Genome-wide identification, characterization, and expression analyses of the HAK/KUP/KT potassium transporter gene family reveals their involvement in K+ deficient and abiotic stress responses in pear rootstock seedlings

High-affinity K+ (HAK)/K+ uptake (KUP)/K+ transporters (KT) play crucial roles in the regulation of cellular K+ levels. However, little is known about these genes in the Rosaceae family. In this study, 56 putative HAK/KUP/KT genes were identified in genome sequences from Pyrus bretschneideri, Fragaria vesca, and Vitis vinifera, 21 of which were from P. bretschneideri (designated PbHAK1-21). HAK/KUP/KTs from these species, as well as from Arabidopsis and rice, were grouped into five major clusters with eight subclades. Whole-genome duplication/segmental duplication and dispersed duplication largely accounts for the expansion of HAK/KUP/KT families in these five species. Orthologous relationships between pear and Arabidopsis genes suggest that some PbHAKs function as high-affinity K+ transporters or mediators of abiotic stress responses. Cis-regulatory motifs upstream of PbHAK genes also suggest that members of this family respond to environment changes. PbHAK2 and PbHAK12 mRNAs are abundant in roots exposed to normal levels of K+ and are rapidly up-regulated under conditions of K+ deficiency, suggesting that they have crucial roles in K+ uptake, especially at low K+ concentrations. PbHAK12 (orthologous to AtHAK5 from Arabidopsis) is predominately localized in the plasma membrane, consistent with a role in mediating K+ uptake. Some PbHAK mRNA levels also change in response to abiotic stresses such as salt, cold, and drought. Our data reveals potential candidate genes for further functional characterization, and may be useful for breeding pear rootstocks that utilize potassium more efficiently.