Background The anionic toxicity of plants under salt stress is mainly caused by chloride (Cl − ). Thus Cl − influx, transport and their regulatory mechanisms should be one of the most important aspects of plant salt tolerance studies, but are often sidelined by the focus on sodium (Na + ) toxicity and its associated adaptations. Plant chloride channels (CLCs) are transport proteins for anions including Cl − and nitrate (NO 3 − ), and are critical for nutrition uptake and transport, adjustment of cellular turgor, stomatal movement, signal transduction, and Cl − and NO 3 − homeostasis under salt stress. Results Among the eight soybean CLC genes, the tonoplast-localized c2 has uniquely different transcriptional patterns between cultivated soybean N23674 and wild soybean BB52. Using soybean hairy root transformation, we found that GsCLC-c2 over-expression contributed to Cl − and NO 3 − homeostasis, and therefore conferred salt tolerance, through increasing the accumulation of Cl − in the roots, thereby reducing their transportation to the shoots where most of the cellular damages occur. Also, by keeping relatively high levels of NO 3 − in the aerial part of the plant, GsCLC-c2 could reduce the Cl − /NO 3 − ratio. Wild type GsCLC-c2, but not its mutants ( S184P , E227V and E294G ) with mutations in the conserved domains, is able to complement Saccharomyces cerevisiae △gef1 Cl − sensitive phenotype. Using two-electrode voltage clamp on Xenopus laevis oocytes injected with GsCLC-c2 cRNA, we found that GsCLC-c2 transports both Cl − and NO 3 − with slightly different affinity, and the affinity toward Cl − was pH-independent. Conclusion This study revealed that the expression of GsCLC-c2 is induced by NaCl-stress in the root of wild soybean. The tonoplast localized GsCLC-c2 transports Cl − with a higher affinity than NO 3 − in a pH-independent fashion. GsCLC-c2 probably alleviates salt stress in planta through the sequestration of excess Cl − into the vacuoles of root cells and thus preventing Cl ...
The salt overly sensitive 1 (SOS1) gene encodes the plasma membrane Na+/H+ antiporter, SOS1, that is mainly responsible for extruding Na+ from the cytoplasm and reducing the Na+ content in plants under salt stress and is considered a vital determinant in conferring salt tolerance to the plant. However, studies on the salt tolerance function of the TrSOS1 gene of recretohalophytes, such as Tamarix, are limited. In this work, the effects of salt stress on cotton seedlings transformed with tobacco-rattle-virus-based virus-induced gene silencing (VIGS) of the endogenous GhSOS1 gene, or Agrobacterium rhizogenes strain K599-mediated TrSOS1-transgenic hairy root composite cotton plants exhibiting VIGS of GhSOS1 were first investigated. Then, with Arabidopsis thaliana AtSOS1 as a reference, differences in the complementation effect of TrSOS1 or GhSOS1 in a yeast mutant were compared under salt treatment. Results showed that compared to empty-vector-transformed plants, GhSOS1-VIGS-transformed cotton plants were more sensitive to salt stress and had reduced growth, insufficient root vigor, and increased Na+ content and Na+/K+ ratio in roots, stems, and leaves. Overexpression of TrSOS1 enhanced the salt tolerance of hairy root composite cotton seedlings exhibiting GhSOS1-VIGS by maintaining higher root vigor and leaf relative water content (RWC), and lower Na+ content and Na+/K+ ratio in roots, stems, and leaves. Transformations of TrSOS1, GhSOS1, or AtSOS1 into yeast NHA1 (Na+/H+ antiporter 1) mutant reduced cellular Na+ content and Na+/K+ ratio, increased K+ level under salt stress, and had good growth complementation in saline conditions. In particular, the ability of TrSOS1 or GhSOS1 to complement the yeast mutant was better than that of AtSOS1. This may indicate that TrSOS1 is an effective substitute and confers enhanced salt tolerance to transgenic hairy root composite cotton seedlings, and even the SOS1 gene from salt-tolerant Tamarix or cotton may have higher efficiency than salt-sensitive Arabidopsis in regulating Na+ efflux, maintaining Na+ and K+ homeostasis, and therefore contributing to stronger salt tolerance.
The Panax ginseng TIP gene PgTIP1 was previously demonstrated to have high water channel activity by its heterologous expression in Xenopus laevis oocytes and in yeast; it also plays a significant role in growth of PgTIP1-transgenic Arabidopsis plants under favorable conditions and has enhanced tolerance toward salt and drought treatment. In this work, we first investigated the physiological effects of heterologous PgTIP1 expression in soybean cotyledon hairy roots or composite plants mediated by Agrobacterium rhizogenes toward enhanced salt tolerance. The PgTIP1-transgenic soybean plants mediated by the pollen tube pathway, represented by the lines N and J11, were analyzed at the physiological and molecular levels for enhanced salt tolerance. The results showed that in terms of root-specific heterologous expression, the PgTIP1-transformed soybean cotyledon hairy roots or composite plants displayed superior salt tolerance compared to the empty vector-transformed ones according to the mitigatory effects of hairy root growth reduction, drop in leaf RWC, and rise in REL under salt stress. Additionally, declines in K+ content, increases in Na+ content and Na+/K+ ratios in the hairy roots, stems, or leaves were effectively alleviated by PgTIP1-transformation, particularly the stems and leaves of composite soybean plants. At the whole plant level, PgTIP1-trasgenic soybean lines were found to possess stronger root vigor, reduced root and leaf cell membrane damage, increased SOD, POD, CAT, and APX activities, steadily increased leaf Tr, RWC, and Pn values, and smaller declines in chlorophyll and carotenoid content when exposed to salt stress compared to wild type. Moreover, the distribution patterns of Na+, K+, and Cl- in the roots, stems, and leaves of salt-stressed transgenic plants were readjusted, in that the absorbed Na+ and Cl- were mainly restricted to the roots to reduce their transport to the shoots, and the transport of root-absorbed K+ to the shoots was simultaneously promoted. PgTIP1 transformation into soybean plants enhanced the expression of some stress-related genes (GmPOD, GmAPX1, GmSOS1, and GmCLC1) in the roots and leaves under salt treatment. This indicates that the causes of enhanced salt tolerance of heterologous PgTIP1-transformed soybean are associated with the positive regulation on water relations, ion homeostasis, and ROS scavenging under salt stress both at root-specific and whole plant levels.
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