The Critical Role of miRNAs in Regulation of Flowering Time and Flower Development

Abstract

Flowering is an important biological process for plants that ensures reproductive success. The onset of flowering needs to be coordinated with an appropriate time of year, which requires tight control of gene expression acting in concert to form a regulatory network. MicroRNAs (miRNAs) are non-coding RNAs known as master modulators of gene expression at the post-transcriptional level. Many different miRNA families are involved in flowering-related processes such as the induction of floral competence, floral patterning, and the development of floral organs. This review highlights the diverse roles of miRNAs in controlling the flowering process and flower development, in combination with potential biotechnological applications for miRNAs implicated in flower regulation.

Keywords: floral induction; flower development; flower regulation; juvenility; microRNAs.

Figure 1

Scheme of plant microRNA biogenesis. Plant microRNA are transcribed by RNA poly II from the transcripts located in intergenic regions. Pri-miRNA was stabilized by DDL, then binds with Serrate (SE), Hyponasty Leaves (HYL1), and cap-binding complex (CBC), and processed by DCL1 into pre-miRNA. The pre-miRNA duplex is methylated by Hua Enhancer 1 (HEN1) and transported from the nucleus to the cytoplasm by HASTY. The guide miRNA strand binds AGO1 and is loaded into the RISC complex to carry out the silencing process.

Figure 2

Analysis of different miRNAs families and their target genes implicated in the regulation of flower induction. The accumulation of miR156 subsequently declines with an increase in plant age, other factors such as sugar (HXK1) accumulation and the ambient temperature gradually repress miRNA156 expression, resulting in rising levels of SPL family genes. MiR172 expression is positively regulated by GI, that leads to the down-regulation of AP2-type floral repressors, miR172 is also regulated by ambient temperature. The miR159 is regulated by the GA pathway and in turn, miR159 regulates the MYB transcription factors. The DELLA protein represses the expression of miR159 and the SPL3/4/5 genes. The miR390-ARF3/4 module regulates floral transition indirectly by inducing the production of ta-siRNAs from the TAS3 locus, through putative control over SPL gene expression. The ambient temperature-induced miR399-PHO2 module and stress-induced miR169-NF-Y module regulate floral induction by regulating the expression of floral meristem identity genes. Overall, miRNAs can either promote or inhibit the expression of the FMI (floral meristem identity genes) and/or FPI (floral pathway integrator), thereby promoting or delaying the onset of flowering, respectively. AP2, APETALA2; ARF, AUXIN RESPONSE FACTOR; HXK1, hexokinase; GI, GIGANTEA; NF-Y, NUCLEAR FACTOR Y; GA, gibberellic acid; SPL, SQUAMOSA PROMOTER-BINDING PROTEIN-LIKE; PHO2, PHOSPHATE 2; ta-siRNA, trans-acting siRNA; TPS1, TREHALOSE-6-PHOSPHATE SYNTHASE1; PHO2, PHOSPHATE 2.

Figure 3

Analysis of different miRNA families and their target genes involved in genes in setting boundaries, flower development, and male/female fertility. MiR319 and miR159 and their target TCP proteins and GAMYB interact with miR167 and in return down-regulates the level of ARF6 and ARF8. GA treatment causes the down-regulation of both miR159 and GAMYB transcription factors through the degradation of DELLA proteins. MiR160 represses the expression of ARF genes which ultimately contribute to flower development. ARF, AUXIN RESPONSE FACTOR; GA, Gibberellic acid; CUC, CUP-SHAPED COTYLEDON; AP2, APETELA 2; GOB, GOBLET; LUG, LEUNIG; LFY, LEAFY; NAM, NO APICAL MERISTEM; NF-Y, NUCLEAR FACTOR Y.