Molecular genetic approaches to leaf development: Knotted and beyond

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Abstract
Molecular genetics provides a promising alternative to other experimental approaches for furthering our understanding of the mechanisms controlling leaf development. We investigated the molecular basis of dominant Knotted (Kn1) mutations in maize, which cause cells associated with the lateral veins of the leaf blade to acquire characteristics of sheath or auricle and sporadically form outgrowths called knots. The kn1 gene encodes a homeodomain, a DNA-binding domain shared by many transcription factors that regulate developmental processes in animals and fungi. In normal plants, the expression of kn1 is confined to the shoot apex, but in Kn1 mutants, the gene is also expressed ectopically in the veins of developing leaves, apparently causing cells to change their developmental fates. The kn1 gene may function in the shoot apex of normal plants to promote indeterminate growth. Consistent with this hypothesis, when kn1 is expressed constitutively at high levels in the leaves of transgenic tobacco, shoots are formed on the leaf surface. Thus, our results indicate that while the kn1 gene may normally have no function in leaf development, it can alter the development of maize and tobacco leaves when it is expressed in the leaf inappropriately. Genes that normally play a role in leaf development are more likely to be defined by recessive mutations that alter leaf morphogenesis and histogenesis. Key words: leaf development, molecular genetics, Knotted.

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