Root-to-Shoot Communication to Modulate Source-Sink Relationship in Tomato Depends on Phytochromes

Phytochromes have been reported as strategic photoreceptors that can modulate the partition of photoassimilates between source and sink. However, so far, it is unknown whether phytochrome accumulation in the root is part of the control mechanisms of the source-sink relationship that modulates root and shoot growth. Thus, the objective of this work was to investigate phytochrome involvement in the source-sink relationship and in the vegetative and reproductive development of tomato plants (Solanum lycopersicum L. cv. Micro-Tom or MT). The experimental design was completely randomized with four treatments, provided by grafting combinations between aurea (au), which is phytochrome deficient, and the near isogenic line MT- (MT/MT, au/au, MT/au and au/MT). We observed differentiated responses for many parameters analyzed. For example, the root dry mass accumulation and stern diameter obtained by MT/MT, MT/au and au/MT grafting were 33% and 31% higher, respectively, than those obtained by au/au. In the au/MT combination, there were greater root dry mass and total dry mass accumulations. Based on the changes in vegetative and reproductive development observed from grafting combinations between MT and the mutant au, we can conclude that phytochromes function in the control of photoassimilate partitioning between roots and stems during tomato growth.