GhCAX3 Gene, a Novel Ca2+/H+ Exchanger from Cotton, Confers Regulation of Cold Response and ABA Induced Signal Transduction

As a second messenger, Ca(2+) plays a major role in cold induced transduction via stimulus-specific increases in [Ca(2+)]cyt, which is called calcium signature. During this process, CAXs (Ca(2+)/H(+) exchangers) play critical role. For the first time, a putative Ca(2+)/H(+) exchanger GhCAX3 gene from upland cotton (Gossypium hirsutum cv. 'YZ-1') was isolated and characterized. It was highly expressed in all tissues of cotton except roots and fibers. This gene may act as a regulator in cotton's response to abiotic stresses as it could be up-regulated by Ca(2+), NaCl, ABA and cold stress. Similar to other CAXs, it was proved that GhCAX3 also had Ca(2+) transport activity and the N-terminal regulatory region (NRR) through yeast complementation assay. Over-expression of GhCAX3 in tobacco showed less sensitivity to ABA during seed germination and seedling stages, and the phenotypic difference between wild type (WT) and transgenic plants was more significant when the NRR was truncated. Furthermore, GhCAX3 conferred cold tolerance in yeast as well as in tobacco seedlings based on physiological and molecular studies. However, transgenic plant seeds showed more sensitivity to cold stress compared to WT during seed germination, especially when expressed in N-terminal truncated version. Finally, the extent of sensitivity in transgenic lines was more severe than that in WT line under sodium tungstate treatment (an ABA repressor), indicating that ABA could alleviate cold sensitivity of GhCAX3 seeds, especially in short of its NRR. Meanwhile, we also found that overexpression of GhCAX3 could enhance some cold and ABA responsive marker genes. Taken together, these results suggested that GhCAX3 plays important roles in the cross-talk of ABA and cold signal transduction, and compared to full-length of GhCAX3, the absence of NRR could enhance the tolerance or sensitivity to cold stress, depending on seedling's developmental stages.