Photosynthetic activity is affected by exogenous and endogenous inputs, including source-sink balance. Reducing the source to sink ratio by partial defoliation or heavy shading resulted in significant elevation of the photosynthetic rate in the remaining leaf of tomato plants within 3 d. The remaining leaf turned deep green, and its area increased by almost 3-fold within 7 d. Analyses of photosynthetic activity established up-regulation due to increased carbon fixation activity in the remaining leaf, rather than due to altered water balance. Moreover, senescence of the remaining leaf was significantly inhibited. As expected, carbohydrate concentration was lower in the remaining leaf than in the control leaves; however, expression of genes involved in sucrose export was significantly lower. These results suggest that the accumulated fixed carbohydrates were primarily devoted to increasing the size of the remaining leaf. Detailed analyses of the cytokinin content indicated that partial defoliation alters cytokinin biosynthesis in the roots, resulting in a higher concentration of trans-zeatin riboside, the major xylem-translocated molecule, and a higher concentration of total cytokinin in the remaining leaf. Together, our findings suggest that trans-zeatin riboside acts as a signal molecule that traffics from the root to the remaining leaf to alter gene expression and elevate photosynthetic activity.

Centre of the Region Haná for Biotechnological and Agricultural Research Department of Genetic Resources for Vegetables Medicinal and Special Plants Crop Research Institute Šlechtitelů 29 Olomouc Czech Republic

Centre of the Region Haná for Biotechnological and Agricultural Research Department of Phytochemistry Faculty of Science Palacky University Šlechtitelů 27 Olomouc Czech Republic

Laboratory of Growth Regulators The Czech Academy of Sciences Institute of Experimental Botany and Palacký University Šlechtitelů 27 Olomouc Czech Republic

The Robert H Smith Institute of Plant Sciences and Genetics in Agriculture The Robert H Smith Faculty of Agriculture Food and Environment The Hebrew University of Jerusalem Rehovot Israel

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Delayed Leaf Senescence by Upregulation of Cytokinin Biosynthesis Specifically in Tomato Roots