Abstract
Plasmodiophora brassicae is a soil borne pathogen and the causal agent of clubroot, a devastating disease of Brassica crops. The pathogen lives inside roots, and hijacks nutrients from the host plants. It is suggested that clubroot galls created an additional nutrient sink in infected roots. However, the molecular mechanism underlying P. brassicae infection and sugar transport is unclear. Here, we analyzed sugar contents in leaves and roots before and after P. brassicae infection using a pair of Chinese cabbage near-isogenic lines (NILs), carrying either a clubroot resistant (CR) or susceptible (CS) allele at the CRb locus. P. brassicae infection caused significant increase of glucose and fructose contents in the root of CS-NIL compared to CR-NIL, suggesting that sugar translocation and P. brassicae growth are closely related. Among 32 B. rapa SWEET homologs, several BrSWEETs belonging to Clade I and III were significantly up-regulated, especially in CS-NIL upon P. brassicae infection. Moreover, Arabidopsis sweet11 mutant exhibited slower gall formation compared to the wild-type plants. Our studies suggest that P. brassicae infection probably triggers active sugar translocation between the sugar producing tissues and the clubbed tissues, and the SWEET family genes are involved in this process.
Highlights
Plasmodiophora brassicae causes clubroot disease, a most challenging disease of the Brassicaceae family worldwide (Dixon, 2009)
The contents of sugars had no significant differences between CS and clubroot resistant (CR) near-isogenic lines (NILs) without P. brassicae inoculation, but showed variance after infection (Figure 1)
The contents of glucose were dramatically increased from 12 to 49% in the roots of the CS-NIL from 1 to 9 dpi, and much less increase of the glucose contents occurred in the roots of the CR-NIL (Figures 1A,B)
Summary
Plasmodiophora brassicae causes clubroot disease, a most challenging disease of the Brassicaceae family worldwide (Dixon, 2009). P. brassicae belongs to an obligate biotrophic protist in the Plasmodiophorids within the Rhizaria. The life cycle of P. brassicae is divided into three stages: the survival of resting spores in soil, the root hair infection (primary infection), and the cortex infection (secondary infection) (Schwelm et al, 2015; Schuller and Ludwig-Müller, 2016). After infection of the plant roots, P. brassicae colonization leads to swelling roots and gall formation, eventually inhibiting roots to uptake nutrients and water from the soil. As one of the most economically serious diseases in Brassica crops, clubroot resulted in significant agricultural losses.
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