Prolonged high drought stress reduces starch concentration in tomato pistils and leaves without activating degradation pathways

Drought stress and yield loss in crops will become more common due to climate change. Starch turnover is one mechanism plants use to mitigate drought stress and has been studied in leaves of various plant species. We compared the effect of drought treatment on starch turnover in flowers and leaves of tomato, a model plant and major crop. Applying a prolonged high drought treatment to tomato plants led to a significant reduction in plant growth and increased flower abortion, with a large reduction in starch concentrations in flower pistils (60 %) and mature leaves (75 %), but not sucrose. However, genes from the starch degradation pathway were not induced. SIBAM3.2 and SlBAM9 were down-regulated in pistils and young leaves. In contrast, SIBAM3.1 was significantly down-regulated in mature leaves, suggesting a different starch degradation response between source and sink tissues. SlESV1, encoding for a protein involved in restricting starch degradation, was also significantly down-regulated in pistils. To explore carbon changes during flower development and put drought stress into context, we investigated metabolite and transcriptional variations in all flower tissues from control plants. In pistils, starch concentrations were lower than in other floral tissues and lower than sucrose in pistils. Transcriptional profiles showed SlBAM3.2 and SlBAM9 were up-regulated and SlESV1 was down-regulated during pistil development. From this we concluded that lower starch concentrations in drought treated pistils were caused by decreased SIESV1 transcription allowing higher starch degradation rates, and that pistil starch turnover is critical for flowering success, fruit development and yield serving as a buffer to maintain stable sucrose concentrations during drought.