AGO1 and AGO2 Act Redundantly in miR408-Mediated Plantacyanin Regulation

Background: In Arabidopsis, AGO1 and AGO2 associate with small RNAs that exhibit a Uridine and an Adenosine at their 59 end, respectively. Because most plant miRNAs have a 59U, AGO1 plays many essential roles in miRNA-mediated regulation of development and stress responses. In contrast, AGO2 has only been implicated in antibacterial defense in association with miR393*, which has a 59A. AGO2 also participates in antiviral defense in association with viral siRNAs. Principal Findings: This study reveals that miR408, which has a 59A, regulates its target Plantacyanin through either AGO1 or AGO2. Indeed, neither ago1 nor ago2 single mutations abolish miR408-mediated regulation of Plantacyanin. Only an ago1 ago2 double mutant appears compromised in miR408-mediated regulation of Plantacyanin, suggesting that AGO1 and AGO2 have redundant roles in this regulation. Moreover, the nature of the 59 nucleotide of miR408 does not appear essential for its regulatory role because both a wildtype 59A-MIR408 and a mutant 59U-MIR408 gene complement a mir408 mutant. Conclusions/Significance: These results suggest that miR408 associates with both AGO1 and AGO2 based on criteria that differ from the 59 end rule, reminiscent of miR390-AGO7 and miR165/166-AGO10 associations, which are not based on the nature of the 59 nucleotide

Site-directed mutagenesis in Arabidopsis thaliana using dividing tissue-targeted RGEN of the CRISPR/Cas system to generate heritable null alleles

Dividing tissue-targeted site-directed mutagenesis using RGEN of CRISPR/Cas system produces heritable mutations in Arabidopsis thaliana. Site-directed genome engineering in higher plants has great potential for basic research and molecular breeding. Here, we describe a method for site-directed mutagenesis of the Arabidopsis nuclear genome that efficiently generates heritable mutations using the RNA-guided endonuclease (RGEN) derived from bacterial clustered regularly interspaced short palindromic repeats (CRISPR)-Cas9 (CRISPR associated) protein system. To induce mutagenesis in proliferating tissues during embryogenesis and throughout the plant life cycle, the single guide RNA (sgRNA) and Cas9 DNA endonuclease were expressed from the U6 snRNA and INCURVATA2 promoters, respectively. After Agrobacterium-mediated introduction of T-DNAs encoding RGENs that targets FLOWERING LOCUS T (FT) and SQUAMOSA PROMOTER BINDING PROTEIN-LIKE 4 genes, somatic mutagenesis at the targeted loci was observed in T1 transformants. In the results of FT-RGEN, T1 plants often showed late flowering indicative of the presence of large somatic sectors in which the FT gene is mutated on both chromosomes. DNA sequencing analysis estimated that about 90 % of independent chromosomal DNA fragments carried mutations in the analyzed tissue of a T1 plant showing late flowering. The most frequently detected somatic polymorphism showed a high rate of inheritance in T2 plants, and inheritance of less frequent polymorphisms was also observed. As a result, late-flowering plants homozygous for novel, heritable null alleles of FT including a 1 bp insertion or short deletions were recovered in the following T2 and T3 generations. Our results demonstrate that dividing tissue-targeted mutagenesis using RGEN provides an efficient heritable genome engineering method in A. thaliana

Advanced Engineering of Lipid Metabolism in Nicotiana benthamiana Using a Draft Genome and the V2 Viral Silencing-Suppressor Protein

The transient leaf assay in Nicotiana benthamiana is widely used in plant sciences, with one application being the rapid assembly of complex multigene pathways that produce new fatty acid profiles. This rapid and facile assay would be further improved if it were possible to simultaneously overexpress transgenes while accurately silencing endogenes. Here, we report a draft genome resource for N. benthamiana spanning over 75% of the 3.1 Gb haploid genome. This resource revealed a two-member NbFAD2 family, NbFAD2.1 and NbFAD2.2, and quantitative RT-PCR (qRT-PCR) confirmed their expression in leaves. FAD2 activities were silenced using hairpin RNAi as monitored by qRT-PCR and biochemical assays. Silencing of endogenous FAD2 activities was combined with overexpression of transgenes via the use of the alternative viral silencing-suppressor protein, V2, from Tomato yellow leaf curl virus. We show that V2 permits maximal overexpression of transgenes but, crucially, also allows hairpin RNAi to operate unimpeded. To illustrate the efficacy of the V2-based leaf assay system, endogenous lipids were shunted from the desaturation of 18:1 to elongation reactions beginning with 18:1 as substrate. These V2-based leaf assays produced ~50% more elongated fatty acid products than p19-based assays. Analyses of small RNA populations generated from hairpin RNAi against NbFAD2 confirm that the siRNA population is dominated by 21 and 22 nt species derived from the hairpin. Collectively, these new tools expand the range of uses and possibilities for metabolic engineering in transient leaf assays. © 2012 Naim et al