Differential Hypocotyl and Root Development of Arabidopsis Auxin-Insensitive Mutants

Seedling development was compared in auxin-insensitive mutants of Arabidopsis, aux1-7, axr1-3 and axr2-1, grown in a natural sandy soil, without sucrose supplementation. The three mutants showed impaired epidermal cell elongation in the hypocotyls of 15-day-old seedlings, with axr2-1 showing the most marked effects. In addition, the roots of axr2-1 elongated faster and presented a more extended meristematic zone than the other genotypes. Unchanged epidermal cell length in the differentiation zone of axr2-1 relative to the wild-type suggested enhancement of cell proliferation. These alterations may have affected the timing and site of emergence of the root hairs, starting later and further from the root tip than in the other genotypes. Similarly to the wild-type, no root hair growth was initiated in axr2-1 drought-induced short roots, although the epidermis was differentiated into trichoblasts and atrichoblasts. On rehydration of the short roots, hair formation occurred from trichoblasts prior to epidermal cell elongation. Therefore, auxin-insensitivity in the axr2-1 mutant did not result in alterations of the hair-forming process itself. The differential development of axr2-1 seedlings, relative to the other auxin-insensitive mutants, suggested that the AXR2 gene has a complex, regulatory function in multiple hormone signaling.