ABA mediation of shoot cytokinin oxidase activity: assessing its impacts on cytokinin status and biomass allocation of nutrient-deprived durum wheat.

Abstract

Although nutrient deprivation alters the concentrations of several plant hormones, the role of each in decreasing shoot-to-root ratio is not clear. A 10-fold dilution of the nutrient concentration supplied to hydroponically-grown 7-day-old durum wheat (Triticum turgidum L. ssp. durum Desf.) plants decreased shoot growth, shoot-to-root ratio and shoot and root cytokinin concentrations, increased shoot ABA concentration and shoot cytokinin oxidase activity, but had no effect on xylem sap ABA and cytokinin concentrations. Nutrient deprivation also increased xylem concentrations of conjugated ABA. The role of ABA in these responses was addressed by adding 11.4 µm ABA to the nutrient solution of well fertilised plants, or 1.2 mm fluridone (an inhibitor of ABA biosynthesis) to the nutrient solution of nutrient-deprived plants. The former induced similar changes in shoot-to-root ratio (by inhibiting shoot growth), shoot ABA concentration, shoot and root cytokinin concentrations and shoot cytokinin oxidase activity as nutrient deprivation. Conversely, fluridone addition to nutrient-deprived plants restored shoot-to-root ratio (by inhibiting root growth), shoot ABA concentration, shoot and root cytokinin concentrations to levels similar to well fertilised plants. Although root growth maintenance during nutrient deprivation depends on a threshold ABA concentration, shoot growth inhibition is independent of shoot ABA status. Although fluridone decreased shoot cytokinin oxidase activity of nutrient-deprived plants, it was still 1.7-fold greater than well fertilised plants, implying that nutrient deprivation could also activate shoot cytokinin oxidase independently of ABA. These data question the root signal basis of cytokinin action, but demonstrate that changes in ABA status can regulate shoot cytokinin concentrations via altering their metabolism.