Difference in root K+ retention ability and reduced sensitivity of K+-permeable channels to reactive oxygen species confer differential salt tolerance in three Brassica species

<em>Brassica</em> species are known to possess significant inter and intraspecies variability in salinity stress tolerance, but the cell-specific mechanisms conferring this difference remain elusive. In this work, the role and relative contribution of several key plasma membrane transporters to salinity stress tolerance were evaluated in three <em>Brassica</em> species (<em>B. napus</em> , <em>B. juncea</em> , and <em>B. oleracea</em>) using a range of electrophysiological assays. Initial root growth assay and viability staining revealed that <em>B. napus</em> was most tolerant amongst the three species, followed by <em>B. juncea</em> and <em>B. oleracea</em> . At the mechanistic level, this difference was conferred by at least three complementary physiological mechanisms: (i) higher Na⁺ extrusion ability from roots resulting from increased expression and activity of plasma membrane SOS1-like Na⁺/H⁺ exchangers; (ii) better root K⁺ retention ability resulting from stress-inducible activation of H⁺-ATPase and ability to maintain more negative membrane potential under saline conditions; and (iii) reduced sensitivity of <em>B. napus</em> root K⁺-permeable channels to reactive oxygen species (ROS). The last two mechanisms played the dominant role and conferred most of the differential salt sensitivity between species. <em>Brassica napus</em> plants were also more efficient in preventing the stress-induced increase in <em>GORK</em> transcript levels and up-regulation of expression of <em>AKT1</em> , <em>HAK5</em>, and <em>HKT1</em> transporter genes. Taken together, our data provide the mechanistic explanation for differential salt stress sensitivity amongst these species and shed light on transcriptional and post-translational regulation of key ion transport systems involved in the maintenance of the root plasma membrane potential and cytosolic K/Na ratio as a key attribute for salt tolerance in <em>Brassica</em> species.