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Smut fungi comprise a large group of biotrophic phytopathogens infecting important crops, such as wheat and corn. U. maydis is a plant pathogenic fungus responsible for common smut in maize and teocintle. Through our analysis of the transcriptome of the yeast-to-mycelium dimorphic transition at acid pH, we determined the number of genes encoding chitin deacetylases of the fungus, and observed that the gene encoding one of them (UMAG_11922; CDA1) was the only one up-regulated. The mutation of this gene and the analysis of the mutants revealed that they contained reduced amounts of chitosan, were severely affected in their virulence, and showed aberrant mycelial morphology when grown at acid pH. When the CDA1 gene was reinserted into the mutants by the use of an autonomous replication plasmid, virulence and chitosan levels were recovered in the retro mutant strains, indicating that the CDA1 gene was involved in these features. These data revealed that chitosan plays a crucial role in the structure and morphogenesis of the cell wall during mycelial development of the fungus, and that in its absence, the cell wall becomes altered and is unable to support the stress imposed by the defense mechanism mounted on by the plant host during the infection process.

The role of the Ustilago maydis putative homolog of the transcriptional repressor ScNRG1, previously described in Saccharomyces cerevisiae, Candida albicans and Cryptococcus neoformans, was analyzed by means of its mutation. In S. cerevisiae this gene regulates a set of stress-responsive genes, and in C. neoformans it is involved in pathogenesis. It was observed that the U. maydisNRG1 gene regulates several aspects of the cell response to acid pH, such as the production of mannosyl-erythritol lipids, inhibition of the expression of the siderophore cluster genes, filamentous growth, virulence and oxidative stress. A comparison of the gene expression pattern of the wild type strain versus the nrg1 mutant strain of the fungus, through RNA Seq analyses, showed that this transcriptional factor alters the expression of 368 genes when growing at acid pH (205 up-regulated, 163 down-regulated). The most relevant genes affected by NRG1 were those previously reported as the key ones for particular cellular stress responses, such as HOG1 for osmotic stress and RIM101 for alkaline pH. Four of the seven genes included WCO1 codifying PAS domain ( These has been shown as the key structural motif involved in protein-protein interactions of the circadian clock, and it is also a common motif found in signaling proteins, where it functions as a signaling sensor) domains sensors of blue light, two of the three previously reported to encode opsins, one vacuolar and non-pH-responsive, and another one whose role in the acid pH response was already known. It appears that all these light-reactive cell components are possibly involved in membrane potential equilibrium and as virulence sensors. Among previously described specific functions of this transcriptional regulator, it was found to be involved in glucose repression, metabolic adaptation to adverse conditions, cellular transport, cell rescue, defense and interaction with an acidic pH environment.

Capsaicin is a chemical compound found in pungent chili peppers (Capsicum spp.). In biotechnology, capsaicin has been proposed as a pathogen control; however, its low solubility in water and high instability limits its uses. The aim of this work was to study the effect of high concentrations of capsaicin on the synthesis of nanoparticles and to evaluate their inhibitory effect on the growth of Rhodotorula mucilaginosa yeast. Bovine serum albumin (BSA)-capsaicin nanoparticles were formulated at 0, 16.2, 32.5, 48.7 and 65.0 µg of capsaicin per mg of BSA. Nanoparticle properties were evaluated and they were added to cultures of R. mucilaginosa to quantify their effect on cell viability. We found that increased capsaicin levels caused several changes to the physicochemical parameters, probably due to changes in the hydrophobicity sites of the albumin during the nanostructuration. The administration of nanoparticles to cultures of R. mucilaginosa produced a maximal viability with nanoparticles at 16.2 µg/mg; on the contrary, nanoparticles at 65.0 µg/mg caused maximal cell death. R. mucilaginosa cells displayed a hormesis effect in response to the nanoparticle dose concentration. The nanoparticles showed different responses during the uptake process, probably as a consequence of the nanostructural properties of capsaicin in the BSA molecules.