3. Post-Transcription Regulation of FT
Post-transcriptional regulation is one the crucial mechanisms responsible for the control of gene expression. It is cleared from the previous studies that TF; MYB, WEREWOLF are the reasons behind theFT mRNA stabilization whereas, at the molecular level the mode of action still need to be described in detail (Seo et al., 2011). It has been reported recently from the study on temperate grasses that microRNA (miRNA) and alternative splicing (AS) can cause modulation in FTexpression.
MicroRNAs (miRNAs) constitute an enormous family of small non-coding RNAs (approx. 21-24 nucleotides) that have emerged an important post-transcriptional regulators of gene expression in plants. Most of the miRNAs have perfectly complementary sequences to that of the targets due to which they become the reason for digestion of mRNA because of their involvement in gene silencing. Taking miR5200 as an example; detected in Brachypodium distachyon for the first time, cleaves the transcripts of two FT orthologs, FT1 and FT2(Figure 2) (Wu et al., 2013). Under different day lengths the study on miR5200 expression patterns and its targets showed that they are specifically identical but temporally different in Brachypodium distachyon . During long days the flowering delays in B. distachyon because of the overexpression of miR5200 (Wu et al., 2013).
In B. distachyon , under short day-conditions (SDs) and long-day conditions (LDs) the accumulation of miR5200 increases and decreases, respectively, because of the vigorous changes in the levels of H3K27 and H3K4me3 at its originator loci. Through a target mimicry strategy, knockdown of miR5200 activity accelerates flowering in B. distachyon under SDs; suggested that there is a negative effect of miR5200 on blooming in the photoperiod pathway (Wu et al., 2013). Due to the absence of miR5200 in maize, rice and other monocots, it was assumed that miR5200 may be a new miRNA with particular functions in Pooideae plants (Wu et al., 2013). In the experimental study on temperate grasses the non-inductive condition-dependent accumulation of mature miR5200 has been observed. Whereas, the involvement of photoperiod mediated regulation of its precursor has been reported in Brachypodium lineage which proposed that in different grasses, transcription or mRNA regulation could be a factor for miR5200 accumulation. (McKeown et al., 2017; Wu et al., 2013).
Alternative splicing (AS) is the reason which expands the diversity of the transcriptome which may result in main consequences in eukaryotes. It has been suggested that for FT regulation, the FT AS may be a common machinery because of the observation of a number ofFT events in various plants (Figure 2). The gene expression mostly regulates with the introduction of non-functional mRNA isoforms with a pre-mature stop codon by AS, which results into the degradation of mRNA. The two functional proteins; FT2α and FT2β , produce in the result of B. distachyon-FT2 AS. Surprisingly, delay of flowering occurs with the ectopic production of FT2β,which is totally opposite to the remarkably bloomed flowering with the overproduction of FT2a, suggesting that FT2α is a normal florigen protein whereas FT2β seems to be a repressor for flowering. The interesting part is that, both the FT2a andFT2β are identical except the absence of some amino acids (Qin et al., 2017). The opposite functions of FT2α and FT2β ; accelerated and delayed flowering respectively, have proved by an experimental study on transgenic plants of B. distachyon (Qin et al., 2017). Molecular analyses showed that FT2β cannot form a homodimer with itself though it form a heterodimer with FT2α,which inhibits florigen activation complex activities by restricting the ability of FT2α to bind 14-3-3s and FDs. The expression ratio ofFT2α / FT2β decreases with the passage of plant age in wheat, barley and B. distachyon (Qin et al., 2017), the blooming during juvenile stage has prevented by the presence of FT2β . Hence, premature flowering resulted in the absence of FT2β which may cause antagonistic effect on plant growth and development. It will be a remarkable study to analyze the expression of FT genes in temperate grasses; that wither they are regulated by these processes, as there is the involvement of 5ʹ capping and 3ʹ polyadenylation in post transcriptional modification of mRNAs.