Plant microRNAs regulate gene expression in a sequence-specific manner by binding to target mRNAs, leading to their degradation. Unlike animal microRNAs, plant microRNAs have a high degree of complementarity to their targets, and the scarcity of microRNA loss-of-function phenotypes in plants implies that redundancy exists between microRNA family members. Now, two papers provide new insights into this redundancy and into microRNA-regulated shoot development in Arabidopsis. Elliot Meyerowitz and colleagues show how the activity of redundant miR164 family members is crucial for the spatial positioning of flowers on the Arabidopsis stem, and for the number and size of floral organs (see ). And Patrick Laufs and co-workers on report how the continued expression of CUP-SHAPED COTYLEDON2 (CUC2), a miR164 target, regulates correct flower positioning. In their study, Meyerowitz and colleagues report that the elimination of all three, but not of individual, miR164 family members severely disrupts flower and shoot development in Arabidopsis. In the absence of the three miR164 microRNAs,the expression domains of their transcription factor-encoding targets, CUC1 and CUC2, become enlarged in the inflorescence meristems of mutant plants. This result, together with other findings reported here, indicate that the miR164 microRNAs not only act to reduce target transcript levels but can also spatially limit target mRNA accumulation- functions, the authors propose, that hint at their possible involvement in developmental robustness. The study also reveals that the miR164family members are not fully redundant but show some functional diversification, raising questions as to whether microRNA gene families evolve in the same way as protein-coding gene families do through gene or genome duplication. Laufs and co-workers' complementary study shows that the miR164-regulated expression of CUC2 is crucial for organ patterning in the stem rather than in the apical meristem. They report that the pattern of flower primordia is irregular in the fully grown stems of plants that express a miR164-resistant version of CUC2. These plants effectively overexpress CUC2, indicating that the timing and level of CUC2 expression is crucial for patterning. Together,these results indicate that patterning is not completely determined in the apical meristem and that redundancy between microRNAs provides the platform for this unexpected level of complexity.
