The Modification of Plant Cell Wall Polysaccharides in Potato Plants during Pectobacterium atrosepticum-Caused Infection

In Vitro Antibacterial Activity and Mode of Action of Piper betle Extracts against Soft Rot Disease-Causing Bacteria

Soft rot disease affects a range of crops in the field and also during transit and storage, resulting in significant yield losses and negative economic impacts. This study evaluated the in vitro antibacterial activities and mode of action of Piper betle extracts against the soft rot disease-causing bacteria, Erwinia caratovora subsp. caratovora (ECC). Dried leaves of P. betle were extracted with water, ethanol, and hexane solvents and evaluated for their antibacterial activity. The results showed the highest antibacterial activity against ECC in the ethanol extract, followed by hexane and water extracts with minimum inhibitory concentration (MIC) 1.562, 6.25, and more than 12.50 mg/mL, respectively. The time-kill assay indicated a bactericidal mode of action. ECC growth was destroyed within 6 and 8 hours after treatment with the ethanol extract at 4-fold MIC and 2-fold MIC, respectively. The ethanol extract of P. betle showed promising activity against ECC, with the potential for further development as a novel alternative treatment to control phytobacteria.

A Switch from Latent to Typical Infection during Pectobacterium atrosepticum-Tobacco Interactions: Predicted and True Molecular Players

Phytopathogenic microorganisms, being able to cause plant diseases, usually interact with hosts asymptomatically, resulting in the development of latent infections. Knowledge of the mechanisms that trigger a switch from latent to typical, symptomatic infection is of great importance from the perspectives of both fundamental science and disease management. No studies to date have compared, at the systemic molecular level, the physiological portraits of plants when different infection types (typical and latent) are developed. The only phytopathogenic bacterium for which latent infections were not only widely described but also at least fluently characterized at the molecular level is Pectobacterium atrosepticum (Pba). The present study aimed at the comparison of plant transcriptome responses during typical and latent infections caused by Pba in order to identify and then experimentally verify the key molecular players that act as switchers, turning peaceful plant-Pba coexistence into a typical infection. Based on RNA-Seq, we predicted plant cell wall-, secondary metabolism-, and phytohormone-related genes whose products contributed to the development of the disease or provided asymptomatic plant-Pba interactions. By treatment tests, we confirmed that a switch from latent to typical Pba-caused infection is determined by the plant susceptible responses mediated by the joint action of ethylene and jasmonates.

Host plant physiological transformation and microbial population heterogeneity as important determinants of the Soft Rot Pectobacteriaceae-plant interactions

Pectobacterium and Dickeya species belonging to the Soft Rot Pectobacteriaceae (SRP) are one of the most devastating phytopathogens. They degrade plant tissues by producing an arsenal of plant cell wall degrading enzymes. However, SRP-plant interactions are not restricted to the production of these "brute force" weapons. Additionally, these bacteria apply stealth behavior related to (1) manipulation of the host plant via induction of susceptible responses and (2) formation of heterogeneous populations with functionally specialized cells. Our review aims to summarize current knowledge on SRP-induced plant susceptible responses and on the heterogeneity of SRP populations. The review shows that SRP are capable of adjusting the host's hormonal balance, inducing host-mediated plant cell wall modification, promoting iron assimilation by the host, stimulating the accumulation of reactive oxygen species and host cell death, and activating the synthesis of secondary metabolites that are ineffective in limiting disease progression. By this means, SRP facilitate host plant susceptibility. During host colonization, SRP populations produce various functionally specialized cells adapted for enhanced virulence, increased resistance, motility, vegetative growth, or colonization of the vascular system. This enables SRP to perform self-contradictory tasks, which benefits a population's overall fitness in various environments, including host plants. Such stealthy tactical actions facilitate plant-SRP interactions and disease progression.

Metagenomic insights into the changes in the rhizosphere microbial community caused by the root-knot nematode Meloidogyne incognita in tobacco

Root-knot nematode (RKN) disease is a destructive soil disease that affects crop health and causes huge losses in crop production. To explore the relationships between soil environments, rhizobacterial communities, and plant health, rhizosphere bacterial communities were analyzed using metagenomic sequencing in tobacco samples with different grades of RKN disease. The results showed that the community structure and function of the plant rhizosphere were significantly correlated to the RKN disease. RKN density and urease content were key factors affecting the rhizosphere bacterial community. Urease accelerated the catabolism of urea and led to the production of high concentrations of ammonia, which directly suppressed the development of RKNs or by improving the nutritional and growth status of microorganisms that were antagonistic to RKNs. Further experiments showed that the suppression role of ammonia should be attributed to the direct inhibition of NH₃. The bacterial members that were positively correlated with RKN density, contained many plant cell wall degrading enzymes, which might destroy plant cell walls and promote the colonization of RKN in tobacco roots. The analysis of metatranscriptome and metabolism demonstrated the role of these cell wall degrading enzymes. This study offers a comprehensive insight into the relationships between RKNs, bacteria, and soil environmental factors and provides new ideas for the biological control of RKNs.

The Role of Pectobacterium atrosepticum Exopolysaccharides in Plant-Pathogen Interactions

The phytopathogenic bacterium Pectobacterium atrosepticum (Pba), one of the members of the soft rot Pectobacteriaceae, forms biofilm-like structures known as bacterial emboli when colonizing the primary xylem vessels of the host plants. The initial extracellular matrix of the bacterial emboli is composed of the host plant's pectic polysaccharides, which are gradually substituted by the Pba-produced exopolysaccharides (Pba EPS) as the bacterial emboli "mature". No information about the properties of Pba EPS and their possible roles in Pba-plant interactions has so far been obtained. We have shown that Pba EPS possess physical properties that can promote the maintenance of the structural integrity of bacterial emboli. These polymers increase the viscosity of liquids and form large supramolecular aggregates. The formation of Pba EPS aggregates is provided (at least partly) by the acetyl groups of the Pba EPS molecules. Besides, Pba EPS scavenge reactive oxygen species (ROS), the accumulation of which is known to be associated with the formation of bacterial emboli. In addition, Pba EPS act as suppressors of the quantitative immunity of plants, repressing PAMP-induced reactions; this property is partly lost in the deacetylated form of Pba EPS. Overall, our study shows that Pba EPS play structural, protective, and immunosuppressive roles during Pba-plant interactions and thus should be considered as virulence factors of these bacteria.

Plant susceptible responses: the underestimated side of plant-pathogen interactions

Plant susceptibility to pathogens is usually considered from the perspective of the loss of resistance. However, susceptibility cannot be equated with plant passivity since active host cooperation may be required for the pathogen to propagate and cause disease. This cooperation consists of the induction of reactions called susceptible responses that transform a plant from an autonomous biological unit into a component of a pathosystem. Induced susceptibility is scarcely discussed in the literature (at least compared to induced resistance) although this phenomenon has a fundamental impact on plant-pathogen interactions and disease progression. This review aims to summarize current knowledge on plant susceptible responses and their regulation. We highlight two main categories of susceptible responses according to their consequences and indicate the relevance of susceptible response-related studies to agricultural practice. We hope that this review will generate interest in this underestimated aspect of plant-pathogen interactions.