RESUMO
Plagiogravitropic growth of roots strongly affects root architecture and topsoil exploration, which are important for the acquisition of water and nutrients. Here we show that basal roots of Phaseolus vulgaris L. develop from 2-3 definable whorls at the root-shoot interface and exhibit position-dependent plagiogravitropic growth. The whorl closest to the shoot produces the shallowest roots, and lower whorls produce deeper roots. Genotypes vary in both the average growth angles of roots within whorls and the range of growth angles, i.e. the difference between the shallowest and deepest basal roots within a root system. Since ethylene has been implicated in both gravitropic and edaphic stress responses, we studied the role of ethylene and its interaction with phosphorus availability in regulating growth angles of genotypes with shallow or deep basal roots. There was a weak correlation between growth angle and ethylene production in the basal rooting zone, but ethylene sensitivity was strongly correlated with growth angle. Basal roots emerging from the uppermost whorl were more responsive to ethylene treatment than the lower-most whorl, displaying shallower angles and inhibition of growth. Ethylene sensitivity is greater for shallow than for deep genotypes and for plants grown with low phosphorus compared with those supplied with high phosphorus. Ethylene exposure increased the range of angles, although deep genotypes grown in low phosphorus were less affected. Our results identify basal root whorl number as a novel architectural trait, and show that ethylene mediates regulation of growth angle by position of origin, genotype and phosphorus availability.
RESUMO
The hypothesis that ethylene participates in the regulation of root hair development by phosphorus availability in Arabidopsis thaliana was tested by chemically manipulating ethylene synthesis and response and with ethylene-insensitive mutants. Low phosphorus-induced root hair development could be mimicked by adding the ethylene precursor, 1-aminocyclopropane-1-carboxylate (ACC), to high phosphorus media, and inhibited by adding ethylene inhibitors to low phosphorus media. Ethylene-insensitive mutants showed a reduced response to low phosphorus, indicating ethylene involvement in root hair responses to phosphorus deficiency. To dissect the nature of this involvement, the morphological and anatomical changes associated with increased root hair density were investigated. Growth in low phosphorus resulted in smaller, more numerous cortical cells, resulting in a larger number of root hair-bearing epidermal cell files. Cortical cell number was not affected by ethylene inhibitors, ACC, or mutations reducing ethylene sensitivity in roots grown with low phosphorus, indicating that ethylene does not participate in this response. The exception was the eir1 mutation, which strongly reduced this change in radial anatomy, supporting a role for polar auxin transport in this process. Trichoblast cell length was reduced by low phosphorus availability in all genotypes, but even more so for ein2-1 and ein4. The proportion of epidermal cells forming hairs and root hair length were reduced in ethylene-insensitive mutants, especially in the presence of low phosphorus. These results demonstrate multiple effects of low phosphorus from the earliest stages of root hair development, and cross-talk between ethylene and phosphorus in the control of a subset of the low phosphorus effects, concentrating on those later in development.