Abstract
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.
Highlights
Aquaporins (AQPs) are integral or intrinsic channel proteins with molecular weights of 21 to 34 kD and a conserved structure
When the empty vector-transformed and PgTIP1-transformed soybean cotyledon hairy roots were exposed to 80 mM NaCl solution for 5 days, root growth was clearly inhibited in comparison with the non-treated ones, especially the empty vector-transformed cotyledon hair roots (Figure 1A-d)
The heterologous overexpression of PgTIP1 resulted in the transformed soybean cotyledon hairy roots or composite plants mediated by A. rhizogenes displaying superior salt tolerance to the empty vector-transformant according to the ameliorative roles in hairy root growth reduction, leaf relative water content (RWC) drop, and relative electrolytic leakage (REL) rise of composite plants under salt stress
Summary
Aquaporins (AQPs) are integral or intrinsic channel proteins with molecular weights of 21 to 34 kD and a conserved structure. They locate in the plasma and intracellular membranes of plant cells, where they facilitate the transport of water and/or a wide range of small neutral or uncharged solutes (glycerol, urea, boric acid, silicic acid, arsenite, ammonia, CO2, and H2O2, etc.) (Katsuhara et al, 2008; Maurel et al, 2008; Chaumont and Tyerman, 2014; Srivastava et al, 2014; Tian et al, 2016; Wang et al, 2016). Tian et al (2016) suggested a pivotal role for an Arabidopsis aquaporin (AtPIP1;4) in the importing of extracellular H2O2 into the cytoplasm for apo-cytoplastic signal transduction and in activating responses for disease immunity or resistance
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