Pea is the fourth largest edible legume crop in the world and is widely cultivated throughtout the world. Several classical leaf mutants have been identified in peas, including unifoliata (uni), afila (af), tendril-less (tl), cocholeata (coch), stipule reduced (st), etc.

In the 1970s, pea breeders introduced the af mutation to develop "half-tendrilless" pea varieties, characterized by all pinnae becoming tendrils but retaining a pair of normal stipules. The af "half-tendrilless" mutation then has since been widely used worldwide. However, the AF gene locus has not been cloned so far.

In a study published in Physiologia Plantarum, researchers from the Xishuangbanna Tropical Botanical Garden (XTBG) of the Chinese Academy of Sciences identified the candidate mutant genes for the classic pea mutant afila as PsPALM1a and PsPALM1b, and further clarified how the conserved molecular regulatory module AF-UNI (pea)/MPL1-CaLFY (chickpea)/PALM1-SGL1 (alfalfa) mediates distinct compound leaf morphogenesis.

Phenotypic statistical analysis of the afila mutant revealed that all the basal side leaves of its compound leaves were transformed into tendril branches, while the tendril structure at the top showed no significant change compared to the wild type.

Molecular analysis revealed that the AF gene locus is highly associated with two tandemly arranged PsPLAM1a and PsPLAM1b genes, both encoding Cys(2)His(2) zinc finger protein transcription factors, which are orthologous to the reported PALM1 in thorny alfalfa, MPL1 in chickpea, and POP in columbine.

Further in situ hybridization showed that PsPALM1a and PsPALM1b are specifically highly expressed in the basal leaf primordia of early compound leaf primordia, while they are low expressed in the upper part of the compound leaf primordia (future tendril development area) and stipule primordia. Knockout of PsPALM1a and PsPALM1b genes by virus-induced gene silencing can promote the transformation of basal side leaves into tendrils.

"Our study provides new insights into the formation mechanism of compound leaf morphological diversity in nature. It also offers important theoretical guidance and genetic resources for the molecular breeding and genetic improvement of semi-leafless traits in pea," said HE Liangliang of XTBG.