Cytokinin-dependent expression of the ARR5::GUS construct during transgenic arabidopsis growth

Growth and glucuronidase (GUS) activity were followed in the cotyledons and rosette leaves of Arabidopsis thaliana (L.) Heynh (ecotype Wassilewskija) plants transformed with the GUS gene under the control of the cytokinin-dependent promoter of the ARR5 gene. The presence of active cytokinins in plant tissues was assessed from GUS activity. Plants were grown for three weeks on the nitrate-or ammonium-containing nutrient medium. In plants grown on ammonium nutrition, cotyledon and leaf growth was substantially suppressed as compared with plants feeding with nitrates. In correspondence with this growth inhibition, GUS activity was markedly lower in plant leaves grown on the ammonium-containing medium. This indicated a reduction in these leaves of active cytokinin forms capable of activation of the promoter for the ARR5 gene. On both nitrogen sources, GUS activity increased during leaf growth and dropped sharply after growth ceasing. This indicated that leaf growth depended on the cytokinin content in them. High GUS activity was detected in petioles and leaf conductive system, indicating leaf providing with cytokinins along the conductive vessels. A sharp drop in the GUS activity after leaf growth stoppage coincided in time with GUS activation in the leaf positioned above this leaf. This indicated possible cytokinin redistribution in the plant; its content could be a limiting factor for leaf growth. A higher growth rate in plants on nitrate nitrogen nutrition and corresponding high GUS activity in them are discussed in terms of cytokinin signaling role in leaf growth regulation mediated by nitrate.