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

Abstract

SUMMARYBiological shape diversity is often manifested in modulation of organ symmetry and modification of the patterned elaboration of repeated shape elements.1–5Whether and how these two aspects of shape determination are coordinately regulated is unclear.5–7Plant 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.1One case of leaf shape diversity is heteroblasty, where the leaf form in a single genotype is modified with progressive plant age.8–11InArabidopsis thaliana,a plant with simple leaves, SQUAMOSA PROMOTER BINDING PROTEIN-LIKE9 (SPL9) controls heteroblasty by activatingCyclinD3expression, thereby sustaining proliferative growth and retarding differentiation in adult leaves.12However, 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, inA. thaliana, proximodistal symmetry of the leaf blade decreases in response to an age-dependentSPL9expression 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 inCardamine hirsuta,a complex-leafed relative ofA. 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.In BriefAge-dependent regulation of proliferative growth and differentiation along the proximodistal axis of leaves modulates organ symmetry and marginal patterning in heteroblasty, and enabled the action of homeobox genes in complex leaf shape evolution.HighlightsAn age-dependent growth gradient underpins regulation ofA. thalianaleaf symmetrySPL9-mediated heteroblastic growth control potentiates leaf histogenic capacityA common growth framework underlies heteroblasty in complex and simple leavesSPL9 enables the action of homeobox genes that promote leaf complexity