Are Homeobox Knotted-Like Genes and Cytokinins the Leaf Architects?

An increasing body of evidence indicates that the products of homeobox genes control the expression of specific target genes that trigger important differentiation processes in a variety of organisms (Gehring, 1987; Skott et al., 1989; Kessel and Gruss, 1990; Kerstelter et al., 1994). In plants the KNAT1 gene, belonging to theknotted1 (kn1) homeobox gene family, has been isolated in Arabidopsis. KNAT1 overexpression in homologous species induces modifications in leaf shape, producing lobed leaves with ectopic meristems in the margins in close vicinity to the veins (Chuck et al., 1996). Similarly, the overexpression of kn1genes in several heterologous species has been reported to affect the architecture of both simple and compound leaves (Hareven et al., 1996;Sinha, 1997). However, neither direct nor indirect relationships have so far been reported between the expression of kn1 genes and the modification of plant biochemical functions that lead to processes of cell differentiation. In our laboratory lettuce (Lactuca sativa) plants overexpressing KNAT1 from Arabidopsis, driven by the pea (Pisum sativum) plastocyanin promoter, have been produced and characterized. Morphological analyses of the transgenic progenies showed dramatic alterations in leaf shape, consisting of a reduction in midvein elongation, an increased complexity of the vascular net, and a regular development of leaf-like structures at the serrations of leaf margins (Fig. 1, a–d). We found a positive correlation between the intensity of the phenotype and KNAT1 expression.