Stress and juvenility in maize. Zea mays

As maize (Zea mays) plants undergo vegetative phase change from juvenile to adult, they both exhibit heteroblasty, an abrupt change in patterns of leaf morphogenesis, and gain the ability to produce flowers. Both processes are under the control of microRNA 156, whose levels decline at the end of the juvenile phase. Gain of ability to flower is conferred by expression of miR156 targets that encode Squamosa Promoter-Binding (SBP) transcription factors, which when derepressed in the adult phase induce the expression of MADS-box transcription factors that promote maturation and flowering. What gene expression differences underlie heteroblasty, as well as what regulates miR156 levels, remain open questions. Here, we compare gene expression in primordia that will develop into juvenile or adult leaves to identify genes that define these two developmental states and may influence vegetative phase change. In comparisons among successive leaves at the same developmental stage of plastochron 6, three-fourths of approximately 1,100 differentially expressed genes were more highly expressed in primordia of juvenile leaves. This juvenile set was enriched in photosynthetic genes, particularly those associated with cyclic electron flow at photosystem I, and in genes involved in oxidative stress and retrograde redox signaling. Pathogen- and herbivory-responsive pathways including jasmonic acid and salicylic acid were also up-regulated in juvenile primordia and indeed, exogenous application of jasmonic acid both delayed the appearance of adult traits and the decline of miR156 levels in maize seedlings. The successful amelioration of stress signals thus plays an important role in inducing vegetative phase change in maize. Overall design: 12 untreated samples (8mm primordia of leaves 1-12) and 1 treated sample (leaf 5 primordium, 15mM JA treatment). 2 or more technical replicates per sample