Regulation of microRNA on plant development and viral infection

Plant microRNAs and their role in defense against viruses: a bioinformatics approach

BACKGROUND

microRNAs (miRNAs) are non-coding short RNAs that regulate gene expression in eukaryotes by translational inhibition or cleavage of complementary mRNAs. In plants, miRNAs are known to target mostly transcription factors and are implicated in diverse aspects of plant growth and development. A role has been suggested for the miRNA pathway in antiviral defense in plants. In this work, a bioinformatics approach was taken to test whether plant miRNAs from six species could have antiviral activity by targeting the genomes of plant infecting viruses.

RESULTS

All plants showed a repertoire of miRNAs with potential for targeting viral genomes. The viruses were targeted by abundant and conserved miRNA families in regions coding for cylindrical inclusion proteins, capsid proteins, and nuclear inclusion body proteins. The parameters for our predicted miRNA:target pairings in the viral genomes were similar to those for validated targets in the plant genomes, indicating that our predicted pairings might behave in-vivo as natural miRNa-target pairings. Our screening was compared with negative controls comprising randomly generated miRNAs, animal miRNAs, and genomes of animal-infecting viruses. We found that plant miRNAs target plant viruses more efficiently than any other sequences, but also, miRNAs can either preferentially target plant-infecting viruses or target any virus without preference.

CONCLUSIONS

Our results show a strong potential for antiviral activity of plant miRNAs and suggest that the miRNA pathway may be a support mechanism to the siRNA pathway in antiviral defense.

Ectopic expression of miR396 suppresses GRF target gene expression and alters leaf growth in Arabidopsis

MicroRNAs (miRNAs) are comprised of approximately 21 nucleotide (nt) RNAs that play important regulatory roles in growth and development by targeting mRNAs for cleavage or translational repression. In Arabidopsis, more than one hundred miRNAs have been identified but the biological functions of only a limited number of them have been determined by molecular genetic analysis. miR396 is a miRNA conserved among the dicot and monocot plants. In Arabidopsis, miR396 has two loci (MIR396a and MIR396b) and targets six Growth-Regulating Factor (GRF) genes encoding putative transcription factors with roles in plant leaf growth. Using a northern blot hybridizations approach, we have found that MIR396 is predominantly expressed in leaf and seedling. To further analyze the role of miR396 in the regulation of target genes and leaf growth, we have generated transgenic Arabidopsis plants that constitutively overexpress MIR396a or MIR396b. These transgenic plants have narrow-leaf phenotypes due to reduction in cell number. Ectopic overexpression of MIR396 represses expression of not only six GRF genes but also GIF1 encoding a GRF-interacting transcription coactivator with a role in cell proliferation in leaf. In addition, transgenic MIR396-overexpressing plants have lower densities of stomata and are more tolerant to drought than wild-type plants. These results strongly support the belief that miR396 plays an important role in plant leaf growth and development, most likely by repressing GRF gene expression.