Topic 25.2

Regulation of Juvenility by the TEOPOD (TP) Genes in Maize

The regulation of the transition from juvenile to adult (i.e., the attainment of competence) is influenced by a wide variety of conditions that affect the growth rate, and by transmissible factors such as carbohydrates and gibberellins. Hence it has been difficult to determine exactly what regulates the phase change during normal development. The analysis of mutations that affect the timing of phase changes, called heterochronic mutations, provides the opportunity to identify specific genes that regulate juvenility. The best-studied example is in maize.

As indicated in the table in Web Topic 24.1, the juvenile and adult phases of corn plants are characterized by differences in vegetative morphology. Scott Poethig and his colleagues at the University of Pennsylvania have been characterizing the heterochronic teopod (tp) mutants of maize: tp1, tp2 and tp3 (Poethig 1990; Dudley and Poethig 1991, 1993). These semidominant mutations cause cells and tissues that would normally develop as adult structures to become more juvenile (Web Figure 25.2.A). In tp plants, leaves in the normally adult part of the shoot are morphologically intermediate between juvenile and adult leaves. Whereas in the wild type, tassels bear male flowers and ears bear female flowers, in tp mutants both tassels and ears possess leaves in addition to flowers. However, tp mutations affect neither the rate nor the duration of growth of the plant, nor do they interfere with the timing of tassel formation. The teopod mutations thus specifically affect the transition from juvenile to vegetative adult phase without affecting the transition to the reproductive phase.

Web Figure 25.2.A Phenotypes of three teopod mutants and the wild-type maize plant. From left to right: tp1, tp2, tp3, and wild type. (Photo from Poethig 1990, courtesy of S. Poethig.) (Click image to enlarge.)

The possible interaction between the TEOPOD genes and the gibberellins has been investigated (Evans and Poethig 1995). In GA-deficient dwarf mutants of maize, the transitions from juvenile to adult vegetative development and from adult vegetative to reproductive development occur later than in wild-type plants. Thus, endogenous GAs are required for the normal timing of phase changes.