Opposing effects of miR156, miR172 and their respective targets on Arabidopsis development. (A) Constitutive overexpression of MIR156 results in prolonged vegetative phase (e.g. an increased number of juvenile leaves) and reduced apical dominance, i.e. the simultaneous bolting of multiple shoots when compared with (B) Col-0 (wild type) control. (C) A plant constitutively overexpressing MIR172, which, by contrast, essentially skips the juvenile vegetative growth phase and flowers after producing only two or three small adult leaves. (D) A plant constitutively overexpressing a miRNA-resistant form of the miR156 target SPL3, which reduces the duration of the juvenile phase and promotes flowering. (E) A plant constitutively overexpressing the miR172 target AP2, which results in late flowering. (F,G) The effects of miRNA target overexpression can be recapitulated by constitutively expressing the target mimics MIM156 and MIM172, which reduce the functional levels of the mature miR156 and miR172 miRNAs, respectively. All plants were grown under long day photoperiod (16 hours of light, 8 hours of darkness). Scale bars: 1 cm. (H) A model of the sequential action of miR156, miR172 and their respective targets in regulating phase transitions in A. thaliana. Abbreviations: 35S, cauliflower mosaic virus promoter; AG, AGAMOUS; AP1, APETALA1; AP2, APETALA2; FUL, FRUITFULL; LFY, LEAFY; MIM156, miR156 mimicry target; MIM172, miR172 mimicry target; miR156, mature miRNA156; miR172, mature miRNA172; MIR156, miRNA156 gene; MIR172, miRNA172 gene; rSPL3, miR156-resistant form of SQUAMOSA PROMOTER BINDING PROTEIN-LIKE 3; SEP3, SEPALLATA3; SOC1, SUPRESSOR OF OVEREXPRESSION OF CONSTANS 1; SPL, SQUAMOSA PROMOTER BINDING PROTEIN-LIKE.

Regulation of phase change in Arabidopsis. During early development, the levels of miR156 are initially high, promoting the juvenile vegetative growth phase in seedlings. Juvenile leaves (light grey, lower left) are almost round in shape and exhibit trichomes only on their adaxial side. As the plant matures, the levels of miR156 steadily decrease, allowing for the production of SPL9 and SPL10 proteins that promote adult leaf traits (dark grey; elongated leaves with abaxial trichomes). At the same time, SPL9 and SPL10 directly induce the expression of MIR172 genes. Increased levels of miR172 result in the downregulation of six AP2-like transcription factors that normally repress flowering. Release from this repression, in combination with the flower-promoting actions of SPL3, SPL4 and SPL5, makes the plant competent to flower and the transition to flowering can occur. During the transition to flowering, the shoot apical meristem does not immediately give rise to flowers but rather to secondary shoots that emerge from the axils of cauline leaves (black, lower right). In addition to its role as a floral repressor (lower right; from above, bottom; in longitudinal section, above), AP2 contributes to the patterning of the emerging flower. Both AP2 and miR172 participate in establishing a sharp boundary between the vegetative outer organs (sepals, petals) and the inner whorls of reproductive organs (stamen, carpels). Abbreviations: AG, AGAMOUS; AP2, APETALA2; miR156, mature miRNA156; miR172, mature miRNA172; SMZ, SCHLAFMÜTZE; SNZ, SCHNARCHZAPFEN; SPL, SQUAMOSA PROMOTER BINDING PROTEIN-LIKE; TOE, TARGET OF EARLY ACTIVATION TAGGED.