Substantial diversity exists for both the size andshape of the leaf,the main photosynthetic organofflowering plants.The two major forms of leaf aresimple leaves,in which the leaf blade is undivided,and compound leaves,which comprise severalleaflets.Leaves form at the shoot apical meristemfrom a group of undifferentiated cells,whichfirstestablish polarity,then grow and differentiate.Each of these processes is controlled by a com-bination of transcriptional regulators,microRNAsand phytohormones.The present review docu-ments recent advances in our understanding ofhow these various factors modulate the develop-ment of both simple leaves(focusing mainly on themodel plantArabidopsis thaliana)and compoundleaves(focusing mainly on the model legumespeciesMedicago truncatula).
