Abstract
Cytokinin flow from roots to shoots can serve as a long-distance signal important for root-to-shoot communication. In the past, changes in cytokinin flow from roots to shoots have been mainly attributed to changes in the rate of synthesis or breakdown in the roots. The present research tested the possibility that active uptake of cytokinin by root cells may also influence its export to shoots. To this end, we collapsed the proton gradient across root membranes using the protonophore carbonyl cyanide m-chlorophenylhydrazone (CCCP) to inhibit secondary active uptake of exogenous and endogenous cytokinins. We report the impact of CCCP on cytokinin concentrations and delivery in xylem sap and on accumulation in shoots of 7-day-old wheat plants in the presence and absence of exogenous cytokinin applied as zeatin. Zeatin treatment increased the total accumulation of cytokinin in roots and shoots but the effect was smaller for the shoots. Immunohistochemical localization of cytokinins using zeatin-specific antibodies showed an increase in immunostaining of the cells adjacent to xylem in the roots of zeatin-treated plants. Inhibition of secondary active cytokinin uptake by CCCP application decreased cytokinin accumulation in root cells but increased both flow from the roots and accumulation in the shoots. The possible importance of secondary active uptake of cytokinins by root cells for the control of their export to the shoot is discussed.
Highlights
Cytokinin hormones influence numerous developmental processes in plants
We report the impact of carbonyl cyanide m-chlorophenylhydrazone (CCCP) on cytokinin concentrations and delivery in xylem sap and on accumulation in shoots of 7-day-old wheat plants in the presence and absence of exogenous cytokinin applied as zeatin
We studied the effects of CCCP on accumulation of cytokinins in root cells, their distribution between roots and shoots, concentration in xylem sap, and rates of delivery to shoots of durum wheat plants in the presence and absence of exogenous zeatin at the roots
Summary
Cytokinin concentrations change in response to external factors such as availability of nitrates (Takei et al, 2002; Rahayu et al, 2005; Vysotskaya et al, 2009), salinity (Ghanem et al, 2011), and water shortage (Kudoyarova et al, 2007; Alvarez et al, 2008) These changes correlate with the cytokinin-controlled processes in the shoots, such as the expression of numerous genes (Brenner and Schmülling, 2012), adjustments in stomatal conductance (Vysotskaya et al, 2010), and the extent of leaf senescence (Dong et al, 2008), suggesting a causal relationship. The balance of evidence favours the view that cytokinin flow from the roots controls a number of developmental processes in shoots
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