Ultrastructural observation of mesophyll cells and temporal expression profiles of the genes involved in transitory starch metabolism in flag leaves of wheat after anthesis.

Transitory starch in cereal plant leaves is synthesized during the day and remobilized at night to provide a carbon source for growth and grain filling, but its mechanistic basis is still poorly understood. The objective of this study is to explore the regulatory mechanism for starch biosynthesis and degradation in plant source organs. Using transmission electron microscopy, we observed that during the day after anthesis, starch granules in mesophyll cells of wheat flag leaves accumulated in chloroplasts and the number of starch granules gradually decreased with wheat leaf growth. During the night, starch granules synthesized in chloroplasts during the day were completely or partially degraded. The transcript levels of 26 starch synthesis-related genes and 16 starch breakdown-related genes were further measured using quantitative real-time reverse transcription polymerase chain reaction. Expression profile analysis revealed that starch metabolism genes were clustered into two groups based on their temporal expression patterns. The genes in the first group were highly expressed and presumed to play crucial roles in starch metabolism. The genes in the other group were not highly expressed in flag leaves and may have minor functions in starch metabolism in leaf tissue. The functions of most of these genes in leaves were further discussed. The starch metabolism-related genes that are predominantly expressed in wheat flag leaves differ from those expressed in wheat grain, indicating that two different pathways for starch metabolism operate in these tissues. This provides specific information on the molecular mechanisms of transitory starch metabolism in higher plants.