Arabidopsis MYB-Related HHO2 Exerts a Regulatory Influence on a Subset of Root Traits and Genes Governing Phosphate Homeostasis

Phosphate (Pi), an essential macronutrient required for growth and development of plants, is often limiting in soils. Pi deficiency modulates the expression of Pi starvation-responsive (PSR) genes including transcription factors (TFs). Here, we elucidated the role of the MYB-related TF HYPERSENSITIVITY TO LOW PHOSPHATE-ELICITED PRIMARY ROOT SHORTENING1 HOMOLOG2 (HHO2, At1g68670) in regulating Pi acquisition and signaling in Arabidopsis thaliana. HHO2 was specifically and significantly induced in different tissues in response to Pi deprivation. Transgenic seedlings expressing 35S:: GFP:: HHO2 confirmed the localization of HHO2 to the nucleus. Knockout mutants of HHO2 showed significant reduction in number and length of first-and higher-order lateral roots and Pi content of different tissues compared with the wild-type irrespective of the Pi regime. In contrast, HHO2-overexpressing lines exhibited augmented lateral root development, enhanced Pi uptake rate and higher Pi content in leaf compared with the wild-type. Expression levels of PSR genes involved in Pi sensing and signaling in mutants and over-expressors were differentially regulated as compared with the wild-type. Attenuation in the expression of HHO2 in the phr1 mutant suggested a likely influence of PHR1 in HHO2-mediated regulation of a subset of traits governing Pi homeostasis.