Cytokinin levels and expression profiles of cytokinin metabolic genes in late stage maize kernels

Cytokinins (CK) are key determinants of seed size and their level can impact crop yield. The biological activities and chemistry of cytokinins are well defined but less is known about their molecular mode of action during maize kernel development, especially in late stage kernels. To understand the regulation of cytokinin levels at late stage kernel development, we examined the levels of active and inactive cytokinins in embryo and endosperm across kernel development. Active trans-zeatin is high during early stage of kernel growth, but decreases during later stages. We also undertook a genome-wide survey of cytokinin metabolic genes involved in biosynthesis, metabolism, conversion, transport, and signaling and examined their expression profiles during kernel development. Several major genes were identified which may contribute to changes in cytokinin levels during the late stage.  Our systematic analysis of cytokinin pathways genes and metabolites provides an overall insight into the regulation of CK level across maize kernel development. Our data on late stage kernel add to the existing knowledge of CK biology in maize and could facilitate the modulation of CK levels using biotechnology.   Key words: Maize, cytokinin, kernel development, metabolism, gene regulation.