KNOX overexpression in transgenic Kohleria (Gesneriaceae) prolongs the activity of proximal leaf blastozones and drastically alters segment fate

Abstract

KNOX (knotted1-like homeobox) genes have a widely conserved role in the generation of dissected leaves. Ectopic KNOX activity in leaves in various angiosperm lineages causes leaf form changes that can elucidate how the configuration of leaf development evolved. We present an analysis of leaf morphology and morphogenesis in transgenic Kohleria lines overexpressing a heterologous KNOX gene. Kohleria, like many members of Gesneriaceae, has simple-serrated leaves with pinnate venation. KNOX overexpression causes prolonged segment proliferation in proximal, but not distal, parts of leaf blades. Elaborate dissected segments reiterate the zonation of the whole leaf, with organogenic activity persisting between a distal maturation zone and a proximal intercalary elongation zone. The architecture of vascular bundles is severely altered, with a reduced midvein and a more palmate venation. The initial establishment of organogenically competent primordial margins (marginal blastozones) and the onset of tissue differentiation in early stages of leaf development were similar in wild-type and KNOX overexpressing lines. However, leaves overexpressing KNOX often failed to fully mature, and persistent marginal blastozones were found at the base of blades in mature portions of the shoot. We conclude that KNOX-mediated perpetuation of marginal blastozones in Kohleria is sufficient to induce a set of processes that result in highly dissected leaflets, which are unusual in this plant family. Spatial confinement of blastozones between an early maturing tip and a late elongating petiole zone reflects the presence of distinct maturation processes that limit the ability of the leaf margins to respond to ectopic KNOX gene expression.