Symbiotic microorganisms such as arbuscular mycorrhizal fungi (AMF) produce both conserved microbial molecules that activate plant defense and lipo-chitooligosaccharides (LCOs) that modulate plant defense. Beside a well-established role of LCOs in the activation of a signaling pathway required for AMF penetration in roots, LCO perception and defense modulation during arbuscular mycorrhiza is not well understood. Here we show that members of the LYRIIIA phylogenetic group from the multigenic Lysin Motif Receptor-Like Kinase family have a conserved role in dicotyledons as modulators of plant defense and regulate AMF colonization in the Solanaceae species Nicotiana benthamiana. Interestingly, these proteins have a high-affinity for LCOs in plant species able to form a symbiosis with AMF but have lost this property in species that have lost this ability. Our data support the hypothesis that LYRIIIA proteins modulate plant defense upon LCO perception to facilitate AMF colonization in mycotrophic plant species and that only their role in plant defense, but not their ability to be regulated by LCOs, has been conserved in non-mycotrophic plants.

Lipo-chitooligosaccharides (LCO) and short-chain chitooligosaccharides (CO) are produced by arbuscular mycorrhizal fungi (AMF) and activate the plant symbiosis signalling pathway, which is essential for mycorrhiza formation. High-affinity LCO receptors belonging to the LysM receptor-like kinase (LysM-RLK) phylogenetic group LYR-IA play a role in AM establishment, but it is unclear which proteins are the plant high-affinity short-chain CO receptors. Here we studied members of the uncharacterized LYR-IB group, and found that they show high affinity for LCO, short- and long-chain CO, and play a complementary role with the LYR-IA receptors for AM establishment. While LYR-IB knock out mutants had a reduced AMF colonization in several species, constitutive/ectopic expression in wheat increased AMF colonization. LYR-IB function is conserved in all tested angiosperms, but in most japonica rice a deletion creates a frameshift in the gene, explaining differences in AM phenotypes between rice and other monocot single LYR-IA mutants. In conclusion, we identified a class of LysM-RLK receptors in angiosperms with unique biochemical properties and a role in both LCO and CO perception for AM establishment.

Establishment of arbuscular mycorrhiza (AM) relies on a plant signaling pathway that can be activated by fungal chitinic signals such as short chain chitooligosaccharides (CO) and lipo-chitooligosaccharides (LCOs). The tomato LysM receptor-like kinase (LysM RLK) SlLYK10 has high affinity for LCOs and is involved in root colonization by arbuscular mycorrhizal fungi (AMF), however its role in LCO responses has not yet been studied. Here, we show that SlLYK10 proteins produced by the Sllyk10-1 and Sllyk10-2 mutant alleles, which both cause decreases in AMF colonization, and carry mutations in LysM1 and 2 respectively, have similar LCO binding affinities compared to the WT SlLYK10. However, the mutant forms were no longer able to induce cell death in Nicotiana benthamiana when co-expressed with MtLYK3, a Medicago truncatula LCO co-receptor, while they physically interacted with MtLYK3 in co-purification experiments. This suggests that the LysM mutations affect the ability of SlLYK10 to trigger signaling through a potential co-receptor rather than its ability to bind LCOs. Interestingly, tomato lines that contain a calcium (Ca²⁺) concentration reporter (Genetically Encoded Ca²⁺ indicators, GECO), showed Ca²⁺ spiking in response to LCO applications, but this occurred only in inner cell layers of the roots, while short chain COs also induced Ca²⁺ spiking in the epidermis. Moreover, LCO-induced Ca²⁺spiking was decreased in Sllyk10-1*GECO plants, suggesting that the decrease in AMF colonization in Sllyk10-1 is due to abnormal LCO signaling.

The growth of young roots is crucial for the development and yield of rice. However, the molecular mechanisms underlying young rice root development remain unclear. Our research indicates that the rice B-type cytokinin response regulator factor ORR3 negatively regulates the development of young rice roots. ORR3 is highly expressed in the root meristematic zone of young rice roots. In transgenic lines overexpressing ORR3, the lengths of primary roots and adventitious roots, as well as the corresponding root meristematic zone lengths, are significantly reduced. This is due to a decrease in both the number and size of longitudinal cells in the root meristematic zone. On the one hand, ORR3 can inhibit root apical cell division and reduce the number of longitudinal cells in the root meristematic zone by affecting the auxin synthesis and transport pathways. On the other hand, ORR3 may directly activate the transcription of cell wall metabolism-related genes, thereby restricting the size of cells in the root meristematic zone. In summary, ORR3 negatively regulates rice young root growth by responding to cytokinin signals to influence auxin signal transduction and cell wall metabolism pathways, thereby negatively regulating the number and size of cells in the root meristematic zone.