Integrative transcriptome analysis revealed the pathogenic molecular basis of Rhizoctonia solani AG-3 TB at three progressive stages of infection

Abstract

Rhizoctonia solani has a broad host range and results in significant losses in agricultural production. Here, an integrated transcriptomic analysis was performed to reveal the critical genes responsible for the pathogenesis of R. solani AG-3 TB on Nicotiana tabacum at different infection stages. The results showed that various differential expressed genes (DEGs) were enriched in fatty acid metabolism, amino sugar, carbon metabolism, and cellular carbohydrate biosynthetic process at the early (6–12 hpi), middle (24–36 hpi), and late stage (48–72 hpi) of infection. Specifically, several critical genes such as shikimate kinase that were involved in the biosynthesis of an important fungal toxin, phenylacetic acid (PAA) showed markedly increase at 24 hpi. Additionally, the genes expression levels of carbohydrate-active enzymes (CAZymes) and cell wall degrading enzymes (CWDEs) were significantly increased at the late infection stage. Furthermore, we identified 807 potential secreted proteins and 78 small cysteine-rich proteins, which may function as fungal effectors and involved in the pathogenicity. These results provide valuable insights into critical and potential genes as well as the pathways involved in the pathogenesis of R. solani AG-3 TB.

Identification of 3-Methoxyphenylacetic Acid as a Phytotoxin, Produced by Rhizoctonia solani AG-3 TB

“…PAA as a toxin compound was first reported from the metabolites of R. solani in 1963 [ 13 ], and the genome prediction and transcriptome analysis of the R. solani AG-1 IA subgroup indicated that PAA may be produced under the five crucial enzymes, including: shikimate kinase, 3-phosphoshikimate 1-carboxyvinyltransferase (EPSP synthase), chorismate synthase, prephenate dehydrogenase, and prephenate dehydratase [ 30 , 31 ]. Other studies of the secondary metabolite gene from R. solani AG-3 TB also revealed that the gene expression level of critical enzymes in PAA synthesis increases during pathogen infection, especially in the early stage (6–12 hpi) and the middle stage (24–36 hpi) [ 32 ]. In addition, studies indicating that PAA produced by R. solani has harmful effects on the host are unavailable, but it has been found to play an important role in the growth of mycelium, while the PAA derivatives can cause the development of necrotic lesions in rice, rape, and potato [ 13 , 15 , 19 , 20 , 33 ].…”

“…In this study, 3-MOPAA was revealed to be a potential phytotoxin in R. solani AG-3 TB. Additionally, PAA can be produced during R. solani AG-3 TB infection [ 32 ]. Therefore, some necessary connection must exist between PAA and 3-MOPAA, and we propose that PAA plays an important role in 3-MOPAA synthesis and that it is first produced by the critical five enzymes; then, 3-MOPAA is finally synthesized by special tailoring enzymes, such as hydroxylases and methyltransferases, during R. solani AG-3 TB infection.…”

“…: NC89, a commonly cultivated susceptible variety). Cotton with sterile water was used for moisturizing, and the symptom was observed after inoculating the acupuncture part at 24, 48, and 72 hpi [ 32 ]. Four leaves of each tobacco plant were inoculated, and each leaf was inoculated at four acupuncture points.…”

RNA interference‐based exogenous double‐stranded RNAs confer resistance to Rhizoctonia solaniAG‐3 on Nicotiana tabacum

“…One leaf of each plant was inoculated at two acupuncture points, respectively, and the necrotic lesion area was measured at 5 dpi. 48 Three plants were used in each treatment group, and three replicates were performed. The relative gene expression levels of endoPGs and RPMK1 were analyzed by quantitative real-time PCR (qRT-PCR).…”

“…The gene expression of cell-wall degrading enzymes (CWDEs) of R. solani AG-3 TB is significantly increased in the late infection stage [48-72 h postinoculation (hpi)] on leaves of N. tabacum. 48 Endopolygalacturonases (endoPGs) are CWDEs capable of degrading pectin and play key roles during fungal infection in plants. The highly pathogenic strain YC-9 of R. solani AG-3 TB produces endoPGs, which degrade the pectin skeleton structure of plant cell walls, and therefore, endoPGs is considered as one of the pathogenic factors of tobacco target spot disease.…”