Phenolic acid content in potato peel determines natural infection of common scab caused by Streptomyces spp

The Streptomyces scabiei Pathogenicity Factor Thaxtomin A Induces the Production of Phenolic Compounds in Potato Tubers

The phytotoxin thaxtomin A (TA) is the key pathogenicity factor synthesized by the bacteria Streptomyces scabiei, the main causal agent of common scab of potato (Solanum tuberosum L.). TA treatment of potato tuber flesh produces a brown color that was attributed to necrosis. The intensity of TA-induced browning was generally thought to correlate with potato sensitivity to the disease. In this study, we found that TA-induced browning was much more intense in the potato tuber flesh of the common scab moderately resistant variety Russet Burbank (RB) than that observed in tubers of the disease-susceptible variety Yukon Gold (YG). However, there was no significant difference in the level of TA-induced cell death detected in both varieties, suggesting that tubers response to TA does not correlate with the level of sensitivity to common scab. TA-treated potato tuber tissues accumulated significantly higher levels of phenolic compounds than untreated controls, with a higher phenol content detected in RB TA-treated tissues than in those of YG. Browning was associated with a significant induction of the expression of genes of the phenylpropanoid pathway in RB tubers, indicating that TA activated this metabolic pathway. These results suggest that tuber flesh browning induced by TA is due to the accumulation of phenolic compounds. These phenolics may play a role in the protection of potato tubers against S. scabiei.

Genetic Parameters and QTLs for Total Phenolic Content and Yield of Wheat Mapping Population of CSDH Lines under Drought Stress

A doubled haploid population of 94 lines from the Chinese Spring × SQ1 wheat cross (CSDH) was used to evaluate additive and epistatic gene action effects on total phenolic content, grain yield of the main stem, grain number per plant, thousand grain weight, and dry weight per plant at harvest based on phenotypic and genotypic observations of CSDH lines. These traits were evaluated under moderate and severe drought stress and compared with well-watered plants. Plants were grown in pots in an open-sided greenhouse. Genetic parameters, such as additive and epistatic effects, affecting total phenolic content, were estimated for eight year-by-drought combinations. Twenty-one markers showed a significant additive effect on total phenolic content in all eight year-by-drought combinations. These markers were located on chromosomes: 1A, 1B, 2A, 2B, 2D, 3A, 3B, 3D, 4A, and 4D. A region on 4AL with a stable QTL controlling the phenolic content, confirmed by various statistical methods is particularly noteworthy. In all years and treatments, three markers significantly linked to QTLs have been identified for both phenols and yield. Thirteen markers were coincident with candidate genes. Our results indicated the importance of both additive and epistatic gene effects on total phenolic content in eight year-by-drought combinations.

Oxygen-Dependent Globin Coupled Sensor Signaling Modulates Motility and Virulence of the Plant Pathogen Pectobacterium carotovorum

Bacterial pathogens utilize numerous signals to identify the presence of their host and coordinate changes in gene expression that allow for infection. Within plant pathogens, these signals typically include small molecules and/or proteins from their plant hosts and bacterial quorum sensing molecules to ensure sufficient bacterial cell density for successful infection. In addition, bacteria use environmental signals to identify conditions when the host defenses are weakened and potentially to signal entry into an appropriate host/niche for infection. A globin coupled sensor protein (GCS), termed PccGCS, within the soft rot bacterium Pectobacterium carotovorum ssp. carotovorum WPP14 has been identified as an O₂ sensor and demonstrated to alter virulence factor excretion and control motility, with deletion of PccGCS resulting in decreased rotting of a potato host. Using small molecules that modulate bacterial growth and quorum sensing, PccGCS signaling also has been shown to modulate quorum sensing pathways, resulting in the PccGCS deletion strain being more sensitive to plant-derived phenolic acids, which can function as quorum sensing inhibitors, and exhibiting increased N-acylhomoserine lactone (AHL) production. These findings highlight a role for GCS proteins in controlling key O₂-dependent phenotypes of pathogenic bacteria and suggest that modulating GCS signaling to limit P. carotovorum motility may provide a means to decrease rotting of plant hosts.

Breeding Vegetables with Increased Content in Bioactive Phenolic Acids

Vegetables represent a major source of phenolic acids, powerful antioxidants characterized by an organic carboxylic acid function and which present multiple properties beneficial for human health. In consequence, developing new varieties with enhanced content in phenolic acids is an increasingly important breeding objective. Major phenolic acids present in vegetables are derivatives of cinnamic acid and to a lesser extent of benzoic acid. A large diversity in phenolic acids content has been found among cultivars and wild relatives of many vegetable crops. Identification of sources of variation for phenolic acids content can be accomplished by screening germplasm collections, but also through morphological characteristics and origin, as well as by evaluating mutations in key genes. Gene action estimates together with relatively high values for heritability indicate that selection for enhanced phenolic acids content will be efficient. Modern genomics and biotechnological strategies, such as QTL detection, candidate genes approaches and genetic transformation, are powerful tools for identification of genomic regions and genes with a key role in accumulation of phenolic acids in vegetables. However, genetically increasing the content in phenolic acids may also affect other traits important for the success of a variety. We anticipate that the combination of conventional and modern strategies will facilitate the development of a new generation of vegetable varieties with enhanced content in phenolic acids.

Suberin Regulates the Production of Cellulolytic Enzymes in Streptomyces scabiei, the Causal Agent of Potato Common Scab

Suberin, a major constituent of the potato periderm, is known to promote the production of thaxtomins, the key virulence factors of the common scab-causing agent Streptomyces scabiei. In the present study, we speculated that suberin affected the production of glycosyl hydrolases, such as cellulases, by S. scabiei, and demonstrated that suberin promoted glycosyl hydrolase activity when added to cellulose-, xylan-, or lichenin-containing media. Furthermore, secretome analyses revealed that the addition of suberin to a cellulose-containing medium increased the production of glycosyl hydrolases. For example, the production of 13 out of the 14 cellulases produced by S. scabiei in cellulose-containing medium was stimulated by the presence of suberin. In most cases, the transcription of the corresponding cellulase-encoding genes was also markedly increased when the bacterium was grown in the presence of suberin and cellulose. The level of a subtilase-like protease inhibitor was markedly decreased by the presence of suberin. We proposed a model for the onset of S. scabiei virulence mechanisms by both cellulose and suberin, the main degradation product of cellulose that acts as an inducer of thaxtomin biosynthetic genes, and suberin promoting the biosynthesis of secondary metabolites including thaxtomins.

Effects of plant antimicrobial phenolic compounds on virulence of the genus Pectobacterium

Pectobacterium spp. are among the most devastating necrotrophs, attacking more than 50% of angiosperm plant orders. Their virulence strategy is based mainly on the secretion of exoenzymes that degrade the cell walls of their hosts, providing nutrients to the bacteria, but conversely, exposing the bacteria to plant defense compounds. In the present study, we screened plant-derived antimicrobial compounds, mainly phenolic acids and polyphenols, for their ability to affect virulence determinants including motility, biofilm formation and extracellular enzyme activities of different Pectobacteria: Pectobacterium carotovorum, P. brasiliensis, P. atrosepticum and P. aroidearum. In addition, virulence assays were performed on three different plant hosts following exposure of the bacteria to selected phenolic compounds. These experiments showed that cinnamic, coumaric, syringic and salicylic acids and catechol can considerably reduce disease severity, ranging from 20 to 100%. The reduced disease severity was not only the result of reduced bacterial growth, but also of a direct effect of the compounds on important bacterial virulence determinants, including pectolytic and proteolytic exoenzyme activities, that were reduced by 50-100%. This is the first report revealing a direct effect of phenolic compounds on virulence factors in a wide range of Pectobacterium strains.