A rapid increase in cytokinin levels and enhanced ethylene evolution precede Al3+-induced inhibition of root growth in bean seedlings (Phaseolus vulgaris L.)

Abstract

The role of plant hormones in aluminium (Al3+)-inducedinhibition of root growth was investigated in roots of Phaseolusvulgaris L. cv. Strike. Changes in ethylene evolution and changes inthe content and composition of cytokinins (CKs), at intervals up to 150min after treatment with Al3+, were determined by gaschromatography (GC) and combined gas chromatography-mass spectrometry (GC-MS),respectively. Seedlings were cultivated in a continuously aerated nutrientsolution at pH 4.5 with and without Al3+. The growth rate ofAl3+-treated roots was significantly decreased after 360min. Ethylene evolution from excised root tips doubled after 15min of Al3+ treatment and reached a maximum 30min after treatment. Levels of CK nucleotides declined 60 to 80%after only 5 min of Al3+ treatment whereas the zeatin(Z) content increased six-fold. The increase in Z continued over the entire 150min-sampling period and reached a level 80 times higher than thatin roots not exposed to Al3+. These results show thatAl3+-induced inhibition of root growth is preceded by significantchanges in CK content and composition and enhanced ethylene evolution. Since CKcan induce ethylene production, the rapid increase in CK, particularly Z anddihydrozeatin (dZ), may contribute to inhibition of root-growth either directlyor indirectly by affecting plant hormone homeostasis.