Characterization of HAK protein family in Casuarina equisetifolia and the positive regulatory role of CeqHAK6 and CeqHAK11 genes in response to salt tolerance

The potassium transporter group of the (high-affinity K )/ (K uptake)/ (K transporter) family plays a crucial role in plant growth and development as well as in environmental adaptation such as tolerance to salt stress. HAK/KUP/KT genes and their functions have been characterized for a number of plant species, but they remain unknown for , an important tree species for coastal protection in southern China and many other countries. In this study, 25 genes were identified in the genome. Their gene structure, conserved motif, phylogeny, and expression were comprehensively and systematically analyzed to understand their functions. All genes were relatively conserved and could be divided into four clusters. The expression level of two particular genes, and , increased significantly with the duration of salt treatment. To further elucidated their function in response to salt stress, subcellular localization, and their functional analysis were developed. Results revealed that and were localized on the plasma membrane, which mainly mediated high-affinity K uptake. Overexpression of or in showed higher germination and survival rates and longer root length than wild-type (WT) under salt stress, suggesting that both genes improve tolerance to salt stress. Moreover, and improved their ability to tolerate salt stress by increasing the K /Na ratio and antioxidant enzyme activities (CAT, POD, and SOD), and decreasing reactive oxygen species (ROS) accumulation. Consequently, and were verified as potassium transport proteins and could be applied for further molecular breeding for salt tolerance in or other crops to increasing salt tolerance.