Genome-wide identification and analysis of the HD-Zip transcription factors in perennial ryegrass highlight LpHOX22 and LpHOX24 as negative regulators of osmotic and heat stresses

Abstract

The homeodomain leucine zipper (HD-Zip) transcription factors are involved in plant responses to abiotic stresses. In this study, we have identified a total of 52 HD-Zip genes from the released genome of perennial ryegrass (Lolium perenne), a cool-season turfgrass and forage species, which is relatively sensitive to heat and drought stresses. These HD-Zip genes are classified into classes I-IV, of which, 49 are evenly distributed on all seven chromosomes of perennial ryegrass whereas other 3 are on scaffolds. Cis-elements analysis showed that phytohormone, light signaling, and stress responsive elements are abundant in the promoter region of the HD-Zip genes. Expression analysis indicated that LpHOX24 and its paralog LpHOX22 were significantly induced by heat stress treatment. Ectopic overexpressing (OE) of LpHOX22 and LpHOX24 in Arabidopsis compromised plant osmotic stress tolerance as evidenced by declined survival rate with elevated electrolyte leakage (EL) and malondialdehyde (MDA) accumulation. Expression levels of several osmotic responsive genes were downregulated in OE lines. Additionally, heat stress tolerance was impaired in LpHOX22 and LpHOX24 overexpression transformants, along with increased EL and MDA content. Further analysis revealed that overexpression of LpHOX22 and LpHOX24 repressed the expressions of AtHSFAs and AtDREB2A when compared to wild type after heat stress treatment. In summary, LpHOX22 and LpHOX24 potentially functioned as negative regulators in perennial ryegrass in response to osmotic and heat stresses.