ATHB-8, -9, -14, -15, and IFL1/REV are members of a small homeodomain-leucine 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 during vascular regeneration after wounding. Here, we demonstrate that although the formation of the vascular system is not affected inathb8 mutants, ectopic expression of ATHB-8 in Arabidopsis plants increased the production of xylem tissue. In particular, a careful anatomical analysis of the transgenic plants indicated that the overexpression of ATHB-8 promotes vascular cell differentiation. First, the procambial cells differentiated precociously into primary xylem. In addition, interfascicular cells also differentiated precociously into fibers. Finally, the transition to secondary growth, mainly producing xylem, was anticipated in transgenic inflorescence stems compared with controls. The stimulation of primary and secondary vascular cell differentiation resulted in complex modifications of the growth and development of the ATHB-8 transgenic plants. Taken together, these results are consistent with the hypothesis that ATHB-8 is a positive regulator of proliferation and differentiation, and participates in a positive feedback loop in which auxin signaling induces the expression of ATHB-8, which in turn positively modulates the activity of procambial and cambial cells to differentiate.

Homeodomain‐leucine zipper (HD‐Zip) proteins are putative transcription factors identified only in plants. Related Arabidopsis homeobox genes, isolated by virtue of sequence conservation within the helix‐3 region of the homeodomain, fall into four families based on sequence similarity. This paper reports the characterization of Athb‐10, a 747 amino acid protein belonging to the fourth HD‐ZIP family. The studies indicate that, although less conserved, the leucine zipper of Athb‐10 can functionally replace that of Athb‐2 in an in vitro DNA‐binding assay. Gene mapping experiments and sequence comparison analysis revealed that Athb‐10 corresponds to GLABRA2, a homeodomain protein involved in trichome development. The mRNA expression analysis revealed that Athb‐10/GLABRA2 is expressed not only in trichome‐bearing organs, but also in the root. The analysis of wild‐type and mutant plants showed that the Athb‐10/GLABRA2 gene expression in the aerial part of the plant and in the root is affected by mutations at the TTG locus. Morphological analysis of the gl2‐1 mutant revealed that the gene is necessary not only for local outgrowth of the trichome, but also for the regulation of root hair development in a subset of epidermal cells. Interestingly, the development of root hair cells in a position normally occupied by non‐hair cells is dependent upon the ethylene regime in which the gl2‐1 plants are grown. Sequence analysis of the gl2‐1 allele revealed that the mutant gene encodes a truncated protein that might still retain a partial activity responsible for the formation of aborted trichomes and for the ethylene‐dependent regulation of root hair formation.

The root-hair pattern of Arabidopsis is determined through a regulatory circuit composed of transcription factor genes. The homeobox gene GLABRA2 ( GL2 ) has been considered a key component, acting farthest downstream in this regulation. GL2 modified to include a transactivating function caused epidermal cells to develop ectopic root hairs or root hair–like structures. With this system, the phospholipase Dζ1 gene ( AtPLD ζ 1 ) was identified as a direct target of GL2. Inducible expression of AtPLD ζ 1 promoted ectopic root-hair initiation. We conclude that GL2 exerts its regulatory effect on root-hair development through modulation of phospholipid signaling.

Plants grown under dense canopies perceive through the phytochrome system a reduction in the ratio of red to far-red light as a warning of competition, and this triggers a series of morphological changes to avoid shade. Several phytochrome signaling intermediates acting as positive regulators of accelerated elongation growth and induction of flowering in shade avoidance have been identified. Here we report that a negative regulatory mechanism ensures that in the presence of far-red-rich light an exaggerated plant response does not occur. Strikingly, this unpredicted negative regulatory mechanism is centrally involved in the attenuation of virtually all plant responses to canopy shade.

A plant grown under canopies perceives the reduction in the ratio of red (R) to far-red (FR) light as a warning of competition, and enhances elongation growth in an attempt to overgrow its neighbors. Here, we report that the same low R/FR signal that induces hypocotyl elongation also triggers a rapid arrest of leaf primordium growth, ensuring that plant resources are redirected into extension growth. The growth arrest induced by low R/FR depends on auxin-induced cytokinin breakdown in incipient vein cells of developing primordia, thus demonstrating the existence of a previously unrecognized regulatory circuit underlying plant response to canopy shade.