The production and secretion of tRNA-derived RNA fragments in the corn smut fungus Ustilago maydis

Transfer RNA and ribosomal RNA fragments - emerging players in plant-microbe interactions

According to current textbooks, the principal task of transfer and ribosomal RNAs (tRNAs and rRNAs, respectively) is synthesizing proteins. During the last decade, additional cellular roles for precisely processed tRNA and rRNAs fragments have become evident in all kingdoms of life. These RNA fragments were originally overlooked in transcriptome datasets or regarded as unspecific degradation products. Upon closer inspection, they were found to engage in a variety of cellular processes, in particular the modulation of translation and the regulation of gene expression by sequence complementarity- and Argonaute protein-dependent gene silencing. More recently, the presence of tRNA and rRNA fragments has also been recognized in the context of plant-microbe interactions, both on the plant and the microbial side. While most of these fragments are likely to affect endogenous processes, there is increasing evidence for their transfer across kingdoms in the course of such interactions; these processes may involve mutual exchange in association with extracellular vesicles. Here, we summarize the state-of-the-art understanding of tRNA and rRNA fragment's roles in the context of plant-microbe interactions, their potential biogenesis, presumed delivery routes, and presumptive modes of action.

RNA-Based Control of Fungal Pathogens in Plants

Our duty to conserve global natural ecosystems is increasingly in conflict with our need to feed an expanding population. The use of conventional pesticides not only damages the environment and vulnerable biodiversity but can also still fail to prevent crop losses of 20-40% due to pests and pathogens. There is a growing call for more ecologically sustainable pathogen control measures. RNA-based biopesticides offer an eco-friendly alternative to the use of conventional fungicides for crop protection. The genetic modification (GM) of crops remains controversial in many countries, though expression of transgenes inducing pathogen-specific RNA interference (RNAi) has been proven effective against many agronomically important fungal pathogens. The topical application of pathogen-specific RNAi-inducing sprays is a more responsive, GM-free approach to conventional RNAi transgene-based crop protection. The specific targeting of essential pathogen genes, the development of RNAi-nanoparticle carrier spray formulations, and the possible structural modifications to the RNA molecules themselves are crucial to the success of this novel technology. Here, we outline the current understanding of gene silencing pathways in plants and fungi and summarize the pioneering and recent work exploring RNA-based biopesticides for crop protection against fungal pathogens, with a focus on spray-induced gene silencing (SIGS). Further, we discuss factors that could affect the success of RNA-based control strategies, including RNA uptake, stability, amplification, and movement within and between the plant host and pathogen, as well as the cost and design of RNA pesticides.

Analysis of gene expression related to polyamine concentration and dimorphism induced in ornithine decarboxylase (odc) and spermidine synthase (spd) Ustilago maydis mutants

Polyamines are ubiquitous small organic cations, and their roles as regulators of several cellular processes are widely recognized. They are implicated in the key stages of the fungal life cycle. Ustilago maydis is a phytopathogenic fungus, the causal agent of common smut of maize and a model system to understand dimorphism and virulence. U. maydis grows in yeast form at pH 7 and it can develop its mycelial form in vitro at pH 3. Δodc mutants that are unable to synthesize polyamines, grew as yeast at pH 3 with a low putrescine concentration, and to complete its dimorphic transition high putrescine concentration was required. Δspd mutants required spermidine to grow and cannot form mycelium at pH 3. In this work, the increased expression of the mating genes, mfa1 and mfa2, on Δodc mutants, was related to high putrescine concentration. Global gene expression analysis comparisons of Δodc and Δspd U. maydis mutants indicated that 2,959 genes were differentially expressed in the presence of exogenous putrescine at pH 7 and 475 genes at pH 3. While, in Δspd mutant, the expression of 1,426 genes was affected by exogenous spermine concentration at pH 7 and 11 genes at pH 3. Additionally, we identified 28 transcriptional modules with correlated expression during seven tested conditions: mutant genotype, morphology (yeast, and mycelium), pH, and putrescine or spermidine concentration. Furthermore, significant differences in transcript levels were noted for genes in modules relating to pH and genotype genes involved in ribosome biogenesis, mitochondrial oxidative phosphorylation, N-glycan synthesis, and Glycosylphosphatidylinositol (GPI)-anchor. In summary, our results offer a valuable tool for the identification of potential factors involved in phenomena related to polyamines and dimorphism.