HD-zip III homeobox genes that include a novel member, ZeHB-13 (Zinnia)/ATHB-15 (Arabidopsis), are involved in procambium and xylem cell differentiation.

HD-Zip III homeobox genes are known to be essential transcriptional factors for vascular development. To further understand the relation of HD-Zip III genes in vascular differentiation, we isolated a new member of the HD-Zip III genes, ZeHB-13, as a Zinnia homolog of ATHB-15, and then characterized the expression profile using a Zinnia xylogenic cell culture and Zinnia plants. We compared the accumulation pattern of transcripts for ZeHB-13 and other HD-Zip III genes and suggested that the expression of ZeHB-13 was restricted to the procambium and was not severely suppressed by brassinazole, an inhibitor of brassinosteroid biosynthesis, unlike other HD-Zip III genes. We also characterized its Arabidopsis counterpart, ATHB-15. A histochemical promoter analysis using ATHB-15::GUS transgenic Arabidopsis plants indicated that ATHB-15 was active specifically in the procambium. These results strongly suggest that ZeHB-13/ATHB-15 is a pivotal transcriptional regulator responsible for early vascular development. Based on these results, we will discuss the regulation of xylem development in light of the functions of HD-Zip III members and brassinosteroids.