Differences in seed germination response of two populations of Phelipanche ramosa (L.) Pomel to a set of GR24 concentrations and durations of stimulation

Phelipanche ramosa is a major weed holoparasite characterized by a broad host range with a suboptimal development on numerous hosts, suggesting inter- or intra-species specificities. Seeds of P. ramosa germinate after exposure to exogenous chemicals exuded by surrounding host roots such as strigolactones, the concentrations of these germination stimulants varying between hosts. In France, P. ramosa is characterized by genetically differentiated populations presenting varying germination rates and a host specificity. The objective of our study was to investigate the sensitivity of seeds of two P. ramosa populations harvested on tobacco and oilseed rape, to a set of GR24 concentrations, a synthetic strigol analogue. The assessment of the germination rate was based on in vitro experiments. Seeds of P. ramosa were placed in Petri dishes with various concentrations of GR24. The cumulative number of germinated seeds of P. ramosa was counted several times after application of the treatment. Cumulative germination curves were analysed using a three-parameter log-logistic model and a time-to-event approach. The results show that the germination rate of P. ramosa seeds depends on the GR24 concentration and the duration of stimulation, but also that the response to these two factors varies greatly according to the origin of the P. ramosa seeds. The difference in germination speed between P. ramosa populations further shows distinct responses at the intraspecific level, thus suggesting that the specialization of P. ramosa probably occurs at least from the first stage of the holoparasite cycle.

Allelopathic Effect of Quercetin, a Flavonoid from Fagopyrum esculentum Roots in the Radicle Growth of Phelipanche ramosa: Quercetin Natural and Semisynthetic Analogues Were Used for a Structure-Activity Relationship Investigation

Allelopathic potential of buckwheat roots on the radicle growth of the broomrape weed species Orobanche cumana and Phelipanche ramosa was studied. Buckwheat root exudates induced a significant growth inhibition in P. ramosa radicles but radicles of O. cumana were not affected. Among the metabolites present in the root organic extract we identified the flavonol quercetin and the stilbene p-coumaric acid methyl ester with only quercetin showing inhibitory effect on P. ramosa. The activity of quercetin was compared with other two similar flavanoids, the flavone apigenin and the dihydroflavanol 3-O-acetylpadmatin extracted respectively from Lavandula stoechas and Dittrichia viscosa plants. In this comparative assay only 3-O-acetylpadmatin besides quercetin, showed inhibition activity of radicle growth while apigenin was inactive. These results indicated that the presence of two ortho-free hydroxy groups of C ring, like catechol, could be an important feature to impart activity while the carbon skeleton of B ring and substituents of both A and B rings are not essential. Besides reduction of radicle growth, haustorium induction was observed at the tip of P. ramosa radicles treated with quercetin which swelled and a layer of papillae was formed. Activity of quercetin on haustorium induction in P. ramosa was assayed in comparison with the known haustorium-inducing factor 2,6-dimethoxy-p-benzoquinone (DMBQ) and a three partial methyl ether derivatives semisynthetized from quercetin. Results indicated that P. ramosa haustorium was induced by DMBQ at concentrations of 1–0.5 mM and quercetin and its derivatives at concentration range 0.1–0.05 mM.

Use of Olive-Mill Wastewater for the Containment of Phelipanche ramosa in Open Field of Processing Tomato Crop

Phelipanche ramosa (L.) Pomel is a root holoparasitic weed that infested many crops, particularly the processing tomato. In semi-arid regions of the world, this problem is increasing, both in intensity and in acreages. In the last few years, researches for the control of this parasite has increased its effort to use suitable methods, as well as the utilization of natural compounds. In this regard, four experimental trials were carried out over three growing seasons (2014, 2016 and 2017), at Foggia (Apulia Region, Southern Italy), in the open field, on processing tomato (cv. “Dres”) crop, infested by P. ramosa, where the olive-mill wastewater (OMW) was incorporated into the soil at the dose of 80 m3 ha-1 40 days prior to tomato seedling transplant in comparison with the untreated control. The numbers of P. ramosa emerged shoots (branched plants) were significantly lower in the OMW treatment than in the untreated control. The tomato marketable yield showed a significantly higher value in the OMW treatment than the untreated control. No significant differences for the fruit qualitative characteristics were observed.

The Physcomitrium (Physcomitrella) patens PpKAI2L receptors for strigolactones and related compounds highlight MAX2 dependent and independent pathways

ABSTRACTIn flowering plants, the α/β hydrolase DWARF14 (D14) perceives strigolactone (SL) hormones and interacts with the F-box protein MORE AXILLARY GROWTH2 (MAX2) to regulate developmental processes. The key SL biosynthetic enzyme, CAROTENOID CLEAVAGE DEOXYGENASE8 (CCD8), is present in the moss Physcomitrium (Physcomitrella) patens, and PpCCD8-derived compounds regulate plant extension. However, perception of these still unknown compounds does not require the PpMAX2 homolog. Putative candidate receptors are among the 13 PpKAI2LIKE-A to -L genes, homologous to the ancestral D14 paralog KARRIKIN INSENSITIVE2 (KAI2). In Arabidopsis, AtKAI2 is the receptor for karrikins and a still elusive endogenous KAI2-Ligand (KL). Based on germination assays using seeds of the parasitic plant Phelipanche ramosa, we propose that PpCCD8-derived compounds are non-canonical SLs. We show that all tested PpKAI2L proteins can bind and cleave SL analogs, some with similar affinities to AtKAI2. The PpKAI2L-H protein shows a strong hydrolytic activity not found for other PpKAI2L. Moss mutants for all PpKAI2L gene subclades were obtained and tested for their response to SL analogs. We show that PpKAI2L-A to -E genes encode redundant proteins that are not involved in PpCCD8-derived compound perception, but rather act in a PpMAX2-dependant pathway. In contrast, mutations in PpKAI2L-G, and -J genes abolish the response to the SL analog (+)-GR24, suggesting that both these encoded proteins are receptors for PpCCD8-derived molecules.

Culturable endophytic bacteria from Phelipanche ramosa (Orobanchaceae) seeds

Endophytic microbiomes play a beneficial role in the development and protection of plants. However, seed-borne endophytic bacteria have not yet been fully explored. Investigation of parasitic plants, whose existence depends on yet poorly understood and complicated relationships with microorganisms and hosts, is particularly crucial. Endophytic bacteria promote seed conservation and facilitate seed germination in soil. Several root holoparasites from the Orobanchaceae family are the most aggressive broomrape species, often causing serious yield losses in important crops. Parasitic plants are characterized by the production of a large number of some of the smallest seeds in the world's flora, allowing them to stay viable in the soil for several dozen years. This study's aim was to isolate and characterize the seed endophyte and surface bacteria of the most aggressive and widespread broomrape weed, Phelipanche ramosa. We isolated two endophytic bacteria from within the seeds which are closely related to Brevibacterium frigoritolerans and Bacillus simplex described as soil bacteria, highly resistant to environmental conditions, and as plant growth-promoting rhizobacteria. Moreover, we isolated three strains from the surface of non-sterile seeds; all three isolates were related to the Bacillus cereus group.

A Phelipanche ramosa KAI2 Protein Perceives enzymatically Strigolactones and Isothiocyanates

Phelipanche ramosa is an obligate root-parasitic weed threatening major crops in central Europe. For its germination, it has to perceive various structurally diverging host-exuded signals, including isothiocyanates (ITCs) and strigolactones (SLs). However, the receptors involved are still uncharacterized. Here, we identified five putative SL receptors in P. ramosa, of which PrKAI2d3 is involved in seed germination stimulation. We established the high plasticity of PrKAI2d3, allowing interaction with different chemicals, including ITCs. The SL perception mechanism of PrKAI2d3 is similar to that of endogenous SLs in non-parasitic plants. We provide evidence that the PrKAI2d3 enzymatic activity confers hypersensitivity to SLs. Additionally, we demonstrated that methylbutenolide-OH binds PrKAI2d3 and stimulates P. ramosa germination with a bioactivity comparable to that of ITCs. This study highlights that P. ramosa has extended its signal perception system during evolution, a fact to be considered in the development of specific and efficient biocontrol methods.