Allene oxide cyclase (AOC) is a key enzyme in the biosynthesis of jasmonic acid (JA), which is involved in plant growth and development as well as adaptation to environmental stresses. We identified the cold- and pathogen-responsive AOC2 gene from Medicago sativa subsp. falcata (MfAOC2) and its homolog MtAOC2 from Medicago truncatula. Heterologous expression of MfAOC2 in M. truncatula enhanced cold tolerance and resistance to the fungal pathogen Rhizoctonia solani, with greater accumulation of JA and higher transcript levels of JA downstream genes than in wild-type plants. By contrast, mutation of MtAOC2 reduced cold tolerance and pathogen resistance, with less accumulation of JA and lower transcript levels of JA downstream genes in the aoc2 mutant than in wild-type plants. The aoc2 phenotype and low levels of cold-responsive C-repeat-binding factor (CBF) transcripts could be rescued by expressing MfAOC2 in aoc2 plants or exogenous application of methyl jasmonate. Compared with wild-type plants, higher levels of CBF transcripts were observed in lines expressing MfAOC2 but lower levels of CBF transcripts were observed in the aoc2 mutant under cold conditions; superoxide dismutase, catalase, and ascorbate-peroxidase activities as well as proline concentrations were higher in MfAOC2-expressing lines but lower in the aoc2 mutant. These results suggest that expression of MfAOC2 or MtAOC2 promotes biosynthesis of JA, which positively regulates expression of CBF genes and antioxidant defense under cold conditions and expression of JA downstream genes after pathogen infection, leading to greater cold tolerance and pathogen resistance.

Alfalfa is one of the most important legume forages in the world and contributes greatly to the improvement of ecosystems, nutrition, and food security. Diseases caused by pathogens and pests severely restrict the production of alfalfa. Breeding resistant varieties is the most economical and effective strategy for the control of alfalfa diseases and pests, and the key to breeding resistant varieties is to identify important resistance genes. Plant innate immunity is the theoretical basis for identifying resistant genes and breeding resistant varieties. In recent years, the framework of plant immunity theory has been gradually formed and improved, and considerable progress has been made in the identification of alfalfa resistance genes and the revelation of the related mechanisms. In this review, we summarize the basic theory of plant immunity and identify alfalfa resistance genes to different pathogens and insects and resistance mechanisms. The current situation, problems, and future prospects of alfalfa resistance research are also discussed. Breeding resistant cultivars with effective resistance genes, together with other novel plant protection technologies, will greatly improve alfalfa production.

Oomycete pathogens can secrete hundreds of effectors into plant cells to interfere with the plant immune system during infection. Here, we identified a RXLR effector protein from the most destructive pathogen of litchi (Litchi chinensis Sonn.), Peronophythora litchii, and named it Peronophythora litchii Avirulence homolog 202 (PlAvh202). PlAvh202 could suppress cell death triggered by Infestin 1 (INF1) or Avirulence protein 3a/Receptor protein 3a (Avr3a/R3a) in Nicotiana benthamiana and was essential for P. litchii virulence. In addition, PlAvh202 suppressed plant immune responses and promoted the susceptibility of N. benthamiana to Phytophthora capsici. Further research revealed that PlAvh202 could suppress ethylene (ET) production by targeting and destabilizing plant S-adenosyl-L-methionine synthetase (SAMS), a key enzyme in the ET biosynthesis pathway, in a 26S proteasome-dependent manner without affecting its expression. Transient expression of LcSAMS3 induced ET production and enhanced plant resistance, whereas inhibition of ET biosynthesis promoted P. litchii infection, supporting that LcSAMS and ET positively regulate litchi immunity towards P. litchii. Overall, these findings highlight that SAMS can be targeted by the oomycete RXLR effector to manipulate ET-mediated plant immunity.

Although the functions of most protein effectors secreted by fungi are not predictable, they are known to modulate host immunity and facilitate infection. Fusarium graminearum is one of the 10 most abundant plant pathogenic fungi worldwide. To date, few effector proteins secreted by F. graminearum have been identified. In this study, we screened a putative effector protein Fg62 from proteins that contain signal peptides and unknown functional domains in F. graminearum secretome. Fg62 expression was highly upregulated during the early stages of F. graminearum infection, and its deletion reduced F. graminearum virulence in wheat and soybean. Transient expression of Fg62 or the recombinant protein led to plant cell death in Nicotiana benthamiana, and the signal peptide of Fg62 was required for cell death activation. Fg62 homologs are distributed in two species of the F. sambucinum species complex, which are also able to induce cell death in N. benthamiana. Fg62 activated plant immunity by increasing the expression of defense-related genes, and the recombinant Fg62 protein induced plant resistance to various pathogens. Overall, our results revealed that the extracellular effector Fg62 contributes to both pathogen virulence and plant immunity induction, providing new avenues for the development of environmentally friendly crop disease control strategies utilizing nonpolluting immune-inducing factors.

The wall-associated kinases (WAKs) can perceive and transmit extracellular signals as one kind of unique receptor-like kinases (RLKs) involved in the regulation of cell expansion, pathogen resistance and abiotic stress tolerance. To understand their potential roles and screen some key candidates in Medicago truncatula (M. truncatula), genome-wide identification and characterization of MtWAKs were conducted in this study. A total of 54 MtWAK genes were identified and classified into four groups based on their protein domains. They were distributed on all chromosomes, while most of them were clustered on chromosome 1 and 3. The synteny analysis showed that 11 orthologous pairs were identified between M. truncatula and Arabidopsis thaliana (A. thaliana) and 31 pairs between M. truncatula and Glycine max (G. max). The phylogenetic analysis showed that WAK-RLKs were classified into five clades, and they exhibited a species-specific expansion. Most MtWAK-RLKs had similar exon–intron organization and motif distribution. Multiple cis-acting elements responsive to phytohormones, stresses, growth and development were observed in the promoter regions of MtWAK-RLKs. In addition, the expression patterns of MtWAK-RLKs varied with different plant tissues, developmental stages and biotic and abiotic stresses. Interestingly, plasm membrane localized MtWAK24 significantly inhibited Phytophthora infection in tobacco. The study provides valuable information for characterizing the molecular functions of MtWAKs in regulation of plant growth, development and stress tolerance in legume plants.