Tryptophan derivatives regulate the seed germination and radicle growth of a root parasitic plant, Orobanche minor

Root parasitic plant germination is induced by the host-derived chemical, strigolactone (SL). We found that a major microbial culture broth component, tryptone, inhibits the SL-inducible germination of a root parasitic plant, Orobanche minor. l-tryptophan (1a, l-Trp) was isolated as the active compound from tryptone. We further found that l-Trp related compounds (1b-11), such as a major plant hormone auxin (8, indole-3-acetic acid; IAA), also inhibit the germination and post-radicle growth of O. minor. We designed a hybrid chemical (13), in which IAA is attached to a part of SL, and found that this synthetic analog induced the germination of O. minor, and also inhibited post-radicle growth. Moreover, we found that N-acetyl Trp (9) showed germination stimulating activity, and introduction of a substitution at C-5 position incresed its activity (12a-12f). Our data, in particular, the discovery of a structurally hybrid compound that has two activities that induce spontaneous germination and inhibit subsequent radical growth, would provide new types of germination regulators for root parasitic plants.

Effects of Benzoquinones on Radicles of Orobanche and Phelipanche Species

The holoparasitic broomrape weeds (Orobanche and Phelipanche species) cause severe yield losses throughout North Africa, the Middle East, and Southern and Eastern Europe. These parasitic weeds form an haustorium at the tip of their radicles to infect the crop upon detection of the host-derived haustorium-inducing factors. Until now, the haustorial induction in the broomrapes remains less studied than in other parasitic plant species. Known haustorium-inducing factors active in hemiparasites, such as Striga and Triphysaria species, were reported to be inefficient for the induction of haustoria in broomrape radicles. In this work, the haustorium-inducing activity of p-benzoquinone and 2,6-dimethoxy-p-benzoquinone (BQ and DMBQ) on radicles of three different broomrapes, namely Orobanche cumana, Orobanche minor and Phelipanche ramosa, is reported. Additional allelopathic effects of benzoquinones on radicle growth and radicle necrosis were studied. The results of this work suggest that benzoquinones play a role in the induction of haustorium in broomrapes. Although dependent on the broomrape species assayed and the concentration of quinones used in the test, the activity of BQ appeared to be stronger than that of DMBQ. The redox property represented by p-benzoquinone, which operates in several physiological processes of plants, insects and animals, is invoked to explain this different activity. This work confirms the usefulness of benzoquinones as haustorium-inducing factors for holoparasitic plant research. The findings of this work could facilitate future studies in the infection process, such as host-plant recognition and haustorial formation.

An account of common broomrape Orobanche minor (Orobanchaceae) in the British Isles

Common broomrape (Orobanche minor Sm.) is the most widespread and variable species in the British Isles and is the subject of much taxonomic confusion. Poor preservation in herbaria, coupled with the presence of cryptic host-specific races, have contributed to this. Here we review the taxonomic status of infraspecific taxa of O. minor in the British Isles, and provide a revised identification key, informed by morphology, ecology and molecular data. We describe two new varieties of O. minor s.l. that are ecologically distinct and discuss within the broader context of cryptic taxa in the subsection Minores (Beck-Mannagetta) Teryokhin in the British Isles and continental Europe. We suggest that delineating taxa objectively and reliably will be important for informing conservation priorities. Host identity and ecology, besides morphology, are essential considerations when identifying infraspecific taxa in this taxonomically challenging species.

Inuloxin E, a New Seco-Eudesmanolide Isolated from Dittrichia viscosa, Stimulating Orobanche cumana Seed Germination

A new sesquiterpenoid belonging to the subgroup seco-eudesmanolides and named inuloxin E was isolated from Dittrichia viscosa, together with the already known sesquiterpenoids inuloxins A–D and α-costic acid. Inuloxin E was characterized by spectroscopic data (essentially NMR and ESI MS) as 3-methylene-6-(1-methyl-4-oxo-pentyl)-3a,4,7,7a-tetrahydro-3H-benzofuran-2-one. Its relative configuration was determined by comparison with the closely related inuloxin D and chemical conversion of inuloxin E into inuloxin D and by the observed significant correlation in the NOESY spectrum. Both inuloxins D and E induced germination of the parasitic weed Orobanche cumana, but were inactive on the seeds of Orobanche minor and Phelipanche ramosa. The germination activity of some hemisynthetic esters of inuloxin D was also investigated.

Effects of Light Level and Nitrogen Supply on the Red Clover–Orobanche Minor Host–Parasite Interaction

Infection by holoparasitic plants typically causes decreases in host mass, thought to be primarily as a result of resource abstraction. Inverse relationships have been noted between the number of Orobanche spp. parasites infecting a host and their mass, suggesting that the parasites compete for a shared resource pool, assumed to be recently fixed carbon (C). In clover, nitrogen (N) fixation requires a high proportion of daily photosynthate and represents a potential competitor for recently fixed C. We grew Trifolium pratense, either singly or parasitised by Orobanche minor, under high or low light levels, and with or without exogenous N supply. Low light and N deficiency led to decreased host biomass, while the damage caused by parasitism was proportionate to host mass. Parasitism caused reductions in host leaf mass, area, photosynthetic rates and shoot N concentration, but did not affect starch accumulation. Parasite mass as a proportion of system biomass was significantly higher when attached to plants grown at high light, which was attributed to higher photoassimilate supply, while the N supply had no effect. While both N limitation and parasitism caused reductions in host growth, little evidence of competition for C between N fixation and the parasites was noted.