Receptor-like proteins (RLPs) are crucial pattern-recognition receptors on the surface of plant cells, which are involved in almost all processes of the plant life cycle. Recently, the evolution of high-throughput sequencing technology has strengthened the appraisal and identification of increasing numbers of RLPs and has primarily improved our understanding of the roles of RLPs in various biological processes. Here, we review the classification and evolutionary characteristics of RLPs and their regulatory roles in pattern-triggered immunity (PTI) and effector-triggered immunity (ETI). In particular, we summarize the ligands recognized by RLPs, their co-receptors, and downstream signalling cascades mediated by RLPs. To summarize, this review offers beneficial guidance for researchers in at-a-glance comprehension of the function of RLPs. It also puts forward the prospect of mining broad-spectrum candidate genes in light of the research on the disease resistance mechanism of RLPs and current challenges in disease resistance breeding.

Steroidal glycoalkaloids (SGAs), which are widely produced by potato, even in other Solanaceae plants, are a class of potentially toxic compounds, but are beneficial to host resistance. However, changes of the other metabolic process along with SGA accumulation are still poorly understood and researched. Based on RNA sequencing (RNA-seq) and bioinformatics analysis, the global gene expression profiles of potato variety Helan 15 (Favorita) was investigated at four-time points during light exposure. The data was further verified by using quantitative Real-time PCR (qRT-PCR). When compared to the control group, 1288, 1592, 1737, and 1870 differentially expressed genes (DEGs) were detected at 6 h, 24 h, 48 h, and 8 d, respectively. The results of both RNAseq and qRT-PCR showed that SGA biosynthetic genes were up-regulated in the potato tuber under light exposure. Functional enrichment analysis revealed that genes related to PS light reaction and Protein degradation were significantly enriched in most time points of light exposure. Additionally, enriched Bins included Receptor kinases, Secondary metabolic process in flavonoids, Abiotic stress, and Biotic stress in the early stage of light exposure, but PS Calvin cycle, RNA regulation of transcription, and UDP glucosyl and glucoronyl transferases in the later stage. Most of the DEGs involved in PS light reaction and Abiotic stress were up-regulated at all four time points, whereas DEGs that participated in biotic stresses were mainly up-regulated at the later stage (48 h and 8 d). Cis-element prediction and co-expression assay were used to confirm the expressional correlation between genes that are responsible for SGA biosynthesis and disease resistance. In conclusion, the expressions of genes involved in PS light reaction, Abiotic stress, and Biotic stress were obviously aroused during the accumulation of SGAs induced by light exposure. Moreover, an increased defense response might contribute to the potato resistance to the infection by phytopathogenic microorganisms.

Rosaceae is an economically important plant family that can be affected by a multitude of pathogenic microbes, which have sometimes caused dramatic losses in production. As a type of pattern recognition receptor, receptor-like proteins (RLPs) are considered vital regulators of plant immunity. Based on genome-wide identification, bioinformatic analysis, and functional determination, we investigated the evolutionary characteristics of RLPs and specifically those that regulate Valsa canker, a devastating fungal disease affecting apple and pear production. A total of 3028 RLPs, from the genome of 19 plant species (including nine Rosaceae plants), were divided into 24 subfamilies. Five subfamilies and seven co-expression modules were involved in the responses of resistant pear rootstock ‘Duli-03’ to Valsa canker signals. Fourteen RLPs were screened as candidate genes that regulate Valsa canker resistance. Among these, PbeRP23 (Chr13.g24394) and PbeRP27 (Chr16.g31400) were identified as key resistance genes that promptly enhanced the resistance of ‘Duli-03’ and strongly initiated immune responses. Additionally, we established ideal methods for the classification and screening of disease-resistant RLPs. Importantly, we obtained potential candidate genes and two key Valsa canker resistance genes for further functional exploration and resistance breeding.

L-type lectin receptor-like kinases (L-LecRKs) act as a sensor of extracellular signals and an initiator for plant immune responses. However, the function of LecRK-S.4 on plant immunity has not been extensively investigated. At present, in the apple (Malus domestica) genome, we identified that MdLecRK-S.4.3, a homologous gene of LecRK-S.4, was differentially expressed during the occursion of Valsa canker. Over-expression of MdLecRK-S.4.3 facilitated the induction of immune response and enhanced the Valsa canker resistance of apple and pear fruit, and ‘Duli-G03’ (Pyrus betulifolia) suspension cells. On the contrary, the expression of PbePUB36, RLCK XI subfamily member, was significantly repressed in the MdLecRK-S.4.3 overexpressed cell lines. Over-expression of PbePUB36 interfered with the Valsa canker resistance and immune response caused by up-regulation of MdLecRK-S.4.3. Furthermore, MdLecRK-S.4.3 interacted with BAK1 or PbePUB36 in vivo. In conclusion, MdLecRK-S.4.3 activated various immune responses and positively regulate Valsa canker resistance, which could be largely compromised by PbePUB36. MdLecRK-S.4.3 interacted with PbePUB36 and/or MdBAK1 to mediate the immune responses. This finding provides a reference for studying the molecular mechanism of resistance to Valsa canker and resistance breeding.