Saffron (Crocus sativus L.) is a geophytic plant belonging to the family Iridaceae. Saffron is a plant with economic significance, not only as a culinary spice but also due to its pharmacological potential (Gohari et al. 2013). The sterile nature of saffron poses a challenge to traditional breeding methods that rely on the exchange of genetic material through sexual reproduction. Saffron plants primarily propagate vegetatively through corms, which are underground storage organs. The limited genetic diversity adds to the ability to introduce desired traits through conventional breeding. In such cases, biotechnological approaches become valuable tools for genetic improvement. A comprehensive understanding of the genetic and molecular mechanisms controlling different developmental pathways can aid biotechnological approaches in saffron genetic improvement. In recent years, transcriptomic studies in saffron have identified numerous genes and transcription factors, but the lack of an efficient genetic transformation system impedes the progression to conclusive functional studies. Agrobacterium-mediated transformation is widely used to manipulate genes in a variety of dicots but with limitations in monocots. Saffron is a monocot and to date, no efficient protocols are available for its genetic transformation. Thus, there is a need to develop alternative methods for functional validation of genes.
Virus-induced gene silencing (VIGS) is a valuable tool in plant molecular biology that offers flexibility in studying gene functions across various plant species (Burch-Smith, Miller et al. 2006). VIGS implies evolutionarily conserved antiviral defence mechanisms present in plants (Baulcombe 2004). In general, the target gene of interest is integrated into a viral vector system. The viral vector acts as a delivery vehicle, carrying the genetic material that triggers the post-translational gene silencing mechanism against the target gene (Vance and Vaucheret 2001). VIGS enables the analysis of gene functions in plant species that are not easily amenable to stable genetic transformation. VIGS has proven to be a valuable tool for examining gene functions in various plant species, including monocot crops like rice (Kant and Dasgupta 2017), maize, wheat (Zhang et al. 2017), and barley (Yuan et al. 2011). VIGS has also been successfully employed for the functional characterization of genes in monocot geophytes such as Gladiolus (Li et al. 2023), Tulips (Sun et al. 2022), Lilium (Xu et al. 2019; Pan et al. 2023).
The availability of advanced VIGS vectors with broad host ranges and high silencing efficiency has significantly facilitated gene function studies in monocot plants (Mei and Whitham 2018; Mei et al. 2016; Ding et al. 2006; Holzberg et al. 2002). For example Tobacco rattle virus (TRV)(Ratcliff et al. 2001), foxtail mosaic virus (FoMV)(Liu et al. 2016), Potato virus X (PVX)(Jones et al. 1999), Barley stripe mosaic virus (BSMV)(Holzberg et al. 2002), Tobacco mosaic virus (TMV) (Kumagai et al. 1995), Cabbage leaf curl virus (CaLCuV) (Muangsan et al. 2004). However, the Tobacco rattle virus (TRV) vector stands out as a valuable tool in plant molecular biology and functional genomics. Its ability to induce efficient gene silencing without causing overt viral symptoms or significant negative effects on host plant growth has made it a preferred choice for researchers studying gene functions in diverse plant species.
In this study, we examined the use of VIGS in saffron via silencing Phytoene Desaturase (PDS), a gene widely used as a marker due to its visible phenotype in the VIGS system (Cunningham Jr and Gantt 1998). PDS encodes for a carotenoid biosynthesis gene and silencing it blocks the production of carotenoids and causes the degradation of chlorophyll, leading to visible photobleached phenotype (Di Stilio et al. 2010). VIGS has not been established nor reported in saffron. Our study provides the first successful demonstration of the Virus-Induced Gene Silencing (VIGS) system in inducing endogenous gene silencing in saffron. The successful use of VIGS lays the foundation for future studies aiming to characterize gene function in saffron.