Using Leaf Starch to Identify Transgenic Tomatoes with Improved Greenhouse Performance

The allocation of leaf assimilate to fruit is a major determinant of tomato yield and fitness. This process is highly regulated to ensure that carbon acquisition in source leaves is balanced with plant growth, developmental and long-term storage needs. We hypothesize that Transcription Factors (TF) as supra-regulatory proteins could partially provide the overarching control that would coordinate source-sink relations in plants and influence yield. We were able to test this using a large proprietary collection of transgenic tomato lines (8500), each ectopically expressing one of the ~1700 TFs cloned from Arabidopsis under 1 of 5 different promoters. We screened this vast population for lines with alterations in leaf starch. While source-sink interactions are complex, leaf starch may act as an indicator of changes in whole plant carbon allocation. Starch accumulates in leaves during periods of carbon sufficiency, but is degraded to sugars for export to the sinks during episodes of carbon starvation. Altered levels in the transgenic lines compared with the transgenic and non-transformed controls, may indicate changes in the carbon status of the plant due to the presence of the ectopically expressed TF. After extensive field and greenhouse screening of these lines, we selected three genotypes for further study. The TF1 and TF2 had a high starch-to-sugar ratio, and a TF3 had double the sugar and starch levels in source leaves. These lines were subjected to careful eco-physiological measurements to evaluate any changes in important agronomic traits under greenhouse conditions. The TF1 line had altered carbon distribution: identical yield but a higher number (28%; P < 0.05) of smaller fruit; TF2 had higher (25%; P < 0.05) yield and TF3 had a higher (15%; P < 0.05) harvest index. Metabolomic and ¹⁴C-flux studies of leaf and fruit provided clues on how the presence of the respective TFs altered plant response. The results of this study will be presented.