Compound leaves begin as simple primordia, with LEAFY (LFY)/FLORICAULA (FLO) orthologs orchestrating subsequent leaflet initiation. Although several repressors of LFY/FLO have been identified, the activating mechanisms remain unknown.

In a study published in Cell Reports, researchers from the Xishuangbanna Tropical Botanical Garden (XTBG) of the Chinese Academy of Sciences have identified a key genetic switch that controls the number of leaflets on a soybean leaf. By activating this switch, they transformed the standard three-leaflet (trifoliate) leaf into a five-leaflet form, potentially boosting the plant’s photosynthetic capacity and overall productivity.

Soybean, an important legume crop used for food and feed production, usually has trifoliate leaves. However, a naturally occurring mutant, known as Lf1, consistently develops five palmately arranged leaflets, increasing total leaf area by approximately 20%, offering a promising target for improving photosynthetic efficiency.

The researchers found that the LF1 gene encodes an AP2 transcription factor, a type of protein that regulates gene expression. In the Lf1 mutant, a single nucleotide change in the gene's coding sequence substitutes one amino acid within a highly conserved DNA-binding domain. This minor alteration dramatically enhances the protein's function, enabling it to bind more efficiently to specific DNA sequences known as GCC-boxes.

They further found that LF1 directly activates two key downstream genes, GmLFYa and GmLFYb, which act as master switches for leaflet initiation. By binding to GCC-boxes within the coding regions of these genes, LF1 activates them. In the Lf1 mutant, this activation is amplified, resulting in the initiation of additional leaflets and the distinctive five-leaflet phenotype.

Crucially, the researchers identified LF1 as a central hub that connects the plant hormone auxin, a key growth regulator, with the LFY/FLO developmental pathway. Although auxin rapidly induces LF1 expression, the researchers discovered a complex, context-dependent feedback loop in which auxin can promote or suppress LF1 activity, depending on the developmental stage and tissue context.

"Our findings identify the long-sought activator of leaflet initiation and reveal how hormonal signals integrate with developmental programs to shape complex leaves," said CHEN Jianghua of XTBG. "This provides us with a precise molecular target for engineering ideal plant architecture."

The study has immediate implications for agricultural biotechnology. By editing the LF1 gene or its regulatory targets, breeders may be able to fine-tune leaf morphology in soybeans and potentially other crops, thereby optimizing leaf area for maximum light capture and photosynthetic output.

"Leaf morphology is a critical determinant of photosynthetic efficiency and ultimately yield. This work gives us a new tool to redesign crop canopies for the future," said HE Liangliang of XTBG.