Small non-coding RNA-mediated epigenetic regulation of gene expression and its implications for wheat adaptation to continuous cultivation

Winter wheat (Triticum aestivum L.) is one of the most important staple crops worldwide. However, continuous wheat cultivation often leads to yield decline which poses a serious threat to sustainable wheat production. This yield decline is generally attributed to the interaction of abiotic and biotic soil stress factors as well as plant adaptation to long-term monoculture systems. However, the underlying molecular mechanisms have not yet been adequately explained. Against this background, this dissertation presents, in addition to a general introduction (Chapter I) and discussion (Chapter V), three research papers that deal with the mechanisms of plant adaptation to continuous cultivation.

Chapter II entitled "RNA-directed DNA methylation (RdDM) modulates the trade-off between growth and defense and contributes to wheat adaptation to continuous cultivation" investigates the possible contribution of epigenetic modifications to wheat adaptation to long-term continuous cultivation. Chapter III entitled "MicroRNAs (miRNAs) and small interfering RNAs (siRNAs) facilitate the adaptation of wheat (Triticum aestivum L.) to continuous cultivation" demonstrates that both miRNAs and 24-nt siRNAs are strongly affected by continuous cultivation and act as key regulators of wheat molecular and physiological processes. Chapter IV entitled "The microRNA Bna-miR1885 interferes with TIR-NLRs and modulates the plant defense response in model plant Arabidopsis thaliana" focuses on the functional characterization of a Brassica-specific miR1885 in model plant Arabidopsis thaliana.

In conclusion, this work shows that both RdDM pathway and miRNA-mediated post-transcriptional gene regulation activate changes in gene expression and physiological processes in plants. This leads to a shift in the balance between growth and defense. Limited energy and resource reserves are increasingly allocated to stress responses, ultimately resulting in a loss of yield. These findings substantially broaden our understanding of plant adaptive strategies at the epigenetic and post-transcriptional levels and provide valuable candidate genes, small RNAs, and epialleles for improving crop tolerance by e.g., targeted breeding or genome-editing approaches.