Although plant diseases are usually characterized by the part of the plant that is affected (e.g., leaf spots, root rots, wilts), surprisingly little is known about the factors that condition the ability of pathogens to colonize different plant tissues. Here we demonstrate that the leaf blast pathogen Magnaporthe grisea also can infect plant roots, and we exploit this finding to distinguish tissue-specific and general requirements for plant infection. Tests of a M. grisea mutant collection identified some mutants that were defective specifically in infection of either leaves or roots, and others such as the map kinase mutant pmk1 that were generally defective in pathogenicity. Conservation of a functional PMK1-related MAP kinase in the root pathogen Gaeumannomyces graminis was also demonstrated. Exploitation of the ability of M. grisea to infect distinct plant tissues thus represents a powerful tool for the comprehensive dissection of genetic determinants of tissue specificity and global requirements for plant infection.

Trichoderma gamsii T6085 has been investigated for many years as a beneficial isolate for use in the biocontrol of Fusarium head blight (FHB) of wheat caused primarily by Fusarium graminearum. Previous work focused on application of T6085 to wheat spikes at anthesis, whereas application to soil before or at sowing has received limited attention. In the present study, the competitive ability of T6085 on plant residues against F. graminearum was investigated. Results showed a significant reduction of wheat straw colonization by the pathogen and of the development of perithecia, not only when T6085 was applied alone but also in the presence of a F. oxysporum isolate (7121), well known as a natural competitor on wheat plant residues. T6085 was able to endophytically colonize wheat roots, resulting in internal colonization of the radical cortex area, without reaching the vascular system, as confirmed by confocal microscopy. This intimate interaction with the plant resulted in a significant increase of the expression of the plant defense-related genes PAL1 and PR1. Taken together, competitive ability, endophytic behavior, and host resistance induction represent three important traits that can be of great use in the application of T6085 against FHB not only on spikes at anthesis but potentially also in soil before or at sowing.

Fusarium Head Blight (FHB) is a cereal disease caused primarily by the ascomycete fungusFusarium graminearumwith public health issues due to the production of mycotoxins including deoxynivalenol (DON). Genetic resistance is an efficient protection means and numerous quantitative trait loci have been identified, some of them related to the production of resistance metabolites. In this study, we have functionally characterized theBrachypodium distachyon BdCYP711A29gene encoding a cytochrome P450 monooxygenase (CYP). We showed thatBdCYP711A29belongs to an oligogenic family of five members. However, following infection byF. graminearum,BdCYP711A29is the only copy strongly transcriptionally induced in a DON-dependent manner. The BdCYP711A29 protein is homologous to theArabidopsis thalianaMAX1 andOryza sativaMAX1-like CYPs representing key components of the strigolactone biosynthesis. We show that BdCYP711A29 is likely involved in orobanchol biosynthesis. Alteration of theBdCYP711A29sequence or expression alone does not modify plant architecture, most likely because of functional redundancy with the other copies.B. distachyonlines overexpressingBdCYP711A29exhibit an increased susceptibility toF. graminearum, although no significant changes in defense gene expression were detected. We demonstrate that both orobanchol and exudates ofBd711A29overexpressing lines stimulate the germination ofF. graminearummacroconidia. We therefore hypothesize that orobanchol is a susceptibility factor to FHB.

Losses due to disease and climate change are among the most important issues currently facing crop production. It is therefore important to establish the impact of climate change, and particularly of high carbon dioxide (hCO₂), on plant immunity in cereals, which provide 60% of human calories. The aim of this study was to determine if hCO₂ impacts Brachypodium distachyon immunity, a model plant for temperate cereals. Plants were grown in air (430 ppm CO₂) and at two high CO₂ conditions, one that is relevant to projections within the coming century (1000 ppm) and a concentration sufficient to saturate photosynthesis (3000 ppm). The following measurements were performed: phenotyping and growth, salicylic acid contents, pathogen resistance tests, and RNAseq analysis of the transcriptome. Improved shoot development was observed at both 1000 and 3000 ppm. A transcriptomic analysis pointed to an increase in primary metabolism capacity under hCO₂. Alongside this effect, up-regulation of genes associated with secondary metabolism was also observed. This effect was especially evident for the terpenoid and phenylpropanoid pathways, and was accompanied by enhanced expression of immunity-related genes and accumulation of salicylic acid. Pathogen tests using the fungus Magnaporthe oryzae revealed that hCO₂ had a complex effect, with enhanced susceptibility to infection but no increase in fungal development. The study reveals that immunity in B. distachyon is modulated by growth at hCO₂ and allows identification of pathways that might play a role in this effect.

The Fusarium metabolite culmorin (CUL) frequently co-occurs with the mycotoxin deoxynivalenol (DON) on cereals. While DON is recognized as a major Fusarium virulence factor on plants, the function of CUL is still unclear. In this study, we show that CUL-deficient F. graminearum mutants created by deletion of the terpene synthase gene CLM1 are less aggressive on wheat than the wild-type, and this is accompanied by increased DON-3-glucoside/DON ratios in infected wheat ears. In root elongation assays with wheat and Brachypodium distachyon, CUL had no effect alone but significantly increased the toxicity of DON. Analysis of roots treated with DON and/or CUL further indicated that both wheat and B. distachyon are able to glucosylate CUL and that its presence impedes DON-glucosylation in both species. We identified two B. distachyon UDP-glucosyltransferases (UGTs) able to glucosylate CUL, and further investigated the effects of CUL on the kinetics of validated DON-glucosylating plant UGTs (BdUGT5g03300, HvUGT13248, OsUGT79). This suggested that CUL inhibits DON-glucosylation either by serving as a competitive substrate with DON or by unproductive binding. BdUGT5g03300 in particular was strongly inhibited by CUL and even its glucosides. Our results indicate that CUL contributes to Fusarium virulence by weakening plant defences related to UGT-catalysed DON-detoxification. As even CUL-glucosides are potentially inhibitory to UGTs, this implies a complex synergy of CUL with DON.