The evolutionary race between hosts and pathogens is based on small RNA interactions. When plants come into contact with pathogenic organisms, they trigger a multi-step defence response based on RNA interference. They synthesise specific miRNAs that regulate the expression of defence genes in the plant and silence the virulence genes in the pathogen, thereby inhibiting its spreading. In contrast, pathogens transport proteins and small RNAs into plant cells to inhibit or modulate RNA interference pathways, thereby preventing recognition by the plant and modulating the amplitude of the plant defence response. This constant communication has led to the coevolution of hosts and their specific pathogens, as exemplified by the relationship between cotton (Gossypium hirsutum) and the fungus Verticillium dahliae. The study and understanding of these complex relationships is an important contribution to the development of agriculture, which is trying to reduce the use of chemical agents to control pathogens, as they are harmful to the environment and health and, above all, their continued use makes pathogens resistant and thus uncontrollable. The transport of small RNAs into the pathogen represents an effective defence in the early stages of disease development and, as such, is of great interest for the development of new protection techniques in agriculture. Therefore, HIGS and SIGS approaches exploiting natural RNA interference mechanisms have been developed as alternatives in sustainable agriculture.
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