Differential hypocotyl and root development of Arabidopsis auxin-insensitive mutants

Seedling development was compared in auxin-insensitive mutants of Arabidops is, aux1-7, axr1-3 and axr2-1, grown in a natural sandy soil, without sucro se supplementation. The three mutants showed impaired epidermal cell elonga tion in the hypocotyls of 15-day-old seedlings, with axr2-1 showing the mos t marked effects. In addition, the roots of axr2-1 elongated faster and pre sented a more extended meristematic zone than the other genotypes. Unchange d epidermal cell length in the differentiation zone of axr2-1 relative to t he wild-type suggested enhancement of cell proliferation These alterations may have affected the timing and site of emergence of the root hairs, start ing later and further from the root tip than in the other genotypes. Simila rly to the wild-type, no root hair growth was initiated in axr2-1 drought-i nduced short roots, although the epidermis was differentiated into trichobl asts and atrichoblasts. On rehydration of the short roots, hair formation o ccurred from trichoblasts prior to epidermal cell elongation. Therefore, au xin-insensitivity in the axr2-1 mutant did not result in alterations of the hair-forming process itself. The differential development of axr2-1 seedli ngs, relative to the other auxin-insensitive mutants, suggested that the AX R2 gene has a complex, regulatory function in multiple hormone signaling.