The specification of leaf identity during shoot development

A single plant produces several different types of leaves or leaf-like orga ns during its life span. This phenomenon, which is termed heteroblasty, is an invariant feature of shoot development but is also regulated by environm ental factors that affect the physiology of the plant. Invariant patterns o f heteroblastic development reflect global changes in the developmental sta tus of the shoot, such as the progression from embryogenesis through juveni le and adult phases of vegetative development, culminating in the productio n of reproductive structures. Genes that regulate these phase-specific aspe cts of leaf identity have been identified by mutational analysis in both ma ize and Arabidopsis. These mutations have revealed that leaf production is regulated independently of leaf identity, implying that the identity of a l eaf at a particular position on the shoot may depend on when the leaf was i nitiated in relation to a temporal program of shoot development.