In contrast to animals, plants utilize different developmental programs from juvenility to maturity. After seed germination, the shoot apical meristem (SAM), a population of pluripotent stem cells at the shoot apex, begins to produce leaves. The plant reaches maturity when it becomes competent to exogenous or endogenous floral-inducing signals such as hormones, light and temperature. Upon the transition to reproductive growth, the SAM gives rise to flowers. While different plants have dramatically different morphology, the program deployed to time developmental transitions is well conserved, which offers us an excellent system to study the reproducibility of developmental programs between individuals.
The plant developmental transitions are under genetic regulation and can be defined by the morphology of lateral organs. In principle, the mutations that affect developmental timing do not appreciably accelerate or retard the life cycle of the plant. Nor do they alter organ identity per se. Rather, they change the temporal identity of lateral organs to one normally expressed at a different time within the same lineage, but usually restricted to a distinct developmental stage. The observed temporal transformations in developmental timing mutants have been likened to the floral homeotic mutants, in which floral organ identities are spatially, rather than temporally, transformed.
The past decade has seen a great advance in our understanding of the molecular basis of plant developmental transitions. Mutant characterizations and genetic analyses in Arabidopsis thaliana reveal a central timing module governed by microRNA156 (miR156) and its targets, SQUAMOSA PROMOTER BINDING-LIKE (SPL) transcription factors. miR156 is extremely conserved in land plants. It accumulates at a high level in seedlings and subsequently declines as plants age. This expression pattern is not only observed in Arabidopsis, but also in other species such as Arabis alpina, Cardamine flexuosa, Chinese cabbage, maize, rice, tobacco, tomato and poplar. The plant developmental transitions (juvenile-to-adult transitions and flowering) are regulated by a decrease in the level of miR156. miR156 is both necessary and sufficient for the maintenance of the juvenile phase; overexpression of miR156 prolongs the juvenile phase and delays flowering, while blocking the function of miR156 by a 'target mimicry' strategy (MIM156) results in a premature phenotype.
Several lines of evidence now reveal a direct link between sugar and miR156 abundance. First, removal of pre-existing leaves causes an increase in miR156 expression and delays plant maturation. Second, miR156 level is reduced in response to exogenous glucose or sucrose treatment, accompanied with an acceleration of juvenile-to-adult phase transition. Third, chlorina1 (ch1), the photosynthetic mutant which shows impaired photosynthesis, prolongs the juvenile phase, in commitment with a high level of miR156. Moreover, the repression of miR156 by sugar is conserved among different plant species. Thus, these result suggest that sugar may accelerate plant developmental transitions by modulating miR156 level.
 Yu S, Lian H, and Wang JW*. (2015). Plant Developmental Transitions: the Role of MicroRNAs and Sugars. Curr Opin Plant Biol. 27: 1-7.
 Yu S, Cao L, Zhou CM, Zhang TQ, Lian H, Sun Y, Wu JQ, Huang JR, Wang GD, and Wang JW*. (2013). Sugar is an Endogenous Cue for Juvenile-to-Adult Phase Transition in Plants. eLife. 2: e00269.
 Wang JW, Czech B, Weigel D*. (2009). miR156-regulated SPL transcription factors define an endogenous flowering pathway in Arabidopsis thaliana. Cell. 138: 738-749.
 Wu G, Park MY, Conway SR, Wang JW, Weigel D, Poethig S*. (2009). The sequential action of miR156 and miR172 regulates developmental timing in Arabidopsis. Cell. 138: 750-759.
In Arabidopsis, the developmental and physiological programs timed by miR156 include leaf shape (length/width ratio of the blade and leaf size), leaf initiation rate, leaf complexity, shoot regenerative capacity, secondary metabolites accumulation, trichome initiation, stress response, innate Immunity, embryonic patterning and flowering.
The biological function of miR156 is exerted by its targets, SQUAMOSA PROMOTER BINDING PROTEIN-LIKEs (SPLs). SPLs encode a family of transcription factors with conserved SBP DNA-binding domain at the amino terminus. SPL9 and SPL15 play predominant roles. Identification of the events downstream of SPL9 in Arabidopsis has revealed that the miR156-SPL9 timer is integrated into diverse developmental and metabolic pathways through either transcriptional activation or repression. For example, SPL9, as a canonical transcription activator, induces flowering through activating MADS-box genes AP1, FUL and SOC1, or promotes terpene biosynthesis through the activation of terpene synthase TPS21. In contrast, SPL9 represses cytokinin response and anthocyanin production through binding with the B-type ARRs, the master regulators in the cytokinin signaling pathway, or PAP1, the MYB transcription factor in the anthocyanin biosynthetic pathway. In a recent study, SPL9 was found to license the CUC2-CUC3 transcription complex that instructs leaf serration formation through binding with TCP4.
In addition to MADS-box genes, the role of SPL9 in flowering is also mediated by another conserved miRNA, miR172. The miR172 level increases with age. It has been shown that one of the miR172 coding loci, MIR172B, is the direct target of SPL9. As a result, miR172 levels are elevated in the absence of miR156 but reduced when miR156 is overexpressed. In the A. thaliana genome, miR172 targets five AP2-like genes which act as flowering repressors through inhibition of the expression of the florigen gene FT. The high level of miR172 leads to early flowering and suppresses the late flowering phenotype caused by miR156 overexpression. Interestingly, the same regulatory circuit is recruited for timing abaxial trichome production. Thus, the miR156-SPL-miR172 module is reminiscent of the lin-4-let-7 cascade in Caenorhabditis elegans, suggesting an evolutionarily conserved timing mechanism in multicellular organisms.
 Zhang TQ, Lian H, Zhou CM, Xu L, Jiao Y, and Wang JW*. (2017). A two-step model for de novo activation of WUSCHEL during plant shoot regeneration. Plant Cell. DOI: 10.1105/tpc.16.0086.
 Zhang TQ, Lian H, Tang H, Dolezal K, Zhou CM, Yu S, Chen JH, Chen Q, Liu H, Ljung K, and Wang JW*. (2015). An intrinsic microRNA timer regulates progressive decline in shoot regenerative capacity in plants. Plant Cell. 27: 349-360.
 Rubio-Somoza I, Zhou CM, Confraria A, Martinho C, Born PV, Baena-Gonzalez E, Wang JW*, and Weigel D*. (2014). Temporal Control of Leaf Complexity by miRNA-Regulated Licensing of Protein Complexes. Curr Biol. 24: 2714-2719.
 Wang JW*. (2014). Regulation of flowering time by miR156-mediated age pathway. J Exp Bot. 65: 4723-4730.
 Yu S, Galvao V, Zhang YC, Horrer D, Zhang TQ, Hao YH, Feng YQ, Wang S, Schmid M, and Wang JW*. (2012). Gibberellin Regulates Arabidopsis Floral Transition through miR156-Targeted SQUAMOSA PROMOTER BINDING-LIKE Transcription Factors. Plant Cell. 24: 3320-3332.
 Gou JY, Felipes FF, Liu CJ, Weigel D, and Wang JW*. (2011). Negative regulation of anthocyanin biosynthesis in Arabidopsis by a miR156-targeted SPL transcription factor. Plant Cell. 23: 1512-1522.
 Wang JW, Schwab R, Czech B, Mica E, Weigel D*. (2008). Dual effects of miR156-Targeted SPL genes and CYP78A5/KLUH on plastochron length and organ size in Arabidopsis thaliana. Plant Cell. 20: 1231-1243.
The life cycle strategy of higher plants can be classified into three forms: annual, biennial and perennial. Under natural conditions, annual plants go from germination to death within one year. By contrast, biennial and perennial plants take at least two years to complete their biological lifecycles. Monocarpic perennials, such as bamboo, bloom only once and subsequently die, whereas polycarpic perennials continue to flower in subsequent years (although not necessarily every year). To dissect the molecular mechanism of seasonal flowering in perennial, we used bitter cress (Cardamine flexuosa), a herbaceous Brassicaceae widely distributed in Europe, and Populus x Canadensis (poplar).
Recently, we demonstrate that the level of miR156 sets a threshold for vernalization response. Age and vernalization pathways coordinately regulate flowering through modulating the expression of CfSOC1, a flower-promoting MADS-box gene. We note that, although both vernalization and age pathways operate in the related annual Arabidopsis thaliana, this species does not confer an age-dependent vernalization response. Thus, our results suggest that the recruitment of age cue in response to environmental signals contributes to the evolution of life cycle in plants.
 Zhou CM, Zhang TQ, Wang X, Yu S, Lian H, Tang H, Feng ZY, Zozomova-Lihova J, and Wang JW*. (2013). Molecular Basis of Age-dependent Vernalization in Cardamine flexuosa. Science. 340: 1097-1100.
 Wang JW*, Park MY, Wang LJ, Chen XY, Weigel D, and Poethig S*. (2011). MiRNA Control of Vegetative Phase Change in Trees. PLoS Genet. 7: e1002012.