**5. Host–pathogen interactions**

### **a. During resting spore germination**

In soil environment, host–pathogen interactions begin at the early seedling stage when host plant root exudates are present, which induces germination of resting spores [41] and releasing of primary zoospores. The role of root exudates as stimulants for resting spore germination was examined and confirmed in different research studies [42–44]. In contrast, substantial studies by Kowalski and Bochow [45] reported that the stimulant effect for germination is not confined to the specific host of *P. brassicae*. This finding was supported by the evidence of root exudates from Brassica host (broccoli) and non-Brassica host (ryegrass), both stimulated spore germination [46]. Studies also reported that some specific stimulants such as caffeic acid, coumalic acid, corilagin, and others could stimulate resting spore germination in Chinese cabbage [47, 48]. All these studies suggest that the Brassica species have unique root charac‐ teristics which permit pathogen invasion and subsequent infection for the disease develop‐ ment. Resting spore germination is observed stimulated by root exudates in other species, but zoospores could not establish primary infection.

#### **b. During disease development and gall formation**

Earlier studies reported that regulation of phytohormones plays an importance role in the formation of massive galls on roots. Rapid increase in both cytokinin and auxin biosynthesis was observed during secondary infection and gall formation in the infected roots of *B. rapa* [49–51]. Brassica species contain high aliphatic, indole, and aromatic glucosinolates may play a vital role in disease development and gall formation because conversion of indole-3-methyl glucosinolate to indole-3-acetonitrile is thought to be the main pathway of auxin synthesis in infected root tissues [52, 53]. Studies suggested that induction in nitrilase activity which cleavages indole-3-acetonitrole to indole-3-acetic acid occurred in infected roots [54]. Elevated cytokinin biosynthesis was also observed in secondary plasmodia during gall formation [51, 55]. Pedras et al. [56] reported production of 45 different metabolites in *B. napus* infected by *P. brassicae*, which suggested that canola roots under biotic stress produce a complex blend of phytoalexins and other antimicrobial metabolites as defensive mechanisms. However, limited information about metabolomic interaction between host and pathogen is available during gall formation in both susceptible and complete clubroot resistance disease reactions.

To enhance our understanding of the pathogenicity factors of *P. brassicae* causing clubroot disease on different Brassica hosts, several molecular techniques and tools are employed to determine *P. brassicae* genome size, structure, and number of possible functional genes in the whole genome. Several studies reported use of pulse-field gel electrophoresis (PFGE) to determine the karyotypes for *P. brassicae*. Ito et al. [32] used sheroplasts and differentiated 13 chromosomal bands in the range of 1.9 Mb to 750 kb. Bryan et al. [33] used isolated plasmodia and differentiated six chromosomal bands in the range of 1.7 Mb to 680 kb. Similarly, Graf et al. [34] distinguished 16 chromosomal bands in the range of 2.2 Mb to 680 kb. Based on these

studies, it is estimated that the *P. brassicae* total genome size can be 18–20.3 Mb [35].

resistance breeding intriguing.

8 Plants for the Future

**5. Host–pathogen interactions**

**a. During resting spore germination**

zoospores could not establish primary infection.

**b. During disease development and gall formation**

On the other hand, several molecular marker techniques were employed to investigate virulent pattern of the *P. brassicae* population derived from single-spore isolate or field isolates [36–38]; however, the number of distinguishing patterns were very low and that could not correlate with virulence patterns. In continuous efforts, two RAPD markers [39] and one SCAR marker [40] were identified, which correlate to isolates of pathotype 1. Yet there are no sets of molecular markers that can distinguish other pathotypes from field isolates which make clubroot-

In soil environment, host–pathogen interactions begin at the early seedling stage when host plant root exudates are present, which induces germination of resting spores [41] and releasing of primary zoospores. The role of root exudates as stimulants for resting spore germination was examined and confirmed in different research studies [42–44]. In contrast, substantial studies by Kowalski and Bochow [45] reported that the stimulant effect for germination is not confined to the specific host of *P. brassicae*. This finding was supported by the evidence of root exudates from Brassica host (broccoli) and non-Brassica host (ryegrass), both stimulated spore germination [46]. Studies also reported that some specific stimulants such as caffeic acid, coumalic acid, corilagin, and others could stimulate resting spore germination in Chinese cabbage [47, 48]. All these studies suggest that the Brassica species have unique root charac‐ teristics which permit pathogen invasion and subsequent infection for the disease develop‐ ment. Resting spore germination is observed stimulated by root exudates in other species, but

Earlier studies reported that regulation of phytohormones plays an importance role in the formation of massive galls on roots. Rapid increase in both cytokinin and auxin biosynthesis was observed during secondary infection and gall formation in the infected roots of *B. rapa* [49–51]. Brassica species contain high aliphatic, indole, and aromatic glucosinolates may play a vital role in disease development and gall formation because conversion of indole-3-methyl glucosinolate to indole-3-acetonitrile is thought to be the main pathway of auxin synthesis in
