Plasmodiophora brassicae-Triggered Cell Enlargement and Loss of Cellular Integrity in Root Systems Are Mediated by Pectin Demethylation

Abstract

Gall formation on the belowground parts of plants infected with Plasmodiophora brassicae is the result of extensive host cellular reprogramming. The development of these structures is a consequence of increased cell proliferation followed by massive enlargement of cells colonized with the pathogen. Drastic changes in cellular growth patterns create local deformities in the roots and hypocotyl giving rise to mechanical tensions within the tissue of these organs. Host cell wall extensibility and recomposition accompany the growth of the gall and influence pathogen spread and also pathogen life cycle progression. Demethylation of pectin within the extracellular matrix may play an important role in P. brassicae-driven hypertrophy of host underground organs. Through proteomic analysis of the cell wall, we identified proteins accumulating in the galls developing on the underground parts of Arabidopsis thaliana plants infected with P. brassicae. One of the key proteins identified was the pectin methylesterase (PME18); we further characterized its expression and conducted functional and anatomic studies in the knockout mutant and used Raman spectroscopy to study the status of pectin in P. brassicae-infected galls. We found that late stages of gall formation are accompanied with increased levels of PME18. We have also shown that the massive enlargement of cells colonized with P. brassicae coincides with decreases in pectin methylation. In pme18-2 knockout mutants, P. brassicae could still induce demethylation; however, the galls in this line were smaller and cellular expansion was less pronounced. Alteration in pectin demethylation in the host resulted in changes in pathogen distribution and slowed down disease progression. To conclude, P. brassicae-driven host organ hypertrophy observed during clubroot disease is accompanied by pectin demethylation in the extracellular matrix. The pathogen hijacks endogenous host mechanisms involved in cell wall loosening to create an optimal cellular environment for completion of its life cycle and eventual release of resting spores facilitated by degradation of demethylated pectin polymers.

The Plant Invertase/Pectin Methylesterase Inhibitor Superfamily

“…The PMEI region of AtPMEI-PME3 could protect pectin in the golgi apparatus from premature de-methylesterification. CW remodeling of Arabidopsis root cells is also exploited by the obligate biotrophic Plasmodiophora brassicae, a protist pathogen that causes clubroot disease in brassica species (Stefanowicz et al, 2021). A pectin de-methylesterification mediated by AtPMEI-PME18 can favor the release of resting spores of the fungus.…”

What Can We Learn from -Omics Approaches to Understand Clubroot Disease?

“…Gene families were investigated in such large data approaches such as UDP-glucosyltransferases [ 97 ], sugar transporter families, including STP and SWEET [ 98 , 99 ], invertase [ 100 ], chitinase [ 101 ], auxin conjugating GH3 genes [ 102 ], pectin-related genes [ 103 ], as well as functional pathways, e.g., flavonoids [ 38 , 104 ] or glucosinolates [ 105 ]. Other defense-related pathways including hormones will be discussed below (see Section 5.4 ).…”

“…Interestingly, an expansin was also on the gene list that was altered after the overexpression of the P. brassicae effector PBZF1 in A. thaliana [ 77 ]. Other cell-wall modifying proteins are encoded by pectin methylesterases, which were analyzed for their role during clubroot development [ 103 ] based on the dataset from [ 80 ]. Functional analyses confirmed that the club development in a pme18 mutant was altered, namely cell wall enlargement was reduced.…”

Biochemical characterization of Pectin Methylesterase Inhibitor 3 from Arabidopsis thaliana

“…Pectins are galacturonic acid (GalA) - containing polysaccharides among which the linear polymer homogalacturonan (HG) is the most abundant matrix polysaccharide, for instance representing 20% of the total cell wall polysaccharides, in Arabidopsis thaliana leaves (Zablackis et al, 1995) or even 50% in onion epidermis cells (Wilson et al, 2021). In vivo studies show that the metabolism of HG plays a critical role in the control of cell expansion and plant morphogenesis (Andres-Robin et al, 2018; K. T. Haas et al, 2020; Jobert et al, 2021; Peaucelle et al, 2015, 2008; Phyo et al, 2017; Qi et al, 2017; Stefanowicz et al, 2021; Wachsman et al, 2020) but the underlying mechanisms are still poorly understood.…”