A new split-root system was established through grafting to study cotton response to non-uniform salinity. Each root half was treated with either uniform (100/100 mM) or non-uniform NaCl concentrations (0/200 and 50/150 mM). In contrast to uniform control, non-uniform salinity treatment improved plant growth and water use, with more water absorbed from the non- and low salinity side. Non-uniform treatments decreased Na+ concentrations in leaves. The [Na+] in the ‘0’ side roots of the 0/200 treatment was significantly higher than that in either side of the 0/0 control, but greatly decreased when the ‘0’ side phloem was girdled, suggesting that the increased [Na+] in the ‘0’ side roots was possibly due to transportation of foliar Na+ to roots through phloem. Plants under non-uniform salinity extruded more Na+ from the root than those under uniform salinity. Root Na+ efflux in the low salinity side was greatly enhanced by the higher salinity side. NaCl-induced Na+ efflux and H+ influx were inhibited by amiloride and sodium orthovanadate, suggesting that root Na+ extrusion was probably due to active Na+/H+ antiport across the plasma membrane. Improved plant growth under non-uniform salinity was thus attributed to increased water use, reduced leaf Na+ concentration, transport of excessive foliar Na+ to the low salinity side, and enhanced Na+ efflux from the low salinity root.

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Leaf senescence varies greatly among genotypes of cotton (Gossypium hirsutium L), possibly due to the different expression of senescence-related genes. To determine genes involved in leaf senescence, we performed genome-wide transcriptional profiling of the main-stem leaves of an early- (K1) and a late-senescence (K2) cotton line at 110 day after planting (DAP) using the Solexa technology. The profiling analysis indicated that 1132 genes were up-regulated and 455 genes down-regulated in K1 compared with K2 at 110 DAP. The Solexa data were highly consistent with, and thus were validated by those from real-time quantitative PCR (RT-PCR). Most of the genes related to photosynthesis, anabolism of carbohydrates and other biomolecules were down-regulated, but those for catabolism of proteins, nucleic acids, lipids and nutrient recycling were mostly up-regulated in K1 compared with K2. Fifty-one differently expressed hormone-related genes were identified, of which 5 ethylene, 3 brassinosteroid (BR), 5 JA, 18 auxin, 8 GA and 1 ABA related genes were up-regulated in K1 compared with K2, indicating that these hormone-related genes might play crucial roles in early senescence of K1 leaves. Many differently expressed transcription factor (TF) genes were identified and 11 NAC and 8 WRKY TF genes were up-regulated in K1 compared with K2, suggesting that TF genes, especially NAC and WRKY genes were involved in early senescence of K1 leaves. Genotypic variation in leaf senescence was attributed to differently expressed genes, particularly hormone-related and TF genes.

Non-uniform root salinity increases the Na(+)efflux, water use, and growth of the root in non-saline side, which may be regulated by some form of signaling induced by the high-salinity side. However, the signaling and its specific function have remained unknown. Using a split-root system to simulate a non-uniform root zone salinity in Gossypium hirsutum L., we showed that the up-regulated expression of sodium efflux-related genes (SOS1, SOS2, PMA1, and PMA2) and water uptake-related genes (PIP1 and PIP2) was possibly involved in the elevated Na(+) efflux and water use in the the roots in the non-saline side. The increased level of indole acetic acid (IAA) in the non-saline side was the likely cause of the increased root growth. Also, the abscisic acid (ABA) and H2O2 contents in roots in the non-saline side increased, possibly due to the increased expression of their key biosynthesis genes, NCED and RBOHC, and the decreased expression of ABA catabolic CYP707A genes. Exogenous ABA added to the non-saline side induced H2O2 generation by up-regulating the RBOHC gene, but this was decreased by exogenous fluridone. Exogenous H2O2 added to the non-saline side reduced the ABA content by down-regulating NCED genes, which can be induced by diphenylene iodonium (DPI) treatment in the non-saline side, suggesting a feedback mechanism between ABA and H2O2.Both exogenous ABA and H2O2 enhanced the expression of SOS1, PIP1;7 ,PIP2;2, and PIP2;10 genes, but these were down-regulated by fluridone and DPI, suggesting that H2O2 and ABA are important signals for increasing root Na(+) efflux and water uptake in the roots in the non-saline side.

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