ETHYLENE AND PHOSPHORUS RESPONSES IN PLANTS

Plant roots react to low phosphorus availability in several ways that may permit them to acquire more phosphorus, including distributing more roots in upper, phosphorus -rich soil horizons, growing relatively more roots, i.e. higher root/shoot ratio, and producing longer and denser root hairs to increase the surface area. These alterations in growth may also influence tolerance to other stresses, such as water stress. The objectives of this study were to examine the involvement of ethylene in phosphorus regulation of root hair density and length and basal root angle. I also evaluated the effects of alumina-buffered phosphorus (Al-P), a novel phosphorus fertilizer that releases phosphorus at a much lower rate than conventional fertilizers, on growth and drought tolerance of woody plants and on seedling establishment of several tree species grown in containers with soilless media. Low phosphorus induced longer and denser root hairs in roots of Arabidopsis thaliana ecotype Columbia. The increase in root hair density partially resulted from increased number of cortical cells compared with roots grown in high phosphorus, which had eight large cortical cells. The increase in cortical cell number resulted in an increased number of trichoblasts. Ethylene manipulation affected root hair length and density, but not cortical cell number or trichoblast number. For example, the ethylene precursor, 1-aminocyclopropane-1-carboxylate (ACC) added to high phosphorus medium could mimic the low phosphorus effect on the root hair growth, while ethylene synthesis and action inhibitors including aminoethoxyvinyl glycine (AVG), MCP (1-methylcyclopropene), or silver thiosulfate (STS) reduced root hair growth under low phosphorus conditions. Ethylene insensitive mutants, including ein4, ein2-1, ein3-1, ein5-1, ein6, ein7, and the mutant eir1-1, showed a reduced but still significant response to low phosphorus. However, results of root cross-sectioning showed that ethylene manipulation did not affect the number of cortical cells or trichoblasts in roots. Ethylene-insensitive mutants grown with low phosphorus produced more and smaller cortical cells, similar to wild-type plants. The pattern of trichoblast length change in response to phosphorus availability for both of the ethylene-insensitive mutants tested (ein2-1 and ein4) was similar to the wild type. These results suggest that low phosphorus and ethylene might operate via separate pathways to influence root hair density. The genotype G19833 of common bean (Phaseolus vulgaris L.) had shallow basal roots that became shallower as phosphorus availability decreased, while DOR364 had deep basal roots that were affected by phosphorus availability to a lesser extent. These genotypes and recombinant inbred lines (RILs) produced from their F1 hybrid were used to study the role of ethylene in basal root gravitropic responses to low phosphorus. Basal roots grew shallower when ACC or gaseous ethylene was added to high phosphorus medium, especially the shallow genotypes. AVG and STS, on the other…