Research progress in studies on the downy blight disease in litchi

Abstract: Litchi (Litchi chinensis Sonn.) is one of the most important tropical and subtropical fruits in China. However, litchi downy blight caused by Peronophythora litchii greatly threatens the healthy development of Chinese litchi industry, which is the most serious disease in litchi production, storage and transportation. Litchi downy blight damages tender leaves, twigs, flowers and fruits, lead-ing to enormous yield loss in the epidemic years. Litchi downy blight usually occurs in April and May. At this time, the weather humidity and rainfall are high, which is suitable for the growth and in-fection of P. litchii. If it rains continuously for two days, it could cause epidemic of this disease. Usu-ally, oospores is the source of primary infection, while sporangia and zoospores can act as the sources of secondary infection. P. litchii always produces sporangia and zoospores rapidly, so serious second-ary infection will happen when the weather is suitable for this disease. P. litchii belongs to oomycetes that are diploid organisms, and the genome sequencing results of P. litchii was officially published in 2016, with a genome size of about 58 MB and a GC content of 49%. Bioinformatics analysis showed that there were 30 NLPs [necrosis- and ethylene-inducing protein 1 (Nep1)-like proteins], 14 CRN ef-fector molecules (crinkler protein) and 245 specific RXLR effectors in P. litchii. The genome se-quencing and CRISPR/Cas9 genome editing system have greatly accelerated the research on the mo-lecular pathogenesis of P. litchii. In the past five years, it has made a series of progresses on molecu-lar mechanism of zoosporogenesis, oospore formation and pathogenesis of P. litchii. In the aspect of functional genomics, A Puf RNA-binding protein encoding gene PlM90 plays a key role in sexual and asexual differentiation of P. litchii; M90 is conserved in oomycete and up-regulated in oospores and sporangia in P. litchii and P. infestans. The silencing of PlMAPK10 decreases the growth rate,production of sporangia, activity of extracellular laccase and the pathogenicity of P. litchii. The heat shock transcription factor PsHSF1 of Phytophthora sojae, also regulates the pathogenicity and laccas-es activities of P. sojae. However, the relationship between MAPK10 and HSF1 in oomycetes needs further studies. Kong reported that PlBZP32 was involved in the oxidative response, sporangium pro-duction, cyst germination and plant infection of P. litchii. However, it is not clear whether the regula-tion of PlBZP32 on sporangium production is directly related to PlMAPK10. Wang found that there were two hybrid histidine kinases (PlHK1, PlHK2) and one response regulator (PlRR1) in the two-component signaling pathway of P. litchii. Among them, PlHK1 and PlHK2 fused an additional phos-phate domain at the C-terminal. These three genes were up-regulated in the infection stage of P. li-tchii and responded to oxidative stress and osmotic stress at transcriptional level. In the functional studies of effector proteins, Kong knocked out the pectin acetylesterase gene PAE5 of P. litchii through CRISPR/Cas9 technology. Phenotypic analysis of the PAE5 mutants showed that this gene was involved in the infection process of P. litchii, and the ectopic expression of PAE5 in tobacco could promote the infection of Phytophthora capsici on tobacco, indicating that PlPAE5 could sup-press the resistance of plants and promote infection. Situ obtained three cell death-inducing RXLR ef-fectors, Avh23, Avh133 and Avh142, through large-scale screening by transient expression system in Benthamiana thaliana. Among them, the knockout mutants of PlAvh142 significantly decreased the virulence of P. litchii, while overexpression of PlAvh142 enhanced the virulence of P. litchii. This indicated that PlAvh142 had dual functions: on one hand, it can be recognized by plants to induce plant immune response, on the other hand, it functioned as a virulence factor. Virus-induced gene silencing assays showed that cell death triggered by PlAvh142 was dependent on the plant transduction compo-nents SGT1 (suppressor of the G2 allele of skp1), RAR1 (require for Mla12 resistance) and HSP90(heat shock protein 90). The interaction between effector and litchi determined the occurrence of dis-ease, however, it is rarely known in litchi-P. litchii interaction. That will be one of the key points for further studies, which will reveal the molecular basis of P. litchii infection and provide potential tar-gets for disease control in the future. Different litchi cultivars showed various resistances to P. litchii.For example, the resistance of Heiye is higher than that of Guiwei. But so far, no resistance gene and high resistant cultivar were reported in litchi. Therefore, the control of litchi diseases must rely on ag-ricultural control, chemicals and biocontrol, but not on resistant cultivar. Several biocontrol bacteria, fungi and actinomyces were found, such as Paenibacillus polymyxa, Pseudomonas, Photobacerium, Eupenicillium brefeldianum, Aspergillus clavatonanicus, Penicillium janthinellum, Talaromyces flavus and Streptomyces sp. DF-5. Further study revealed that several species produced volatile organic com-pounds (VOCs) to suppress the infection of P. litchii. Overall, this paper reviewed the identification,classification, biological characterization and pathogenesis of P. litchii. We also summarized the symptoms, occurrence, epidemic and control strategy of the disease, putting forward the study trend and problems to be solved. We hope this review would provide reference for the further studies on the pathogen and the control of this disease.