Long-Read Genome Sequence of the Sugar Beet Rhizosphere Mycoparasite Pythium oligandrum.

Pythium oligandrum is a soil born free living oomycete able to parasitize fungi and oomycetes prey, including important plant and animals pathogens. Pythium oligandrum can colonize endophytically the root tissues of diverse plants where it induces plant defenses. Here we report the first long-read genome sequencing of a P. oligandrum strain sequenced by PacBio technology. Sequencing of genomic DNA loaded onto six SMRT cells permitted the acquisition of 913,728 total reads resulting in 112X genome coverage. The assembly and polishing of the genome sequence yielded180 contigs (N50 = 1.3 Mb; L50 = 12). The size of the genome assembly is 41.9 Mb with a longest contig of 2.7 Mb and 15,007 predicted protein-coding genes among which 95.25% were supported by RNAseq data, thus constituting a new Pythium genome reference. This data will facilitate genomic comparisons of Pythium species that are commensal, beneficial or pathogenic on plant, or parasitic on fungi and oomycete to identify key genetic determinants underpinning their diverse lifestyles. In addition comparison with plant pathogenic or zoopathogenic species will illuminate genomic adaptations for pathogenesis toward widely diverse hosts.

Pectin Demethylesterification Generates Platforms that Anchor Peroxidases to Remodel Plant Cell Wall Domains.

Plant cell walls are made of polysaccharidic-proteinaceous complex matrices. Molecular interactions governing their organization remain understudied. We take advantage of the highly dynamic cell walls of Arabidopsis seed mucilage secretory cells to propose a hierarchical multi-molecular interaction model within a cell wall domain. We show that the PECTINMETHYLESTERASE INHIBITOR6 activity creates a partially demethylesterified pectin pattern acting as a platform allowing positioning of PEROXIDASE36 in a remote primary cell wall domain during early development. This allows triggering the loosening of this domain during later development, in turn leading to proper physiological function upon mature seed imbibition and germination. We anticipate that this pioneer example of molecular scaffold within a cell wall domain is more widespread through other combinations of the individual molecular players all belonging to large multigenic families. These results highlight the role of cell wall polysaccharide-protein interactions in the organization of cell wall domains.

Cadmium-induced changes in antioxidative systems and differentiation in roots of contrasted Medicago truncatula lines.

Defense pathways and stress responses induced under Cd stress were illustrated in roots of hydroponically grown Medicago truncatula seedlings. Actually, the ascorbate-glutathione and antioxidative system, secondary metabolism events including peroxidases, phenolic compounds, and lignification launching, and developmental modifications were described. Cd (100 µM) initially increased reactive oxygen species, enhanced antioxidative (total SOD, CAT, and PRX) and ascorbate-glutathione-related metabolism enzymes (APX and MDAR), except in A17 and TN1.11. In agreement with peroxidase enhancement, physiological measurement and in situ observation illustrated soluble phenolic compound accumulation under Cd treatment. However, lignification was restricted to recently created protoxylem elements established in the root tip area, usually constituting the elongation zone. Cell death was increased. In the absence of necrotic reactions, developmental changes including lignin deposition, increase in cellulose and pectin contents, intercellular meatus, and condensed and deformed hairs were noticed in Cd-treated roots.

Deciphering the route of Ralstonia solanacearum colonization in Arabidopsis thaliana roots during a compatible interaction: focus at the plant cell wall.

The compatible interaction between the model plant, Arabidopsis thaliana, and the GMI1000 strain of the phytopathogenic bacterium, Ralstonia solanacearum, was investigated in an in vitro pathosystem. We describe the progression of the bacteria in the root from penetration at the root surface to the xylem vessels and the cell type-specific, cell wall-associated modifications that accompanies bacterial colonization. Within 6 days post inoculation, R. solanacearum provoked a rapid plasmolysis of the epidermal, cortical, and endodermal cells, including those not directly in contact with the bacteria. Plasmolysis was accompanied by a global degradation of pectic homogalacturonanes as shown by the loss of JIM7 and JIM5 antibody signal in the cell wall of these cell types. As indicated by immunolabeling with Rsol-I antibodies that specifically recognize R. solanacearum, the bacteria progresses through the root in a highly directed, centripetal manner to the xylem poles, without extensive multiplication in the intercellular spaces along its path. Entry into the vascular cylinder was facilitated by cell collapse of the two pericycle cells located at the xylem poles. Once the bacteria reached the xylem vessels, they multiplied abundantly and moved from vessel to vessel by digesting the pit membrane between adjacent vessels. The degradation of the secondary walls of xylem vessels was not a prerequisite for vessel colonization as LM10 antibodies strongly labeled xylem cell walls, even at very late stages in disease development. Finally, the capacity of R. solanacearum to specifically degrade certain cell wall components and not others could be correlated with the arsenal of cell wall hydrolytic enzymes identified in the bacterial genome.