HASTY-dependent miRNA cell-to-cell movement is required for systemic pathogen resistance in Arabidopsis.

Plant defenses against pathogens are tightly regulated through complex gene expression control mechanisms. Precise activation and repression of defense-related genes are crucial for balancing the trade-off between growth and immunity. MicroRNAs (miRNAs) play a well-established role in the local regulation of plant-microbe interactions. While some miRNAs are also essential for systemic defense responses, their mechanisms of action, biogenesis, and long-distance mobility remain largely unexplored. Here, we show that HASTY (HST), a key factor in miRNA biogenesis and intercellular movement, is required for systemic defense activation. Impaired miRNA mobility in hst mutants correlates with the absence of systemic responses. In infected tissues, HST may enhance the co-transcriptional processing of specific primary miRNAs, thereby promoting cell-to-cell movement of their mature forms and contributing to systemic defense activation. Furthermore, two miRNAs that exhibit increased mobility during systemic defense induction are required for a proper systemic response. Notably, restoring miRNA mobility in the hst mutant without altering miRNA abundance, or complementing it with a version of HST expressed exclusively in companion cells, is sufficient to restore systemic defense induction, highlighting the critical role of miRNA cell-to-cell movement. Together, these findings elucidate the role of HST in plant immunity, linking miRNA biogenesis and mobility to the fine-tuned regulation of systemic defenses.