Recently, CRISPR/Cas9 technology has emerged as a powerful approach for targeted genome modification in eukaryotic organisms from yeast to human cell lines. Its successful application in several plant species promises enormous potential for basic and applied plant research. However, extensive studies are still needed to assess this system in other important plant species, to broaden its fields of application and to improve methods. Here we showed that the CRISPR/Cas9 system is efficient in petunia (Petunia hybrid), an important ornamental plant and a model for comparative research. When PDS was used as target gene, transgenic shoot lines with albino phenotype accounted for 55.6%–87.5% of the total regenerated T0 Basta-resistant lines. A homozygous deletion close to 1 kb in length can be readily generated and identified in the first generation. A sequential transformation strategy—introducing Cas9 and sgRNA expression cassettes sequentially into petunia—can be used to make targeted mutations with short indels or chromosomal fragment deletions. Our results present a new plant species amenable to CRIPR/Cas9 technology and provide an alternative procedure for its exploitation.

Venation is a common anthocyanin pattern displayed in flowers that confers important ornamental traits to plants. An anthocyanin-related R2R3-MYB transcription factor, DPL, has been proposed to regulate corolla tube venation in petunia plants. Here, however, we provide evidence redefining the role of DPL in petunia. A CRISPR/Cas9-mediated mutation of DPL resulted in the absence of the vein-associated anthocyanin pattern above the abaxial surface of the flower bud, but not corolla tube venation, thus indicating that DPL did not regulate the formation of corolla tube venation. Alternately, quantitative real-time PCR analysis demonstrated that the spatiotemporal expression pattern of another R2R3-MYB gene, AN4, coincided with the formation of corolla tube venation in petunia. Furthermore, overexpression of AN4 promoted anthocyanin accumulation by increasing the expression of anthocyanin biosynthesis genes. CRISPR/Cas9-mediated mutation of AN4 led to an absence of corolla tube venation, suggesting that this gene in fact determines this key plant trait. Taken together, the results presented here redefine the prime regulator of corolla tube venation, paving the way for further studies on the molecular mechanisms underlying the various venation patterns in petunia.

Wintersweet (Chimonanthus praecox) is a popular garden plant because of its flowering time, sweet fragrance, and ornamental value. However, research into the molecular mechanism that regulates flower development in wintersweet is still limited. In this study, we sought to investigate the molecular characteristics, expression patterns, and potential functions of two C3H-type zinc finger (CZF) protein genes, CpCZF1 and CpCZF2, which were isolated from the wintersweet flowers based on the flower developmental transcriptome database. CpCZF1 and CpCZF2 were more highly expressed in flower organs than in vegetative tissues, and during the flower development, their expression profiles were associated with flower primordial differentiation, especially that of petal and stamen primordial differentiation. Overexpression of either CpCZF1 or CpCZF2 caused alterations on stamens in transgenic Arabidopsis. The expression levels of the stamen identity-related genes, such as AGAMOUS (AG), PISTILLATA (PI), SEPALLATA1 (SEP1), SEPALLATA2 (SEP2), SEPALLATA3 (SEP3), APETALA1 (AP1), APETALA2 (AP2), and boundary gene RABBIT EAR (RBE) were significantly up-regulated in CpCZF1 overexpression lines. Additionally, the transcripts of AG, PI, APETALA3 SEP1-3, AP1, and RBE were markedly increased in CpCZF2 overexpressed plant inflorescences. Moreover, CpCZF1 and CpCZF2 could interact with each other by using yeast two-hybrid and bimolecular fluorescence complementation assays. Our results suggest that CpCZF1 and CpCZF2 may be involved in the regulation of stamen development and cause the formation of abnormal flowers in transgenic Arabidopsis plants.

Although artificial microRNA (amiRNA) technology has been used frequently in gene silencing in plants, little research has been devoted to investigating the accuracy of amiRNA precursor processing. In this work, amiRNAchs1 (amiRchs1), based on the Arabidopsis miR319a precursor, was expressed in order to suppress the expression of CHS genes in petunia. The transgenic plants showed the CHS gene-silencing phenotype. A modified 5′ RACE technique was used to map small-RNA-directed cleavage sites and to detect processing intermediates of the amiRchs1 precursor. The results showed that the target CHS mRNAs were cut at the expected sites and that the amiRchs1 precursor was processed from loop to base. The accumulation of small RNAs in amiRchs1 transgenic petunia petals was analyzed using the deep-sequencing technique. The results showed that, alongside the accumulation of the desired artificial microRNAs, additional small RNAs that originated from other regions of the amiRNA precursor were also accumulated at high frequency. Some of these had previously been found to be accumulated at low frequency in the products of ath-miR319a precursor processing and some of them were accompanied by 3′-tailing variant. Potential targets of the undesired small RNAs were discovered in petunia and other Solanaceae plants. The findings draw attention to the potential occurrence of undesired target silencing induced by such additional small RNAs when amiRNA technology is used. No appreciable production of secondary small RNAs occurred, despite the fact that amiRchs1 was designed to have perfect complementarity to its CHS-J target. This confirmed that perfect pairing between an amiRNA and its targets is not the trigger for secondary small RNA production. In conjunction with the observation that amiRNAs with perfect complementarity to their target genes show high efficiency and specificity in gene silencing, this finding has an important bearing on future applications of amiRNAs in gene silencing in plants.

MIGS (miRNA-induced gene silencing) is a straightforward and efficient gene silencing technique in Arabidopsis. It works by exploiting miR173 to trigger the production of phasiRNAs (phased small interfering RNAs). MIGS can be used in plant species other than Arabidopsis by co-expression of miR173 and target gene fragments fused to an upstream miR173 target site. However, the efficiency and technical mechanisms have not been thoroughly investigated in other plants. In this work, two vectors, pMIGS-chs and pMIGS-pds, were constructed and transformed into petunia plants. The transgenic plants showed CHS (chalcone synthase) and PDS (phytoene desaturase) gene-silencing phenotypes respectively, indicating that MIGS functions in petunia. MIGS-chs plants were used to investigate the mechanisms of this technique in petunia. Results of 5′- RACE showed that the miR173 target site was cleaved at the expected position and that endogenous CHS genes were cut at multiple positions. Small RNA deep sequencing analysis showed that the processing of Arabidopsis miR173 precursors in MIGS-chs transgenic petunia plants did not occur in exactly the same way as in Arabidopsis, suggesting differences in the machinery of miRNA processing between plant species. Small RNAs in-phase with the miR173 cleavage register were produced immediately downstream from the cleavage site and out-of-phase small RNAs were accumulated at relatively high levels from processing cycle 5 onwards. Secondary siRNAs were generated from multiple sites of endogenous CHS-A and CHS-J genes, indicating that miR173 cleavage induced siRNAs have the same ability to initiate siRNA transitivity as the siRNAs functioning in co-suppression and hpRNA silencing. On account of the simplicity of vector construction and the transitive amplification of signals from endogenous transcripts, MIGS is a good alternative gene silencing method for plants, especially for silencing a cluster of homologous genes with redundant functions.