The obligate root parasite Orobanche cumana exhibits several rbcL sequences

Prohexadione calcium is herbicidal to the sunflower root parasite Orobanche cumana

BACKGROUND

The obligatory sunflower root parasite Orobanche cumana Wallr. deprives its host of essential nutrients, resulting in a dramatic reduction in yield and biomass. A post-emergence application with an imidazolinone herbicide on an imidazolinone-tolerant sunflower is highly effective against O. cumana. The herbicide inhibits the enzyme acetohydroxy acid synthase and consequently, growth of the parasite is inhibited, although the sunflower survives the treatment through mutations in the target enzyme. Interestingly, field studies have shown that a combined application of an imidazolinone herbicide with prohexadione resulted in reduced emergence of O. cumana compared with the sole application of the herbicide. The aim of this study was to investigate whether prohexadione is herbicidal to O. cumana.

RESULTS

Prohexadione was rapidly distributed within the sunflower, reaching the roots, the site of O. cumana attack, as early as 6 h after application (HAA) on sunflower leaves. A direct impact of prohexadione on O. cumana germination was investigated and a half-maximal inhibitory concentration (IC₅₀ ) of 84 μm prohexadione was found. In addition, the inhibition of germination by prohexadione was terminal, meaning that O. cumana seeds died after prohexadione contact as soon as they were primed for germination. Additionally, excretion studies showed that a small proportion of the applied prohexadione was excreted by sunflower roots.

CONCLUSION

We show that prohexadione is an inhibitor of O. cumana germination and that the growth regulator is found in sunflower roots shortly after application. We hypothesize that prohexadione is excreted in sufficient amounts from the sunflower roots, therefore having a direct impact on O. cumana germination. © 2020 Society of Chemical Industry.

Primary metabolic profiling of Egyptian broomrape (Phelipanche aegyptiaca) compared to its host tomato roots

Broomrape (Phelipanche aegyptiaca) is a root holoparasitic plant considered among the most destructive agricultural weeds worldwide. In order to acquire more knowledge about the metabolism of broomrape and its interaction with its tomato host, we performed primary metabolic profiling using GCMS analysis for the early developmental stage of the parasite and of infected and non-infected roots. The analysis revealed that out of 59 metabolites detected, the levels of 37 significantly increased in the parasite while the levels of 10 significantly decreased compared to the infected roots. In addition, the analysis showed that the levels of total protein in the albumin fraction, reducing sugars (representing starch) and total phenols increased by 9.8-, 4.6- and 3.3-fold, respectively, in the parasite compared to the roots. These changes suggest that P. aegyptiaca has its own metabolism that differs significantly in its regulation from those found in their host. In addition, the results have shown that the levels of most of the metabolites in the infected roots were similar to levels detected in the non-infected roots, except for seven metabolites whose levels increased in the infected versus the non-infected roots. This suggests that the parasite did not significantly affect the host primary metabolic pathways.

Knowing the Parasite: Biology and Genetics of Orobanche

Due to their forms and colors, parasitic plants are most often considered to be botanical curiosities. However, in some cases, these are proved to be also deadly pests with the capacity to exploit other plants. Among the obligate root parasitic weeds, the holoparasites that are devoid of chlorophyll and thus unable to carry out photosynthesis totally rely on their hosts for their water, mineral, and carbohydrate supplies. Members of the genus Orobanche and Phelipanche, belonging to the Orobanchaceae family (the broomrape family), are thus the final result of this evolutionary transition from autotrophism to heterotrophism. The underlying process of this trophic exploitation, governed by a fine-tuned molecular dialogue between both partners, is an extraordinary example of adaptive plant biology operated by these parasitic organisms in the course of evolution. This transition is associated with remarkable morphological and physiological adaptations, such as the requirement for the seeds to germinate to perceive molecules produced by host roots, the development of a novel organ, the haustorium, which invades host tissues and establishes a physiological continuum between the parasite and the host, the establishment of a sink strength required for translocation of host resources, the loss of photosynthesis, and a reduced leaf and root architecture.

Multiple and different genomic rearrangements of the rbcL gene are present in the parasitic orchid Neottia nidus-avis

In parasitic plants that have lost most, if not all, of their photosynthetic genes, the genome of their plastids has also undergone a dramatic reduction. For example, photosynthetic genes, such as rbcL, frequently become pseudogenes, in which large portions of the gene have been found to be deleted. Orchids are flowering plants with several parasitic lineages. This is consistent with the observation that parasitic orchids can invade pre-existing mutualistic associations between ectomycorrhizal trees and fungi to obtain fixed carbon and nutrients. In addition, some parasitic species are devoid of chlorophyll, and consequently, have lost their photosynthetic capacity. Here, the organization of the plastid genome of the parasitic orchid Neottia nidus-avis (L.) Rich. was investigated using sequencing and hybridization experiments. In particular, genomic rearrangements in the rbcL region of this parasitic orchid were analyzed. At least three distinct rbcL sequences were found to be present as pseudogenes and were likely located in the plastid genome. Based on these results, it is hypothesized that N. nidus-avis contains different plastomes, each with a different pseudogene, and these can exist within the same individual plant.

Plastid genome evolution in mycoheterotrophic Ericaceae

Unlike parasitic plants, which are linked to their hosts directly through haustoria, mycoheterotrophic (MHT) plants derive all or part of their water and nutrients from autothrophs via fungal mycorrhizal intermediaries. Ericaceae, the heather family, are a large and diverse group of plants known to form elaborate symbiotic relationships with mycorrhizal fungi. Using PHYA sequence data, we first investigated relationships among mycoheterotrophic Ericaceae and their close autotrophic relatives. Phylogenetic results suggest a minimum of two independent origins of MHT within this family. Additionally, a comparative investigation of plastid genomes (plastomes) grounded within this phylogenetic framework was conducted using a slot-blot Southern hybridization approach. This survey encompassed numerous lineages of Ericaceae with different life histories and trophic levels, including multiple representatives from mixotrophic Pyroleae and fully heterotrophic Monotropeae and Pterosporeae. Fifty-four probes derived from all categories of protein coding genes typically found within the plastomes of flowering plants were used. Our results indicate that the holo-mycoheterotrophic Ericaceae exhibit extensive loss of genes relating to photosynthetic function and expression of the plastome but retain genes with possible functions outside photosynthesis. Mixotrophic taxa tend to retain most genes relating to photosynthetic functions but are varied regarding the plastid ndh gene content. This investigation extends previous inferences that the loss of the NDH complex occurs prior to becoming holo-heterotrophic and it shows that the pattern of gene losses among mycoheterotrophic Ericaceae is similar to that of haustorial parasites. Additionally, we identify the most desirable candidate species for entire plastome sequencing.

Host-driven divergence in the parasitic plant Orobanche minor Sm. (Orobanchaceae)

Many parasitic angiosperms have a broad host range and are therefore considered to be host generalists. Orobanche minor is a nonphotosynthetic root parasite that attacks a range of hosts from taxonomically disparate families. In the present study, we show that O. minor sensu lato may comprise distinct, genetically divergent races isolated by the different ecologies of their hosts. Using a three-pronged approach, we tested the hypothesis that intraspecific taxa O. minor var. minor and O. minor ssp. maritima parasitizing either clover (Trifolium pratense) or sea carrot (Daucus carota ssp.gummifer), respectively, are in allopatric isolation. Morphometric analysis revealed evidence of divergence but this was insufficient to define discrete, host-specific taxa. Intersimple sequence repeat (ISSR) marker-based data provided stronger evidence of divergence, suggesting that populations were isolated from gene flow. Phylogenetic analysis, using sequence-characterized amplified region (SCAR) markers derived from ISSR loci, provided strong evidence for divergence by clearly differentiating sea carrot-specific clades and mixed-host clades. Low levels of intrapopulation SCAR marker sequence variation and floral morphology suggest that populations on different hosts are probably selfing and inbreeding. Morphologically cryptic Orobanche taxa may therefore be isolated from gene flow by host ecology. Together, these data suggest that host specificity may be an important driver of allopatric speciation in parasitic plants.

Susceptibility of the tomato mutant high pigment-2dg (hp-2dg) to Orobanche spp. infection

The consumption of natural products with potential health benefits has been continuously growing, and enhanced pigmentation is of major economic importance in fruits and vegetables. The tomato hp-2 ( dg ) is an important mutant line that has been introgressed into commercial tomato cultivars marketed as lycopene rich tomatoes (LRT) because of their enhanced fruit pigmentation, attributed to higher levels of carotenoids, including lycopene. Strigolactones are signaling compounds that mediate host finding in root parasitic plants and are biosynthetically derived from carotenoids. Considering the high carotenoid content of the hp-2 ( dg ) mutant, we studied its susceptibility to the root parasite Orobanche. In a field experiment, the average number of Orobanche aegyptiaca plants growing on hp-2 ( dg ) was surprisingly significantly reduced compared with its isogenic wild-type counterpart. In vitro assays and LC-MS/MS analysis showed that this reduction was associated with a lower production of strigolactones, which apparently renders the high-carotenoid hp-2 ( dg ) mutant less susceptible to Orobanche.

Tomato strigolactones are derived from carotenoids and their biosynthesis is promoted by phosphate starvation

* Strigolactones are rhizosphere signalling compounds that mediate host location in arbuscular mycorrhizal (AM) fungi and parasitic plants. Here, the regulation of the biosynthesis of strigolactones is studied in tomato (Solanum lycopersicum). * Strigolactone production under phosphate starvation, in the presence of the carotenoid biosynthesis inhibitor fluridone and in the abscisic acid (ABA) mutant notabilis were assessed using a germination bioassay with seeds of Orobanche ramosa; a hyphal branching assay with Gigaspora spp; and by liquid chromatography coupled to tandem mass spectrometry (LC-MS/MS) analysis. * The root exudates of tomato cv. MoneyMaker induced O. ramosa seed germination and hyphal branching in AM fungi. Phosphate starvation markedly increased, and fluridone strongly decreased, this activity. Exudates of notabilis induced approx. 40% less germination than the wild-type. The LC-MS/MS analysis confirmed that the biological activity and changes therein were due to the presence of several strigolactones; orobanchol, solanacol and two or three didehydro-orobanchol isomers. * These results show that the AM branching factors and parasitic plant germination stimulants in tomato root exudate are strigolactones and that they are biosynthetically derived from carotenoids. The dual activity of these signalling compounds in attracting beneficial AM fungi and detrimental parasitic plants is further strengthened by environmental conditions such as phosphate availability.

Phylogeography of the endangered Eryngium alpinum L. (Apiaceae) in the European Alps

We studied the phylogeography of Eryngium alpinum by sequencing two intergenic chloroplast spacers, trnH-psbA and trnS-trnG (1322 bp). The sampling design included 36 populations and 397 individuals spanning the entire distribution range of the species, from France to Bosnia. Twenty-one haplotypes were characterized and polymorphism was observed both within and among populations. Population differentiation was strong (F(ST) = 0.92) and largely explained by the distinction of five geographic regions: Southwestern, Western, Middle, Eastern Alps and Balkans (F(CT) = 0.62). Moreover, N(ST) was significantly higher than G(ST) (P < 0.05), showing the existence of a phylogeographic pattern. Six major lineages were recognized using samova and median-joining networks. One lineage, highly divergent from the other ones, was only found in the Balkans and probably persisted in situ during last glaciations. All other lineages might have survived in a Southwestern refugium (Mercantour) and colonized the entire Alpine arc (Southwestern, Western, Middle and Eastern Alps) through repeated colonization events at different time periods. This is the first empirical study suggesting Southern refugia for calcareous Alpine plants, although the existence of a secondary refugium in northern Italy/Austria is also suspected. We also observed recent haplotype diversification, especially in the Southwestern Alps.

Horizontal gene transfer of a plastid gene in the non-photosynthetic flowering plants Orobanche and Phelipanche (Orobanchaceae)

Plastid sequences are among the most widely used in phylogenetic and phylogeographic studies in flowering plants, where they are usually assumed to evolve like non-recombining, uniparentally transmitted, single-copy genes. Among others, this assumption can be violated by intracellular gene transfer (IGT) within cells or by the exchange of genes across mating barriers (horizontal gene transfer, HGT). We report on HGT of a plastid region including rps2, trnL-F, and rbcL in a group of non-photosynthetic flowering plants. Species of the parasitic broomrape genus Phelipanche harbor two copies of rps2, a plastid ribosomal gene, one corresponding to the phylogenetic position of the respective species, the other being horizontally acquired from the related broomrape genus Orobanche. While the vertically transmitted copies probably reside within the plastid genome, the localization of the horizontally acquired copies is not known. With both donor and recipient being parasitic plants, a possible pathway for the exchange of genetic material is via a commonly attacked host.

Phylogeny and intraspecific variability of holoparasitic Orobanche (Orobanchaceae) inferred from plastid rbcL sequences

The rbcL sequences of 106 specimens representing 28 species of the four recognized sections of Orobanche were analyzed and compared. Most sequences represent pseudogenes with premature stop codons. This study confirms that the American lineage (sects. Gymnocaulis and Myzorrhiza) contains potentially functional rbcL-copies with intact open reading frames and low rates of non-synonymous substitutions. For the first time, this is also shown for a member of the Eurasian lineage, O. coerulescens of sect. Orobanche, while all other investigated species of sects. Orobanche and Trionychon contain pseudogenes with distorted reading frames and significantly higher rates of non-synonymous substitutions. Phylogenetic analyses of the rbcL sequences give equivocal results concerning the monophyly of Orobanche, and the American lineage might be more closely related to Boschniakia and Cistanche than to the other sections of Orobanche. Additionally, species of sect. Trionychon phylogenetically nest in sect. Orobanche. This is in concordance with results from other plastid markers (rps2 and matK), but in disagreement with other molecular (nuclear ITS), morphological, and karyological data. This might indicate that the ancestor of sect. Trionychon has captured the plastid genome, or parts of it, of a member of sect. Orobanche. Apart from the phylogenetically problematic position of sect. Trionychon, the phylogenetic relationships within sect. Orobanche are similar to those inferred from nuclear ITS data and are close to the traditional groupings traditionally recognized based on morphology. The intraspecific variation of rbcL is low and is neither correlated with intraspecific morphological variability nor with host range. Ancestral character reconstruction using parsimony suggests that the ancestor of O. sect. Orobanche had a narrow host range.