imply that Slc26-STAS domains may all interact with (R)CFTR but that the physiological outcome is specific to differing Slc26 proteins, allowing for dynamic and acute fine tuning of ion transport for various epithelia.

Plant non-symbiotic hemoglobins (nsHbs) play important roles in a variety of cellular processes. Previous evidence from this laboratory indicates that the expression of a class 1 nsHb gene ( GhHb1 ) from cotton is induced in cotton roots challenged with the Verticillium wilt fungus. The present study examined further the expression patterns of the GhHb1 gene in cotton plants and characterized its in vivo function through ectopic overexpression of the gene in Arabidopsis thaliana . Expression of GhHb1 in cotton plants was induced by exogenously applied salicylic acid, methyl jasmonic acid, ethylene, hydrogen peroxide (H ₂ O ₂ ) and nitric oxide (NO). Ectopic overproduction of GhHb1 in Arabidopsis led to constitutive expression of the defense genes PR-1 and PDF1.2 , and conferred enhanced disease resistance to Pseudomonas syringae and tolerance to V . dahliae . GhHb1 -transgenic Arabidopsis seedlings were more tolerant to exogenous NO and contained lower levels of cellular NO than the wild-type control. Moreover, transgenic plants with relatively high levels of expression of the GhHb1 gene developed spontaneous hypersensitive lesions on the leaves in the absence of pathogen inoculation. Our results indicate that GhHb1 proteins play a role in the defense responses against pathogen invasions, possibly by modulating the NO level and the ratio of H ₂ O ₂ /NO in the defense process.