Auxin transport inhibitor induced low complexity petiolated leaves and sessile leaf-like stipules and architectures of heritable leaf and stipule mutants in Pisum sativum suggest that its simple lobed stipules and compound leaf represent ancestral forms in angiosperms

In angiosperms, leaf and stipule architectures are inherited species-specific traits. Variation in leaf and stipule sizes, and forms result from the interaction between abiotic and biotic stimuli, and gene regulatory network(s) that underlie the leaf and stipule developmental programme(s). Here, correspondence between variation in leaf and stipule architectures described for extant angiosperms and that induced mutationally and by imposition of stress in model angiosperm species, especially in Pisum sativum, was detected. Following inferences were drawn from the observations. (i) Several leaf forms in P. sativum have origin in fusion of stipule and leaf primordia. Perfoliate (and amplexicaul and connate) simple sessile leaves and sessile adnate leaves are the result of such primordial fusions. Reversal of changes in the gene regulatory network responsible for fusion products are thought to restore original stipule and leaf conditions. (ii) Compound leaf formation in several different model plants, is a result of promotion of pathways for such condition by gene regulatory networks directed by KNOX1 and LEAFY transcription factors or intercalation of the gene networks directed by them. (iii) Gene regulatory network for compound leaves in P. sativum when mutated generates highly complex compound leaves on one hand and simple leaves on other hand. These altered conditions are mutationally reversible. (vi) Simple leaves in model plants such as Arabidopsis thaliana despite overexpression of KNOX1 orthologues do not become compound. (v) All forms of leaves, including simple leaf, probably have origins in a gene regulatory network of the kind present in P. sativum.