Effects of Drought on Gene Expression in Maize Reproductive and Leaf Meristem Tissues as Revealed by Deep Sequencing

Drought is a major environmental stress factor that poses a serious threat to food security. The effects of drought on early reproductive tissue at 1-2 DAP (days after pollination) is irreversible in nature and leads to embryo abortion, directly affecting the grain yield production. We developed a working RNA-Seq pipeline to study maize (Zea mays) drought transcriptome sequenced by Illumina GSIIx technology to compare drought treated and well- watered fertilized ovary (1-2DAP) and basal leaf meristem tissue. The pipeline also identified novel splice junctions - splice variants of previously known gene models and potential novel transcription units. An attempt was also made to exploit the data to understand the drought mediated transcriptional events (e.g. alternative splicing). Gene Ontology (GO) enrichment analysis revealed massive down-regulation of cell division and cell cycle genes in the drought stressed ovary only. Among GO categories related to carbohydrate metabolism, changes in starch and sucrose metabolism-related genes occurred in the ovary, consistent with a decrease in starch levels, and in sucrose transporter function, with no comparable changes occurring in the leaf meristem. ABA-related processes responded positively, but only in the ovaries. GO enrichment analysis also suggested differential responses to drought between the two tissues in categories such as oxidative stress-related and cell cycle events. The data are discussed in the context of the susceptibility of maize kernel to drought stress leading to embryo abortion, and the relative robustness of actively dividing vegetative tissue taken at the same time from the same plant subjected to the same conditions. A hypothesis is formulated, proposing drought-mediated intersecting effects on the expression of invertase genes, glucose signaling (hexokinase 1-dependent and independent), ABA-dependent and independent signaling, antioxidant responses, PCD, phospholipase C effects, and cell cycle related processes.

This work was supported by the National Science Foundation Plant Genome Research Pro- gram (grant no. DBI0922747), iPlant Collaborative (NSF DBI-0735191) and also NSF ABI1062472.