Regulation of Polar Auxin Transport by AtPin1 in Arabidopsis Vascular Tissue

Polar auxin transport controls multiple developmental processes in plants, including the formation of vascular tissue. Mutations affecting the PIN-FORMED (PIN1) gene diminish polar auxin transport in Arabidopsis thaliana inßorescence axes. The AtPIN1 gene was found to encode a 67-kilodalton protein with similarity to bacterial and eukaryotic carrier proteins, and the AtPIN1 protein was detected at the basal end of auxin transport competent cells in vascular tissue. AtPIN1 may act as a transmembrane component of the auxin efflux carrier

The Arabidopsis ATHB-8 HD-Zip Protein Acts as a Differentiation-Promoting Transcription Factor of the Vascular Meristems

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 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 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

Five components of the ethylene-response pathway identified in a screen for weak ethylene-insensitive mutants in Arabidopsis

Five ethylene-insensitive loci (wei1–wei5) were identified by using a low-dose screen for “weak” ethylene-insensitive mutants. wei1, wei2, and wei3 seedlings showed hormone insensitivity only in roots, whereas wei4 and wei5 displayed insensitivity in both roots and hypocotyls. The genes corresponding to wei1, wei4, and wei5 were isolated using a positional cloning approach. The wei1 mutant harbored a recessive mutation in TIR1, which encodes a component of the SCF protein ubiquitin ligase involved in the auxin response. wei4, a dominant mutant, resulted from a mutation in the ethylene receptor ERS, whereas wei5, a semidominant mutant, was caused by a mutation in the EIN3-related transcription factor gene EIL1. The simultaneous loss of functional WEI5/EIL1 and EIN3 nearly completely abolished the ethylene response in etiolated seedlings, and adult plants were highly susceptible to infection by the necrotrophic fungal pathogen Botrytis cinerea. Moreover, wei5/eil1 ein3 double mutants were able to fully suppress constitutive signaling caused by ctr1, suggesting a synergistic interaction among these gene products. Unlike previously known root ethylene-insensitive mutants, wei2 and wei3 were not affected in their response to auxin and showed a normal response to gravity. Genetic mapping studies indicate that wei2 and wei3 correspond to previously unidentified ethylene pathway genes that may control cell-elongation processes functioning at the intersection of the ethylene and auxin response pathways