Transcriptomic and functional characterization reveals CsHAK5;3 as a key player in K plus homeostasis in grafted cucumbers under saline conditions

Grafting can improve the salt tolerance of many crops. However, critical genes in scions responsive to rootstock under salt stress remain a mystery. We found that pumpkin rootstock decreased the content of Na+ by 70.24 %, increased the content of K+ by 25.9 %, and increased the K+/Na+ ratio by 366.0 % in cucumber scion leaves. RNA-seq analysis showed that ion transport-related genes were the key genes involved in salt stress tolerance in grafted cucumber. The identification and analysis of the expression of K+ transporter proteins in cucumber and pumpkin revealed six and five HAK5 members, respectively. The expression of CsHAK5;3 in cucumber was elevated in different graft combinations under salt stress and most notably in cucumber scion/pumpkin root-stock. CsHAK5;3 was localized to the plasma membrane, and a yeast complementation assay revealed that it can transport K+. CsHAK5;3 knockout in hairy root mutants decreased the K+ content of leaves (45.6 %) and roots (50.3 %), increased the Na+ content of leaves (29.3 %) and roots (34.8 %), and decreased the K+/Na+ ratio of the leaves (57.9 %) and roots (62.9 %) in cucumber. However, CsHAK5;3 overexpression in hairy roots increased the K+ content of the leaves (31.2 %) and roots (38.3 %), decreased the Na+ content of leaves (17.2 %) and roots (14.3 %), and increased the K+/Na+ ratio of leaves (58.9 %) and roots (61.6 %) in cucumber. In conclusion, CsHAK5;3 in cucumber can mediate K+ transport and is one of the key target pumpkin genes that enhance salt tolerance of cucumber grafted.

Automated summary

Grafting with pumpkin rootstock can enhance salt tolerance in cucumber scions by decreasing Na+ content and increasing K+ content. The key genes involved in salt stress tolerance were found to be ion transport-related genes. CsHAK5;3, a pumpkin gene, was identified as a key target gene that improves salt tolerance in grafted cucumber.