Philippe Simier

Parasitic weeds are major agricultural threats through fine-tuned chemical interactions with their hosts. We investigated the germination-stimulating metabolites released by Arabidopsis thaliana germinating seeds into the spermosphere, the zone surrounding the seeds and showed that host–parasite chemical recognition can be initiated at the earliest stages of host seed germination. Through co-germination assays, biochemical fractionation of host germinating seed exudates and a mutant approach targeting the glucosinolate (GSL) and strigolactone (SL) pathways, we demonstrated that germinating A. thaliana Col-0 seeds specifically stimulated P. ramosa germination via GSL-derived isothiocyanates (ITCs), not SLs. This activity was MYB28, MYB29-dependent (aliphatic GSLs) and MYB34, MYB51, MYB122-independent (indolic GSLs). These results were confirmed throughout developmental stages of A. thaliana GSL-deficient lines using minirhizotron assays. The primary detected bio-active compound was 4-methylsulfinylbutyl isothiocyanate (4MSOB-NCS, sulforaphane). Additionally, 7-methylsulfinylheptyl isothiocyanate (7MSOH-NCS) was detected as another significant bio-active compound, although it was released in its GSL form and required myrosinase-mediated hydrolysis for activation. By contrast, SL-dependent inhibition of GSLs accumulation in seeds and P. ramosa germination stimulation operated through SLs AtD14 and KARs HTL-3 receptors partially independent on MAX2 integrator. These findings reinforce and complement our views on the major role played by A. thaliana exudated ITC and GSLs in triggering P. ramosa seed germination notably early in the spermosphere of the germinating host seeds. It also reveals a novel regulatory network where SLs indirectly modulate plant-plant interaction chemistry, representing an evolutionary trade-off between pathways. Understanding host–parasite chemical communication offers new avenues for controlling parasitic plants in Brassica crops.

Broomrapes (Phelipanche and Orobanche spp.) and witchweeds (Striga spp.) are parasitic weeds that are increasingly threatening crops worldwide. Seeds of these species rely on host-derived signals such as strigolactones (SL) to germinate. While cytokinins (CKs) were also reported as germination inducers of witchweeds, their role during the germination of broomrapes remains unexplored. Our study shows that some but not all CKs stimulate Striga hermonthica germination independently of SL and that high concentrations of bioactive CKs trigger Striga seedlings to differentiate into fully extruded embryo-like structures. In contrast, CK-free bases but not ribosylated or glycosylated conjugates are extremely potent inhibitors of broomrapes germination. Germination inhibition upon the CK signaling inhibitor PI-55 and inhibitor of CK degradation INCYDE suggest that the CK perception and degradation machinery is conserved in parasitic weeds. In Phelipanche ramosa, gene expression analyses combined with targeted quantification of CK contents revealed that SL first induce an increase in abscisic acid catabolism, then a modification of the CK endogenous pool in favor of inactive conjugates. Overall, this study provides valuable insights into the hormonal interplay governing seed germination in broomrapes and witchweeds, paving the way for future studies aimed at developing novel strategies for parasitic weed control.

BACKGROUND: Phelipanche ramosa L. (Pomel) is a noxious parasitic weed in field and vegetable crops in Mediterranean countries. Control of this pest is complex and far from being achieved, and new environmentally‐friendly strategies are being sought. The present study evaluates the possibility of using (±)‐catechins as a natural herbicide against broomrapes.RESULTS: The results show that (±)‐catechins have no effect on GR24‐induced germination over a wide concentration range (10⁻⁴ to 10⁻¹⁰ m), nor on radicle elongation after germination, but strongly inhibit, at 10⁻⁴ and 10⁻⁵ m, prehaustorium formation in response to the haustorium‐inducing factor, cis/trans‐zeatin. Accordingly, pot experiments involving the supplies of 10⁻⁵ m of (±)‐catechins to tomato plants infested or not with P. ramosa demonstrate that (±)‐catechins do not influence growth of non‐parasitized tomato plants and prevent heavy infestation by strongly reducing parasite attachments and inducing parasite necrosis once they are attached.CONCLUSION: This study points the potential use of (±)‐catechins for parasitic weed control. It raises also the question of the mechanisms involved in the inhibition of prehaustorium formation and the necrosis of parasite attachments in response to (±)‐catechins application. © 2024 The Author(s). Pest Management Science published by John Wiley & Sons Ltd on behalf of Society of Chemical Industry.

Phelipanche ramosa is a root parasitic plant fully dependent on host plants for nutrition and development. Upon germination, the parasitic seedling develops inside the infected roots a specific organ, the haustorium, thanks to the cell wall-degrading enzymes of haustorial intrusive cells, and induces modifications in the host’s cell walls. The model plant Arabidopsis thaliana is susceptible to P. ramosa; thus, mutants in cell wall metabolism, particularly those involved in pectin remodeling, like Atpme3-1, are of interest in studying the involvement of cell wall-degrading enzymes in the establishment of plant–plant interactions. Host–parasite co-cultures in mini-rhizotron systems revealed that parasite attachments are twice as numerous and tubercle growth is quicker on Atpme3-1 roots than on WT roots. Compared to WT, the increased susceptibility in AtPME3-1 is associated with reduced PME activity in the roots and a lower degree of pectin methylesterification at the host–parasite interface, as detected immunohistochemically in infected roots. In addition, both WT and Atpme3-1 roots responded to infestation by modulating the expression of PAE- and PME-encoding genes, as well as related global enzyme activities in the roots before and after parasite attachment. However, these modulations differed between WT and Atpme3-1, which may contribute to different pectin remodeling in the roots and contrasting susceptibility to P. ramosa. With this integrative study, we aim to define a model of cell wall response to this specific biotic stress and indicate, for the first time, the role of PME3 in this parasitic plant–plant interaction.

Phelipanche ramosa is a root parasitic plant fully dependent on host plants for nutrition and development. Once germinated, the parasitic seedling develops inside of the infected roots a specific organ, the haustorium, thanks to notably cell wall degrading enzymes of haustorial intrusive cells and induced modifications in host’s cell walls. The model plant Arabidopsis thaliana is susceptible to P. ramosa, thus mutants in cell wall metabolism, notably in pectin remodeling like Atpme3-1, are of interest in studying the involvement of cell wall degrading host’s enzymes in establishment of the plant-plant interaction. Host-parasite co-cultures in mini-rhizotrons systems revealed that parasite attachments are twice more numerous and tubercle growth is quicker on Atpme3-1 roots than on WT roots. When compared to the WT, increase in susceptibility in the mutant is associated to reduction in PME activity in roots and to lowering in the degree of pectin methylation at the host-parasite interface, as detected immunohistochemically in infected roots. In addition, both WT and Atpme3-1 responded to infestation by modulations in expression of PAE and PME encoding genes and in global related enzymes activities in roots, before and after parasite attachment. However, modulations differ in WT and Atpme3-1 and this may contribute to different pectin remodeling in roots and contrasted susceptibility to P. ramosa. By this integrative study, we could define a model of the cell wall response to this particular biotic stress and indicate for the first time the role of PME3 in this parasitic plant-plant interaction.

One-sentence summaryRecent advances provide insight into the molecular mechanisms underlying host-dependent seed germination and haustorium formation in parasitic plants.

The plant-parasitic plant interaction is a interesting model to study sink-source relationship and phloem unloading. The parasitic plants, such as the achlorophyllous plant Phelipanche ramosa, connect to the host phloem through the haustorium and act as supernumerary sinks for the host-derived photoassimilates, primarily sucrose. The application of the fluorescent symplastic tracer, carboxyfluorescein (CF) derived from carboxyfluorescein diacetate (CFDA), to the leaves of the host plant (Brassica napus) showed direct phloem connections at the host-parasite interface. These experiments also evidenced the dominant apoplastic pathway for phloem unloading in major vegetative sinks of the parasite, including tubercles and shoots, except the adventitious root apices. The CF experiments showed also the symplastic isolation of the phloem tissues from the sink tissues in tubercle and shoot of the parasite, then suggesting the pivotal role of sucrose transporters in sucrose unloading in P. ramosa sinks. Three cDNAs encoding sucrose transporters (PrSUT) were isolated from the parasitic plant. PrSUT1 transcripts accumulated at the same level in the tubercle throughout the parasite growth while a significant increase in transcript accumulation occurred after emergence in the flowering shoot, notably in the growing apical part. The in situ hybridization experiments revealed the PrSUT1 transcript accumulation in the mature phloem cells of both subterranean and flowering shoots, as well as in shoot terminal sinks corresponding to apical meristem, scale leaf primordia and immature vasculature. The transient expression experiments in Arabidopsis protoplasts showed that PrSUT1 was localized at the plasma membrane, suggesting its role in phloem functioning and sucrose uptake by the sink cells in P. ramosa. Conversely, the PrSUT2 transcript accumulation was constantly low in tubercles and shoots but PrSUT3 transcripts accumulated markedly in the subterranean and flowering shoots, in concordance with the PrSUT3 mRNA accumulation in multiple sink areas including apical meristem, scale-leaf primordia, immature vasculature and even storage parenchyma. However, the PrSUT3 transcripts did not accumulate in the mature phloem cells. The transient expression experiments in Arabidopsis protoplasts suggested a tonoplast localization of PrSUT3, for which nevertheless the involvement in intracellular sucrose transport needs clarification.