A soybean plant may look still and quiet, but inside it is constantly listening for danger. Its leaves are exposed to airborne attackers, while its roots meet a different community of microbes in the soil. To survive, the plant needs more than one general alarm.

In a study published in Nature Plants, a research group led by Dr. LI Lei from the Institute of Genetics and Developmental Biology (IGDB) of the Chinese Academy of Sciences discovered a flexible warning system that helps soybean defend different tissues.

There are tiny molecules at the heart of the system called peptides, working like short alarm messages sent by the plant itself. A matching sensor on the surface of a cell receives the message and tells the cell to prepare for attack. These messages are called GmSubPEPs hidden inside much larger proteins. When needed, the short message can be released and used to activate defense.

Based on the group's earlier high-throughput work, researchers created a scalable way to match soybean peptides with their receptors in this study. And they traced the hidden SubPEP signals, identified their receptor partners, and examined how the system evolved across legumes.

Researchers discovered several GmSubPEP variants with very different amino-acid sequences. Each peptide could activate disease resistance, and showed a preferred place of action. GmSubPEP3 and GmSubPEP4 were especially active in leaves and reduced lesions caused by the fungal pathogen Rhizoctonia solani. GmSubPEP1 and GmSubPEP2 were more active in roots, while several family members improved resistance to the soil-borne pathogen Phytophthora sojae.

The matching receptors, named GmSubPEP receptors (GSPRs), explain how the plant distinguishes these messages. GSPR1 recognizes GmSubPEP1 and GmSubPEP2, while GSPR2 and GSPR3 mediate the activities of GmSubPEP3 and GmSubPEP4, respectively. These receptors show different expression patterns in leaves and roots, helping to explain the tissue-specific responses.

Moreover, researchers found that genes encoding the peptides and their receptors are not scattered randomly; instead, they were interspersed within the same genomic region, forming a local collection of signals and sensors. They also found the related clusters in other legumes, including Medicago and chickpea.

These results suggest that peptide and receptor genes expand together during legume evolution. Over time, individual pairs acquire different recognition properties and roles in different tissues, which may allow plants to preserve a common immune framework while continually generating new, specialized defenses.

This study reveals the role of SubPEP-GSPR modules in plant-microbe interactions, and provides a framework for mining and engineering peptide-receptor modules to strengthen crop immunity.