Symposium IV: Evolution of Plant Development

Abs # 40002: The evolution of leaves in vascular plants

Presenter:	Bowman, John L, jlbowman@ucdavis.edu
Authors	Bowman, John L (A)   Floyd, Sandra K (A)   Emery, John F (A)   Alvarez, John P (A)   Oldenhof, Harriette  (A)   Hawker, Nathaniel P (A)   Izhaki, Anat  (A)   Engstrom, Eric M (A)
Affiliations:	(A): Section of Plant Biology, UC Davis

Lateral organs of plants, such as leaves and floral organs, are formed from the flanks of apical meristems. Therefore, an inherent positional relationship exists between them: organ primordia have an adaxial side adjacent to the meristem, and an abaxial one away from the meristem. In angiosperms, surgical and genetic studies suggest that a morphogenetic gradient, whose source is the meristem, converts the inherent polarity into a functional one. Once an adaxial-abaxial axis of polarity is established within organ primordia, it provides cues for proper lamina growth and asymmetric development. Several key players involved in this process were identified recently, and analyses of these genes support and refine our views of axis formation in plants. The complex relationships between and within various members of these plant-specific gene families (class III HD-ZIPs, YABBYs and KANADIs) may account for a significant portion of the morphological variation in lateral organs of angiosperms. Despite common positional, anatomical, and functional aspects of leaves in vascular plants, interpretation of the fossil record indicates that leaves evolved independently in the major extant groups of plants (lycophytes, ferns and seed plants) from leafless ancestors in each clade. Thus, a phylogenetic interpretation of the fossil evidence implies that developmental mechanisms for producing leaves evolved several times in vascular plants. To investigate whether similar developmental programs operate in leaves of other taxa, we have cloned orthologues of the three gene families (class III HD-ZIPs, YABBYs and KANADIs) from gymnosperms, ferns, lycopods and mosses. Our analyses support the hypothesis that leaves evolved independently in these lineages, and provide insight into the origin of the genetic program that patterns polarity in seed plants.