The Control of Pinna Morphology in Wildtype and Mutant Leaves of the Garden Pea (Pisum sativum L.)

The compound leaf of the wildtype pea exhibits basal stipules, proximal leaflets, and distal tendrils, which are profoundly modified by several recessive genes thought to act as homeotic mutations. The acacia (tl) gene appears to replace simple tendrils with leaflets in the distal region, while the afila (af) gene appears to replace the leaflets with branched tendrils in the proximal region. Morphological analyses of all homozygous and heterozygous combinations of this genetic system indicate that these genes affect many other features of leaf morphology, including leaf length, petiole length, pinna number, and branching order. Moreover, these genes are apparently expressed within the context of preexisting proximal and distal regions in the developing leaf. Pinna identity on the leaves from different genotypes is therefore regulated by the interaction among relative pinna position, morphological region, and genetic background. Current models of pea leaf development fail to account for several characteristics of pea leaves, such as (1) the leaf phenotype of the double recessive mutant called pleiofila, which consists of novel compound pinnae with very small terminal leaflets, (2) mixed pairs of proximal and distal pinnae, and (3) intermediate narrow leaflets/broad tendrils in certain heterozygotes. Starting with the assumption that the pleiofila leaf represents the ground state of the pea leaf, the available evidence indicates that the dominant Af and Tl genes are acting as heterochronic mutations that determine the timing of lamina formation and axis termination, with the result that these genes cause the observed morphological changes in pinna identity.