Heteroblastic features of leaf anatomy in maize and their genetic regulation

Heteroblastic features of leaf anatomy in maize were identified by conducting a quantitative analysis of leaf anatomy. Heteroblastic variation in cuticle thickness and epidermal cell shape paralleled changes in previously defined juvenile- and adult-specific traits. The other traits examined in this study (thickness of the leaf blade, epidermal and bundle sheath cell size, vascular area, interveinal distance, mesophyll area:bundle sheath area ratio) varied in a more complex fashion. To determine which of these traits are regulated by genes involved in shoot maturation, we examined the effect of the Teopod2 (Tp2) mutation on their expression. Tp2 increases the number of leaves that express the juvenile form of cuticle thickness, epidermal cell shape, and vascular area and causes all other leaves to produce intermediate (juvenile/adult) forms of these traits. It has little or no effect on any of the other traits we examined. Thus, much of the heteroblastic variation in the internal anatomy of the maize leaf appears to be regulated by factors that are unrelated to the developmental phenomenon of phase change. The effect of Tp2 on leaf anatomy is interesting not only because it provides a criterion for distinguishing between different types of heteroblastic traits, but also because it provides some insight into the nature of the developmental processes involved in shoot maturation. In particular, the observation that Tp2 leaves are quantitatively intermediate between juvenile and adult leaves supports the hypothesis that some phase-specific aspects of leaf identity are regulated in a combinatorial fashion rather than by mutually exclusive patterns of gene expression.