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Vegetative phase change is regulated by a decrease in the abundance of the miRNAs, miR156 and miR157, and the resulting increase in the expression of their targets, SQUAMOSA PROMOTER BINDING PROTEIN-LIKE (SPL) transcription factors. To determine how miR156/miR157 specify the quantitative and qualitative changes in leaf morphology that occur during vegetative phase change, we measured their abundance in successive leaves and characterized the phenotype of mutations in different MIR156 and MIR157 genes. miR156/miR157 decline rapidly between leaf 1&2 and leaf 3 and decrease more slowly after this point. The amount of miR156/miR157 in leaves 1&2 greatly exceeds the threshold required to specify their identity. Subsequent leaves have relatively low levels of miR156/miR157 and are sensitive to small changes in their abundance. In these later-formed leaves, the amount of miR156/miR157 is close to the threshold required to specify juvenile vs. adult identity; a relatively small decrease in the abundance of miR156/157 in these leaves produces a disproportionately large increase in SPL proteins and a significant change in leaf morphology. miR157 is more abundant than miR156 but has a smaller effect on shoot morphology and SPL gene expression than miR156. This may be attributable to the inefficiency with which miR157 is loaded into AGO1, as well as to the presence of an extra nucleotide at the 5' end of miR157 that is mis-paired in the miR157:SPL13 duplex. miR156 represses different targets by different mechanisms: it regulates SPL9 by a combination of transcript cleavage and translational repression and regulates SPL13 primarily by translational repression. Our results offer a molecular explanation for the changes in leaf morphology that occur during shoot development in Arabidopsis and provide new insights into the mechanism by which miR156 and miR157 regulate gene expression. [ABSTRACT FROM AUTHOR]

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.