Leaves exhibit two primary forms: simple (a single blade) or compound (composed of multiple leaflets). The intricate and varied morphology of compound leaves has long fascinated plant biologists, driving their research into how plants develop such complex structures and the genetic foundations of their remarkable diversity.

Recently, researchers from the Xishuangbanna Tropical Botanical Garden (XTBG) of the Chinese Academy of Sciences focused on studies on species such as tomato, Cardamine hirsuta, and key legumes including Medicago and pea, and summarized how compound leaves develop. Their study, published in Current Opinion in Plant Biology, offers a clearer view of the genetic “toolkit” that shapes these elaborate leaves.

Researchers showed that the development of a compound leaf begins with a simple peg-like primordium at the periphery of the shoot apical meristem (SAM). A key feature is the transient activity of the marginal blastozone, which is a lateral meristematic zone that allows leaflets to initiate.

A deeply conserved genetic toolkit guides a leaf bud to become a multi-leaflet structure. The KNOXI and LFY/FLO gene families are central to this process. "In most plants like tomato, KNOXI genes are reactivated to promote leaflet formation. But in many legumes, the floral gene LFY/FLO is responsible for this task, showing remarkable flexibility," said CHEN Jianghua from XTBG, one corresponding author of this study.

These core genes do not work alone. They interact with hormone signals like auxin that pinpoint where leaflets emerge, and with boundary genes like CUC/NAM transcription factors that demarcate regions between emerging leaflets, ensuring that proper separation and individualization separate the leaflets.

In the model legume Medicago truncatula, repressors including PALM1 finely modulate the activity of the LFY-type gene SGL1 to establish the correct three-leaflet pattern. Mutations in these repressors result in strikingly altered leaf morphology. The regulation also involves adaxial-abaxial polarity genes which interact with auxin to influence leaflet arrangement. 

These genetic cues direct localized cell growth to form the mature leaf architecture. Researchers proposed that future research should employ systematic biology approaches including single-cell and spatial transcriptomics to map the complete regulatory network across species. Integrating phylogenetics with evolutionary developmental biology will clarify how genetic rewiring drives the stunning variety of leaf forms observed in nature.