A well-adjusted regulation of vascular development is crucial for plant growth and development. CLASS III HOMEODOMAIN LEUCINE ZIPPER (HD-ZIP III) genes play a decisive role in cambial stem-cell maintenance, xylem cell fate determination, and ensuring proper patterning of vasculature. LITTLE ZIPPER (ZPR) microproteins are involved in the post-translational regulation of HD-ZIP III via a negative feedback mechanism. The protein-protein docking has predicted that HD-ZIP III and ZPR interact at the b-ZIP domain, which is an evolutionarily conserved region. The expression and mutant phenotype of ZPR in root vasculature haven’t been previously described. In this study, we developed the reporter lines and checked the expression in the Arabidopsis root. ZPR1 and ZPR3 showed a distinct and continuous expression from root tip to elongation and differentiation zone. In the cross-section, the expression was detected in cambium and xylem cells. Unlike ZPR1, ZPR3 showed a stronger expression towards the phloem side. Although ZPR2 didn’t show any endogenous expression, expression was detected in the primary root tip and lateral root initiation site upon xylemin treatment. Exogenous auxin application increased the expression of ZPR3, but it required a longer response time. The expression results suggest that ZPR follows the HD-ZIP III expression pattern, however, each ZPR shows some variation from the others. In the mutant analysis, zpr1 showed increased cellular proliferation in the vasculature, but zpr2 had no difference compared to the wild type. The results suggest that ZPR and HD-ZIP III have their function in a similar region in the root vasculature. However, further empirical evidence is necessary to elucidate the interaction of HD-ZIP III and ZPR, and its role in the regulation of root vascular development.
