The Arabidopsis ATHB-8 HD-zip protein acts as a differentiation-promoting transcription factor of the vascular meristems

ATB-8, -9, -14, -15, and IFL1/REV are members of a small homeodomain-leucin e zipper family whose genes are characterized by expression in the vascular tissue. ATHB-8, a gene positively regulated by auxin (Baima et al., 1995), is considered an early marker of the procambial cells and of the cambium d uring vascular regeneration after wounding. Here, we demonstrate that altho ugh the formation of the vascular system is not affected in athb8 mutants, ectopic expression of ATHB-8 in Arabidopsis plants increased the production of xylem tissue. In particular, a careful anatomical analysis of the trans genic plants indicated that the overexpression of ATHB-8 promotes Vascular cell differentiation. First, the procambial cells differentiated precocious ly into primary xylem. In addition, interfascicular cells also differentiat ed precociously into fibers. Finally, the transition to secondary growth, m ainly producing xylem, was anticipated in transgenic inflorescence stems co mpared with controls. The stimulation of primary and secondary vascular cel l differentiation resulted in complex modifications of the growth and devel opment of the ATHB-8 transgenic plants. Taken together, these results are c onsistent with the hypothesis that ATHB-8 is a positive regulator of prolif eration and differentiation, and participates in a positive feedback loop i n which auxin signaling induces the expression of ATHB-8, which in turn pos itively modulates the activity of procambial and cambial cells to different iate.