The pre- and early post-pollination phases of maize (Zea mays L.) reproductive development are critical for seed set and subsequent grain yield. During this time, the plant is especially sensitive to abiotic stresses, such as drought, which can reduce pollination efficiency or lead to kernel abortion. A key determinant of reproductive success is carbohydrate allocation and use in the developing female inflorescence, which is often disrupted by stress. Understanding the mechanisms that underlie regulation of sink strength during normal progression of maize floral development is thus central to improving seed set under adverse environmental conditions.

In this work, we tested the hypothesis that expression of genes related to specific metabolic or regulatory pathways would change in association with carbohydrate allocation and use during silk exsertion and pollination in maize. Individual stages of pre- and early post-pollination maize female florets were characterized based on physical characteristics and expression of a molecular marker for development. Subsequent analyses revealed a shift in sink strength, as approximated by dry and fresh weights, during the pollination period from rapidly expanding silks and subtending floral structures (lemma, palea, and glumes) to the developing ovary and pedicel. This shift coincided with isoform-specific expression of sucrose metabolizing invertases, which provide hexose substrates essential for turgor-based expansion prior to pollination and also for post-pollination growth of symplastically-isolated filial tissues. In addition, accumulation of sucrose and hexoses in the pedicel and ovary, respectively, indicated that invertases could contribute to spatial regulation of cell expansion and differentiation during development.

We used a genome-wide transcript profiling approach to determine whether co-expressed genes were related to specific functional processes or associated with relevant metabolic pathways during pre- and early post-pollination ovary development. A gene-specific, sequencebased, 3'-UTR profiling strategy was developed and tested in parallel to microarray analyses. We resolved co-expression profiles for key genes related to nitrogen and amino acid metabolism, carbohydrate metabolism, lignin biosynthesis, and cell growth during ovary development. Transcript profiles were combined with sugar and metabolite analyses and fresh/dry weight quantifications to further support relevance of key sets of co-expressed genes during ovary sink establishment in maize. Results from this study provide evidence for testable roles of such genes in kernel set.