Root-Shoot Communication in the Control over Photosynthetic Activity - ISRR 2018 10th Symposium for the International Society of Root Research

Noga Glanz Idan ¹ Petr Tarkowski ^2,3 Shmuel Wolf ¹

¹The Robert H. Smith Institute of Plant Sciences and Genetics in Agriculture, The Hebrew University of Jerusalem, The Robert H. Smith Faculty of Agriculture, Food and Environment, Israel
²Centre of the Region Haná for Biotechnological and Agricultural Research, Central Laboratories and Research Support Faculty of Science, Palacky University, Czech Republic
³Centre of the Region Haná for Biotechnological and Agricultural Research, Department of Genetic Resources for Vegetables, Medicinal and Special Plants, Crop Research Institute, Czech Republic

By some projections, a 40% increase in crop yield will be necessary by 2050 to keep pace with the world’s expanding population. To date, major increases in yield have resulted from growth habit modifications and improved agronomic practices. Further increases will require improvement in the fundamental parameter of photosynthesis per unit leaf area. The key to our current research is the well-known fact that the rate of carbon fixation can be elevated by decreasing the source-sink ratio.

Partial defoliation or heavily shading caused a significant elevation in the rate of photosynthesis in the remaining leaf of tomato plants within 3-4 days. The remaining leaf became deep green, and its area increased by about 50% within 8 days.

These findings indicate that photosynthetic activity is regulated at the whole plant level and, therefore, must be governed by long-distance signaling mechanism. Since the directionality of phloem flow is from the remaining leaf outwards, we assume that signals coming into the leaf are carried in the xylem and originate in the roots. We propose that partial leaf removal results in production of additional cytokinin (CK) in the roots that is transported to the shoot to promote leaf expansion, increase photosynthetic activity and delay senescence. Detailed analyses of CK content indicated that partial defoliation altered CK biosynthesis in the roots, resulting in higher concentration of tZ-type, the major xylem long-distance signaling molecules. Moreover, manipulation of CK biosynthesis in the roots only had a significant effect over leaf senescence and photosynthetic activity. The presented results support the notion that alteration of CK biosynthesis in the roots has stronger influence over leaf functioning than alteration of CK biosynthesis in the leaf.

Understanding of the endogenous mechanisms stimulating up-regulation of photosynthesis will enable the development of biotechnology strategies to increase productivity and crop yield.