Overexpression of an ABA-inducible Homeodomain-Leucine Zipper I Gene MsHB7 Confers Salt Stress Sensitivity to Alfalfa

Alfalfa (Medicago sativa L.) is the foremost forage legume crop worldwide for diary production and ecological balance. Salinity has become a crucial factor affecting crop productivity. Plant unique homeodomain-leucine zipper (HD-Zip) transcription factors are reported to play significant roles in abiotic stress signaling transduction and regulation. This study was conducted to elucidate the regulatory roles of an alfalfa HD-Zip I transcription factor, Medicago sativa homeobox 7 (MsHB7), on salt stress response. MsHB7 was indicated to be located in nucleus with transactivation activity. Expression pattern analysis revealed that MsHB7 was induced by abscisic acid (ABA) and salt (NaCl) stress. Enhanced salt sensitivity in MsHB7-overexpressing alfalfa was reflected by the morphological and physiological performances. In addition, genes regarding antioxidation (MspxdC and MsCatalase4) and ABA response (MsABI1 and MsSnRK2.6) were down-regulated in MsHB7-overexpressing alfalfa compared with wild type. These data suggested that MsHB7 played a negative role on alfalfa salt stress regulation, which could be through repressing expressions of ABA-responsive genes, MsABI1 and MsSnRK2.6, and antioxidant enzyme coding genes, MspxdC and MsCatalase4. This study could enlarge knowledge of alfalfa HD-Zip I gene regulation mechanism, and provide important strategy for salt-tolerant alfalfa breeding.