Phytoalexins in defense against pathogens

Lignin, the most abundant aromatic biopolymer on Earth, is extremely recalcitrant to degradation. By linking to both hemicellulose and cellulose, it creates a barrier to any solutions or enzymes and prevents the penetration of lignocellulolytic enzymes into the interior lignocellulosic structure. Some basidiomycetes white-rot fungi are able to degrade lignin efficiently using a combination of extracellular ligninolytic enzymes, organic acids, mediators and accessory enzymes. This review describes ligninolytic enzyme families produced by these fungi that are involved in wood decay processes, their molecular structures, biochemical properties and the mechanisms of action which render them attractive candidates in biotechnological applications. These enzymes include phenol oxidase (laccase) and heme peroxidases [lignin peroxidase (LiP), manganese peroxidase (MnP) and versatile peroxidase (VP)]. Accessory enzymes such as H2O2-generating oxidases and degradation mechanisms of plant cell-wall components in a non-enzymatic manner by production of free hydroxyl radicals (·OH) are also discussed.

Fungal biodegradation and enzymatic modification of lignin

Miscanthus represents a key candidate energy crop for use in biomass-to-liquid fuel-conversion processes and biorefineries to produce a range of liquid fuels and chemicals; it has recently attracted considerable attention. Its yield, elemental composition, carbohydrate and lignin content and composition are of high importance to be reviewed for future biofuel production and development. Starting from Miscanthus, various pre-treatment technologies have recently been developed in the literature to break down the lignin structure, disrupt the crystalline structure of cellulose, and enhance its enzyme digestibility. These technologies included chemical, physicochemical, and biological pre-treatments. Due to its significantly lower concentrations of moisture and ash, Miscanthus also represents a key candidate crop for use in biomass-to-liquid conversion processes to produce a range of liquid fuels and chemicals by thermochemical conversion. The goal of this paper is to review the current status of the technology for biofuel production from this crop within a biorefinery context.

Miscanthus: a fast-growing crop for biofuels and chemicals production

Plant resistance to pathogens relies on a complex network of constitutive and inducible defensive barriers. The plant cell wall is one of the barriers that pathogens need to overcome to successfully colonize plant tissues. The traditional view of the plant cell wall as a passive barrier has evolved to a concept that considers the wall as a dynamic structure that regulates both constitutive and inducible defense mechanisms, and as a source of signaling molecules that trigger immune responses. The secondary cell walls of plants also represent a carbon-neutral feedstock (lignocellulosic biomass) for the production of biofuels and biomaterials. Therefore, engineering plants with improved secondary cell wall characteristics is an interesting strategy to ease the processing of lignocellulosic biomass in the biorefinery. However, modification of the integrity of the cell wall by impairment of proteins required for its biosynthesis or remodeling may impact the plants resistance to pathogens. This review summarizes our understanding of the role of the plant cell wall in pathogen resistance with a focus on the contribution of lignin to this biological process.

https://biblio.ugent.be/publication/5866457/file/5866474.pdf

The role of the secondary cell wall in plant resistance to pathogens

The battle between plants and microbes is evolutionarily ancient, highly complex and often co-dependent. A primary challenge for microbes is to breach the physical barrier of host cell walls whilst avoiding detection by the plant’s immune receptors. While some receptors sense conserved microbial features, others monitor physical changes caused by an infection attempt. Detection of microbes leads to activation of appropriate defense responses that then challenge the attack. Plant cell walls are formidable and dynamic barriers. They are constructed primarily of complex carbohydrates joined by numerous distinct connection types, and are subject to extensive post-synthetic modification to suit prevailing local requirements. Multiple changes can be triggered in cell walls in response to microbial attack. Some of these are well described, but many remain obscure. The study of the myriad of subtle processes underlying cell wall modification poses special challenges for plant glycobiology. In this review we describe the major molecular and cellular mechanisms that underlie the roles of cell walls in plant defense against pathogen attack. In so doing, we also highlight some of the challenges inherent in studying these interactions, and briefly describe the analytical potential of molecular probes used in conjunction with carbohydrate microarray technology.

/pdf/the-role-of-the-cell-wall-in-plant-immunity-1kaoqgk39a.pdf

The role of the cell wall in plant immunity.

Isoflavonoid accumulation in soybean hairy roots upon treatment with Fusarium solani

Soybean [Glycine max (L.) Merr.] seed isoflavone concentration has been shown to be highly dependent on environmental conditions, but isoflavone concentrations have not been studied under controlled conditions to quantify the effects of specific factors. To determine the effect of air temperature and soil moisture status during soybean seed development on seed isoflavone concentration and composition, soybean plants were grown in the greenhouse under intermediate (18/28 degrees C), 9.5 h night/14.5 h daytime temperatures with high soil moisture conditions. Beginning at the R6 growth stage plants were subjected to either intermediate (18/28 degrees C), low (13/23 degrees C), or high (23/33 degrees C) 9.5 h night/14.5 h daytime temperatures with either low or high soil moisture conditions. Two French cultivars, Imari and Queen, and three U.S. cultivars, Dwight, Jack and Loda, all in maturity group II were studied. The overall results show that low temperatures and high soil moisture conditions produced the highest seed isoflavone concentrations with changes in temperature having the larger effect. The changes in daidzein and genistein concentrations were similar to changes in total isoflavones but the glycitein concentration was much less affected. The three U.S. cultivars were much less responsive to soil moisture than the two French cultivars. All five cultivars showed a two- to threefold increase in total isoflavone concentrations at the low temperature regime compared to the high temperature regime. Plant height was greatest under the intermediate temperatures; whereas, low temperatures and low soil moisture hastened maturity. Seed size was not significantly affected by any treatment. Soil moisture and air temperature have clear effects on the isoflavone concentrations in mature soybean seeds, but the ranking of all the cultivars based on average isoflavone concentration remained the same with all treatments. Environmental factors can have a large effect on isoflavone concentration, but the potential for isoflavone production is largely under genetic control.

Effect of temperature and soil moisture status during seed development on soybean seed isoflavone concentration and composition

The composition and concentrations of cell wall polysaccharides and phenolic compounds were analyzed in mature stems of several Miscanthus genotypes, in comparison with switchgrass and reed (Arundo donax), and biomass characteristics were correlated with cell wall saccharification efficiency. The highest cellulose content was found in cell walls of M. sinensis‘Grosse Fontaine’ (55%) and in A. donax (47%) and lowest (about 32%) in M. sinensis‘Adagio’. There was little variation in lignin contents across M. sinensis samples (all about 22–24% of cell wall), however, Miscanthus×giganteus (M × g) cell walls contained about 28% lignin, reed – 23% and switchgrass – 26%. The highest ratios of cellulose/lignin and cellulose/xylan were in M. sinensis‘Grosse Fontaine’ across all samples tested. About the same total content of ester-bound phenolics was found in different Miscanthus genotypes (23–27 μg/mg cell wall), while reed cell walls contained 17 μg/mg cell wall and switchgrass contained a lower amount of ester-bound phenolics, about 15 μg/mg cell wall. Coumaric acid was a major phenolic compound ester-bound to cell walls in plants analyzed and the ratio of coumaric acid/ferulic acid varied from 2.1 to 4.3, with the highest ratio being in M × g samples. Concentration of ether-bound hydroxycinnamic acids varied greatly (about two-three-fold) within Miscanthus genotypes and was also the highest in M × g cell walls, but at a concentration lower than ester-bound hydroxycinnamic acids. We identified four different forms of diferulic acid esters bound to Miscanthus cell walls and their concentration and proportion varied in genotypes analyzed with the 5-5-coupled dimer being the predominant type of diferulate in most samples tested. The contents of lignin and ether-bound phenolics in the cell wall were the major determinants of the biomass degradation caused by enzymatic hydrolysis.

https://onlinelibrary.wiley.com/doi/pdf/10.1111/j.1757-1707.2011.01091.x

Composition of cell wall phenolics and polysaccharides of the potential bioenergy crop - Miscanthus.

Soybean hairy roots, transformed with the soybean chalcone synthase (CHS6) or isoflavone synthase (IFS2) genes, with dramatically decreased capacity to synthesize isoflavones were produced to determine what effects these changes would have on susceptibility to a fungal pathogen. The isoflavone and coumestrol concentrations were decreased by about 90% in most lines apparently due to gene silencing. The IFS2 transformed lines had very low IFS enzyme activity in microsomal fractions as measured by the conversion of naringenin to genistein. The CHS6 lines with decreased isoflavone concentrations had 5 to 20-fold lower CHS enzyme activities than the appropriate controls. Both IFS2 and CHS transformed lines accumulated higher concentrations of both soluble and cell wall bound phenolic acids compared to controls with higher levels found in the CHS6 lines indicating alterations in the lignin biosynthetic branch of the pathway. Induction of the soybean phytoalexin glyceollin, of which the precursor is the isoflavone daidzein, by the fungal pathogen Fusarium solani f. sp. glycines (FSG) that causes soybean sudden death syndrome (SDS) showed that the low isoflavone transformed lines did not accumulate glyceollin while the control lines did. The (iso)liquritigenin content increased upon FSG induction in the IFS2 transformed roots indicating that the pathway reactions before this point can control isoflavonoid synthesis. The lowest fungal growth rate on hairy roots was found on the FSG partially resistant control roots followed by the SDS sensitive control roots and the low isoflavone transformants. The results indicate the importance of phytoalexin synthesis in root resistance to the pathogen.

Modification of phenolic metabolism in soybean hairy roots through down regulation of chalcone synthase or isoflavone synthase

Sudden death syndrome (SDS), caused by the soilborne fungal pathogen Fusarium solani f. sp. glycines, is one of the most important diseases of soybean. Lignin degradation may play a role in the infection, colonization, and survival of the fungus in root tissue. Lignin degradation by F. solani f. sp. glycines was shown by the catalyzed release of 14CO2 from purified 14C-labeled Klason lignin, the degradation of polymeric aromatic dyes in culture (a method commonly used to test the ligninolytic capacity of microorganisms), and the production of laccase and lignin peroxidase (the major fungal lignin degrading enzymes). The laccase and lignin peroxidase activities and the amount of decolorization of aromatic polymeric dyes (Poly R-478 and Remazol Brilliant Blue R) by F. solani f. sp. glycines were intermediate or greater than that found with two known lignin-degrading fungi, Polyporus tulipifera and Schizophyllum commune. Studies of lignin synthesis from [14C]phenylalanine with soybean hairy root cultures showed that F. solani f. sp. glycines treatment stimulated lignin synthesis in 2 h, and by 24 h, some lignin degradation had occurred. These results indicate that F. solani f. sp. glycines was capable of degrading lignin which may be important in infection, colonization, and survival of the fungus.

https://apsjournals.apsnet.org/doi/pdfplus/10.1094/PD-90-0077

Anatoliy V. Lygin

Papers

Isoflavonoid accumulation in soybean hairy roots upon treatment with Fusarium solani

Effect of temperature and soil moisture status during seed development on soybean seed isoflavone concentration and composition

Composition of cell wall phenolics and polysaccharides of the potential bioenergy crop - Miscanthus.

Modification of phenolic metabolism in soybean hairy roots through down regulation of chalcone synthase or isoflavone synthase

Lignin degradation by Fusarium solani f. sp. glycines