The LysM Receptor-Like Kinase SlLYK10 Controls Lipochitooligosaccharide Signaling in Inner Cell Layers of Tomato Roots

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.