miR172 regulates stem cell fate and defines the inner boundary of APETALA3 and PISTILLATA expression domain in Arabidopsis floral meristems

In Arabidopsis, two floral homeotic genes APETALA2 (AP2) and AGAMOUS (AG) specify the identities of perianth and reproductive organs, respectively, in flower development. The two genes act antagonistically to restrict each other to their proper domains of action within the floral meristem. In addition to AG, which antagonizes AP2, miR172, a microRNA, serves as a negative regulator of AP2. In this study, we showed that AG and miR172 have distinct functions in flower development and that they largely act independently in the negative regulation of AP2. We uncovered functions of miR172-mediated repression of AP2 in the regulation of floral stem cells and in the delineation of the expression domain of another class of floral homeotic genes. Given the antiquity of miR172 in land plants, our findings have implications for the recruitment of a microRNA in the building of a flower in evolution.     

Repression of Flowering by the miR172 Target SMZ

A small mobile protein, encoded by the FLOWERING LOCUS T (FT) locus, plays a central role in the control of flowering. FT is regulated positively by CONSTANS (CO), the output of the photoperiod pathway, and negatively by FLC, which integrates the effects of prolonged cold exposure. Here, we reveal the mechanisms of regulation by the microRNA miR172 target SCHLAFMÜTZE (SMZ), a potent repressor of flowering. Whole-genome mapping of SMZ binding sites demonstrates not only direct regulation of FT, but also of many other flowering time regulators acting both upstream and downstream of FT, indicating an important role of miR172 and its targets in fine tuning the flowering response. A role for the miR172/SMZ module as a rheostat in flowering time is further supported by SMZ binding to several other genes encoding miR172 targets. Finally, we show that the action of SMZ is completely dependent on another floral repressor, FLM, providing the first direct connection between two important classes of flowering time regulators, AP2- and MADS-domain proteins.     

miR172: a messenger between nodulation and flowering

Legumes coordinate nodulation and plant development to maximize reproductive success, but the underlying molecular mechanisms are not well understood. A recent study by Yun et al. has revealed that nodulation drives root-to-shoot movement of microRNA172 (miR172) to accelerate flowering time, thus building a new bridge between nodulation and plant growth regulation.     

APETALA 2‐like genes AP2L2 and Q specify lemma identity and axillary floral meristem development in wheat

The spikelet is the basic unit of the grass inflorescence. In tetraploid (Triticum turgidum) and hexaploid wheat (Triticum aestivum), the spikelet is a short indeterminate branch with two proximal sterile bracts (glumes) followed by a variable number of florets, each including a bract (lemma) with an axillary flower. Varying levels of miR172 and/or its target gene Q (AP2L5) result in gradual transitions of glumes to lemmas, and vice versa. Here, we show that AP2L5 and its related paralog AP2L2 play critical and redundant roles in the specification of axillary floral meristems and lemma identity. AP2L2, also targeted by miR172, displayed similar expression profiles to AP2L5 during spikelet development. Loss‐of‐function mutants in both homeologs of AP2L2 (henceforth ap2l2) developed normal spikelets, but ap2l2 ap2l5 double mutants generated spikelets with multiple empty bracts before transitioning to florets. The coordinated nature of these changes suggest an early role of these genes in floret development. Moreover, the flowers of ap2l2 ap2l5 mutants showed organ defects in paleas and lodicules, including the homeotic conversion of lodicules into carpels. Mutations in the miR172 target site of AP2L2 were associated with reduced plant height, more compact spikes, promotion of lemma‐like characters in glumes and smaller lodicules. Taken together, our results show that the balance in the expression of miR172 and AP2‐like genes is crucial for the correct development of spikelets and florets, and that this balance has been altered during the process of wheat and barley (Hordeum vulgare) domestication. The manipulation of this regulatory module provides an opportunity to modify spikelet architecture and improve grain yield.