Ms. Hagar Fox

In trees, nonstructural carbohydrates (NSC), which include starch and soluble sugars, serve in long-term carbon storage and long-distance carbon transport from source to sink. Like many tree species, Populus are known to deploy a passive sugar loading strategy, relying on sugars’ concentration gradient rather than on active sugar transport. The molecular regulation of these processes is largely unknown, especially under drought stress when NSC metabolism is shifted and in high demand. To this aim, P. alba’s leaves, stems, and roots were sampled under drought and ambient conditions, followed by NSC and mRNA quantification. Drought-stressed trees were depleted of starch across organs, while soluble sugars (Sucrose, Glucose, and Fructose) were mainly accumulated in the leaves. Major gene families related to sugar sensing, translocation, and metabolism were identified in our transcriptome, of which we focused on the expression of drought-related transcripts. Under drought stress, genes involved in sucrose breakdown were found mainly up-regulated in leaves and stem, while sucrose biosynthesis genes were elevated in the roots. The expression of passive sugar transporters was mostly down-regulated, while active transporters were up-regulated across organs and highly correlated with measured sugar levels. Our results underpin the role of sub-cellular sugar sequestration in determining NSC levels and composition under drought, specifically in passive phloem loading species. This research highlights potential key regulators of available sugar pools, improving our understanding of trees’ drought responses and resilience mechanisms.