Wheat microRNAs and Their Association with Hybrid Incompatibility
Recently, the association of microRNA (miRNA) networks with developmental plasticity in higher plants has been discussed (Rubio-Somoza and Weigel 2011). miRNAs have been defined as a highly conserved class of small non-coding RNA molecules acting in post-transcriptional gene repression (Bartel 2009). The mature miRNA coupled with an RNA-induced silencing complex directs repression of mRNAs containing the complementary sequence, usually by mRNA cleavage in higher plants. Modified expression levels of small RNAs including miRNAs have been reported in the allopolyploidization process of Arabidopsis suecica Fries (Ha et al. 2009; Ng et al. 2012). A number of miRNA molecules have been identified in common wheat by next generation sequencing of small RNA molecules (Kanter et al. 2012; Yao and Sun 2012). The percentage of small RNAs corresponding to miRNAs increases with wheat polyploidy level, with the abundance of most miRNA species similar to midparent values in an interspecific wheat hybrid between tetraploid wheat and Ae. tauschii (Kenan-Eichler et al. 2011). Levels of accumulation of some miRNAs, such as miR168, miR156 and miR390, respective to the midparent values were distinct in ABD hybrids (Kenan-Eichler et al. 2011), though a direct relationship between the altered miRNA levels and phenotypic changes during allopolyploid evolution of wheat has not been demonstrated.
To clarify temperature-dependent changes in expression profiles of miRNAs in type II necrosis plants, we conducted deep sequencing using small RNAs isolated from crown tissues (Matsuda et al. in preparation). A comparative study of miRNA expression profiles showed that growth temperature dramatically changed the expression profiles of miRNAs, and that more than 200 (15 %) of the identified 1,600 miRNAs were differentially expressed between the wild type and type II necrosis plants. Among the differentially expressed miRNAs, miR156 was upregulated in the crown tissues of type II necrosis plants under normal temperatures. In maize, the grass clump phenotype of Corngrass1 mutants is caused by the overexpression of miR156, which induces altered expression of SQUAMOSA PROMOTER BINDING PROTEIN LIKE (SPL) transcription factor genes (Chuck et al. 2007). SPL genes, post-transcriptionally regulated by miR156, control tillering in higher plants (Xie et al. 2006; Fu et al. 2012). Overexpression of miR156 was also observed in synthetic hexaploid wheat relative to the miR156 midparental value (KenanEichler et al. 2011). These observations imply significant association of miRNAs with temperature-dependent phenotypic plasticity in the Net1-Net2 interaction at the crown tissues. In fact, transcript accumulation of some wheat SPLs containing the miR156 target site were significantly reduced in the crown tissues of type II necrosis plants only at the normal temperature (Matsuda et al. in preparation). Therefore, we presumed that, at the normal temperature, Net1-Net2 epistatic interaction increased the miR156 level, and that the enhanced levels of miR156 led to digestion of SPL transcripts, resulting in an excessive increase in tiller numbers in type II necrosis (Fig. 17.1). Therefore, gene expression profiles including miRNAs in SAM in response to growth temperature could be dramatically altered in wheat hybrids and allopolyploids, resulting in phenotypic plasticity. Further studies of interspecific hybrids in Triticum and Aegilops species should offer new knowledge about hybrid incompatibility.
Acknowledgments This work was supported by grants from the Ministry of Education, Culture, Sports, Science and Technology of Japan (Grants-in-Aid for Scientific Research (B) Nos. 21380005 and 25292008).