Abstract
Glucosinolate (GSL) is associated with clubroot disease, which is caused by the obligate biotrophic protist Plasmodiophora brassicae. Due to the complicated composition of GSLs, their exact role in clubroot disease development remains unclear. By investigating clubroot disease resistance in cruciferous plants and characterizing the GSL content in seeds, we can determine if clubroot disease development is related to the components of GSLs. The difference in the infection process between Matthiola incana L. (resistant) and Brassica napus L. (susceptible) was determined. Root hair infection was definitely observed in both resistant and susceptible hosts, but no infection was observed during the cortical infection stage in resistant roots; this finding was verified by molecular detection of P. brassicae via PCR amplification at various times after inoculation. Based on the time course detection of the contents and compositions of GSLs after P. brassicae inoculation, susceptible roots exhibited increased accumulation of aliphatic, indolic, and aromatic GSLs in B. napus, but only aromatic GSLs were significantly increased in M. incana. Gluconapin, which was the main aliphatic GSL in B. napus and present only in B. napus, was significantly increased during the secondary infection stage. Quantification of the internal jasmonic acid (JA) concentration showed that both resistant and susceptible plants exhibited an enhanced level of JA, particularly in susceptible roots. The exogenous JA treatment induced aliphatic GSLs in B. napus and aromatic GSLs in M. incana. JA-induced aromatic GSLs may be involved in the defense against P. brassicae, whereas aliphatic GSLs induced by JA in B. napus likely play a role during the secondary infection stage. Three candidate MYB28 genes regulate the content of aliphatic GSLs identified in B. napus; one such gene was BnMYB28.1, which was significantly increased following both the treatment with exogenous JA and P. brassicae inoculation. In summary, the increased content of JA during the secondary infection stage may induce the expression of BnMYB28.1, which caused the accumulation of aliphatic GSLs in clubroot disease development.
Highlights
Clubroot disease in cruciferous crops, those belonging to the Brassicaceae family, is caused by the obligate biotrophic protist Plasmodiophora brassicae (Dixon, 2009)
High auxin levels are involved in gall formation during the late infection stage, and indolic GSLs are in turn precursors for indole-3-acetic acid (IAA) biosynthesis and are correlated with clubroot disease severity (Ludwig-Muller et al, 1999b; Ludwig-Muller, 2009)
A relatively low level of the pathogen was detected in resistant roots, and the pathogen level decreased after 21 days after inoculation (DAI), suggesting that the infection in M. incana was at the root hair infection stage
Summary
Clubroot disease in cruciferous crops, those belonging to the Brassicaceae family, is caused by the obligate biotrophic protist Plasmodiophora brassicae (Dixon, 2009). The glucosinolates (GSLs), which are synthetized from amino acids and sugars, compose one of the largest known groups of secondary metabolites in the Brassicaceae family (Ishida et al, 2014). The GSLs are classified into three groups (aliphatic, aromatic, and indolic GSL) according to their amino acid precursors (Wittstock and Halkier, 2002). In Brassica cultivars and A. thaliana mutants, clubroot disease severity is correlated with the content of indole GSLs, which are considered precursors for auxin biosynthesis (Ludwig-Muller et al, 1999b). High auxin levels are involved in gall formation during the late infection stage, and indolic GSLs are in turn precursors for indole-3-acetic acid (IAA) biosynthesis and are correlated with clubroot disease severity (Ludwig-Muller et al, 1999b; Ludwig-Muller, 2009)
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