Résumé
Effector proteins secreted via the type III secretion system (T3SS) of nitrogen-fixing rhizobia are key determinants of symbiotic compatibility in legumes. Previous report revealed that the T3SS of Bradyrhizobium sp. DOA9 plays negative effects on Arachis hypogaea symbiosis. In this study, we characterized the symbiotic role of 4 effector proteins (p0490, p0871, SkP48, and p0903) containing the small ubiquitin-like modifier (SUMO) protease domain identified in DOA9 during symbiosis. While the DOA9 strain and the two mutants of SUMO-proteases, p0490 and p0871, induced inefficient nodulation in A. hypogaea, the mutation of SUMO-proteases SkP48 or p0903 promoted efficient symbiosis comparable to the type strain Bradyrhizobium arachidis CCBAU051107. Complementation study of ∆p0903 with various mutated forms of p0903 highlighted importance of ubiquitin-like protein (ULP) domain in restriction of nodulation in A. hypogaea. We observed the accumulation of jasmonic acid (JA) and upregulation of several defence genes involved in the JA/ethylene (ET) signalling pathway at the early stage of infection in roots inoculated with DOA9 strain compared with those inoculated with the DOA9-∆p0903 strain. Our data highlight the importance of SUMO-protease effectors during the symbiotic interaction between bradyrhizobia and A. hypogaea, which could be useful for the development of high-performance inocula to improve its growth.
The symbiotic relationship between rhizobia and legumes represents a fascinating and complex biological process with significant agronomical and ecological importance. This interaction begins when nitrogen-fixing rhizobia release signalling molecules called Nod factors (NFs), which trigger the formation of root nodules and facilitate bacterial infection. Within these nodules, rhizobia convert atmospheric nitrogen into a plant-accessible form in exchange for photosynthates and other nutrients from the host plant. Among rhizobia, Bradyrhizobium strains are particularly valuable in agriculture because of their ability to form symbioses with economically important legume crops such as soybeans, peanuts, and cowpeas 1-3 . This mutually beneficial partnership has attracted considerable scientific interest, as it plays a crucial role in sustainable agriculture and global nitrogen cycling.
In several strains of Bradyrhizobium, a type III secretion system (T3SS) plays a crucial role during symbiosis, significantly influencing the interaction with host plants 4,5 . The T3SS serves as a nanosyringe structure found in Gram-negative bacteria. Its function is to deliver type III effectors (T3Es) into eukaryotic host cells during interaction with the host 6 . As observed in pathogenic bacteria, the T3Es identified in rhizobia, often referred to as nodulation outer proteins (Nops), can act as double-edged swords, depending on the host plant 7 . They can promote symbiosis by suppressing the plant's immune system; however, if recognized by plant resistance proteins, they can trigger an immune response known as effector-triggered immunity (ETI), which can block