Trans-acting siRNA-mediated repression of ETTIN and ARF4 regulates heteroblasty in Arabidopsis

Mutations in the ARGONAUTE gene ZIPPY(ZIP)/AGO7 in Arabidopsis accelerate the juvenile-to-adult transition. A screen for mutations that suppress this precocious phenotype yielded alleles of two auxin-related transcription factors known to be upregulated in zip: ETTIN (ETT)/ARF3 and ARF4. Mutations in ETT/ARF3 and ARF4 delay the expression of adult traits, demonstrating that these genes have non-redundant roles in shoot maturation. ZIP is not generally required for the production of trans-acting (ta) siRNAs, but is required for the production and/or stability of tasiR-ARF, a ta-siRNA that targets both ETT/ARF3 and ARF4. tasiR-ARF is absent in zip-2, and overexpression of a tasiR-ARF-insensitive form of ETT mimics the zip phenotype. We conclude that the precocious phenotype of zip is attributable to the absence of tasiR-ARF-mediated repression of ETT and ARF4. The abundance of tasiR-ARF, ETT/ARF3 and ARF4 RNA does not change during vegetative development. This result suggests that tasiR-ARF regulation establishes the threshold at which leaves respond to a temporal signal, rather than being a component of this signal.