The cellular basis for synergy between RCO and KNOX1 homeobox genes in leaf shape diversity

Leaves of seed plants provide an attractive system to study the development and evolution of form. Leaves show varying degrees of margin complexity ranging from simple, as in Arabidopsis thaliana, to fully dissected into leaflets in the closely related species Cardamine hirsuta. Leaflet formation requires actions of Class I KNOTTED1-LIKE HOMEOBOX (KNOX1) and REDUCED COMPLEXITY (RCO) homeobox genes, which are expressed in the leaves of C. hirsuta but not A. thaliana. Evolutionary studies indicate that diversification of KNOX1 and RCO genes was repeatedly associated with increased leaf complexity. However, whether this gene combination represents a developmentally favored avenue for leaflet formation remains unknown, and the cell-level events through which the combined action of these genes drives leaflet formation are also poorly understood. Here we show, through a genetic screen, that when a C. hirsuta RCO transgene is expressed in A. thaliana, then ectopic KNOX1 expression in leaves represents a preferred developmental path for leaflet formation. Using time-lapse growth analysis, we demonstrate that KNOX1 expression in the basal domain of leaves leads to prolonged and anisotropic cell growth. This KNOX1 action, in synergy with local growth repression by RCO, is instrumental in generating rachises and petiolules, the linear geometrical elements, that bear leaflets in complex leaves. Our results show how the combination of cell-level growth analyses and genetics can help us understand how evolutionary modifications in expression of developmentally important genes are translated into diverse leaf shapes.