Age-associated growth control modifies leaf proximodistal symmetry and enables leaf shape diversification

Abstract

Biological shape diversity is often manifested in modulation of organ symmetry and modification of the patterned elaboration of repeated shape elements.1-5 Whether and how these two aspects of shape determination are coordinately regulated is unclear.5-7 Plant leaves provide an attractive system to investigate this problem, because they usually show asymmetries along their proximodistal axis, along which they can also produce repeated marginal outgrowths such as serrations or leaflets.1 One case of leaf shape diversity is heteroblasty, where the leaf form in a single genotype is modified with progressive plant age.8-11 In Arabidopsis thaliana, a plant with simple leaves, SQUAMOSA PROMOTER BINDING PROTEIN-LIKE9 (SPL9) controls heteroblasty by activating CyclinD3 expression, thereby sustaining proliferative growth and retarding differentiation in adult leaves.12 However, the precise significance of SPL9 action for leaf symmetry and marginal patterning is unknown. By combining genetics, quantitative shape analyses, and time-lapse imaging, we show that, in A. thaliana, proximodistal symmetry of the leaf blade decreases in response to an age-dependent SPL9 expression gradient, and that SPL9 action coordinately regulates the distribution and shape of marginal serrations and overall leaf form. Using comparative analyses, we demonstrate that heteroblastic growth reprogramming in Cardamine hirsuta, a complex-leafed relative of A. thaliana, also involves prolonging the duration of cell proliferation and delaying differentiation. We further provide evidence that SPL9 enables species-specific action of homeobox genes that promote leaf complexity. In conclusion, we identify an age-dependent layer of organ proximodistal growth regulation that modulates leaf symmetry and has enabled leaf shape diversification.