Broomrapes ( Phelipanche aegyptiaca and Orobanche spp.) are obligate plant parasites that cause extreme damage to crop plants. The parasite seeds have strict requirements for germination, involving preconditioning and exposure to specific chemicals strigolactones [SLs] exuded by the host roots. SLs are plant hormones derived from plant carotenoids via a pathway involving the Carotenoid Cleavage Dioxygenase 8 ( CCD8) . Having no effective means to control parasitic weeds in most crops, and with CRISPR/Cas9 being an effective gene-editing tool, here we demonstrate that CRISPR/Cas9-mediated mutagenesis of the CCD8 gene can be used to develop host resistance to the parasitic weed P . aegyptiaca . Cas9/single guide (sg) RNA constructs were targeted to the second exon of CCD8 in tomato ( Solanum lycopersicum L .) plants. Several CCD8 Cas9 mutated tomato lines with variable insertions or deletions in CCD8 were obtained with no identified off-targets. Genotype analysis of T1 plants showed that the introduced CCD8 mutations are inherited. Compared to control tomato plants, the CCD8 Cas9 mutant had morphological changes that included dwarfing, excessive shoot branching and adventitious root formation. In addition, SL-deficient CCD8 Cas9 mutants showed a significant reduction in parasite infestation compared to non-mutated tomato plants. In the CCD8 Cas9 mutated lines, orobanchol (SL) content was significantly reduced but total carotenoids level and expression of genes related to carotenoid biosynthesis were increased, as compared to control plants. Taking into account, the impact of plant parasitic weeds on agriculture and difficulty to constitute efficient control methods, the current study offers insights into the development of a new, efficient method that could be combined with various collections of resistant tomato rootstocks.

All living organisms engage in parasitic relations, as either parasites or hosts, and these interactions play an essential role in biological evolution and the functioning of the biosphere (Combes, 2001). Even among photosynthetic organisms, parasitic interactions are often established by both algae and land plants (Oborník, 2019). The most widespread form of parasitism among plants is characterized by the ability to develop a specialized organ called the haustorium (Kuijt, 1969). This organ differentiates the species commonly referred to as parasitic flowering plants from other non-autotrophic plants that require fungal hosts or partners to fulfill their nutritional needs (Feild and Brodribb, 2005; Merckx et al., 2009). Usually developed soon after germination, the haustorium enables attachment AJB REVIEWS

Root parasitic weeds in Orobanchaceae pose a tremendous threat to agriculture worldwide. We used an in vitro assay to screen libraries of small molecules for those capable of inhibiting or enhancing haustorium development in the parasitic plant Triphysaria versicolor. Several redox-modifying molecules and one structural analog of 2,6-dimethoxybenzoquine (DMBQ) inhibited haustorium development in the presence of the haustorium-inducing factor DMBQ, some of these without apparent growth inhibition to the root. Triphysaria seedlings were able to acclimate to some of these redox inhibitors. Transcript levels of four early-stage haustorium genes were differentially influenced by inhibitors. These novel haustorium inhibitors highlight the importance of redox cycling for haustorium development and suggest the potential of controlling parasitic weeds by interrupting early-stage redox-signaling pathways.

Triphysaria is a facultative parasitic plant in the Orobanchaceae that parasitizes the roots of a wide range of host plants including Arabidopsis , Medicago , rice and maize. The important exception to this broad host range is that Triphysaria rarely parasitize other Triphysaria . We explored self and kin recognition in Triphysaria versicolor and showed that exudates collected from roots of host species, Arabidopsis thaliana and Medicago truncatula , induced haustorium development when applied to the roots of Triphysaria seedlings in vitro while those collected from Triphysaria did not. In mixed exudate experiments, Triphysaria exudates did not inhibit the haustorium-inducing activity of those from host roots. Interestingly, when roots of Triphysari a seedlings were treated with either horseradish peroxidase or fungal laccase, the extracts showed haustorium-inducing factor (HIF) activity, suggesting that Triphysaria roots contain the proper substrates for producing HIFs. Transgenic Triphysaria roots overexpressing a fungal laccase gene TvLCC1 showed an increased responsiveness to a known HIF, 2,6-dimethoxy benzoquinone (DMBQ), in developing haustoria. Our results indicate kin recognition in Triphysaria is associated with the lack of active HIFs in root exudates. Treatment of Triphysaria roots with enzymatic oxidases activates or releases molecules that are HIFs. This study shows that exogenously applied oxidases can activate HIFs in Triphysaria roots that had no previous HIF activity. Further studies are necessary to determine if differential oxidase activities in host and parasite roots account for the kin recognition in haustorium development.