Parasitic plants thrive by infecting other plants. Flowering plants evolved parasitism independently at least 12 times, in all cases developing a unique multicellular organ called the haustorium that forms upon detection of haustorium-inducing factors derived from the host plant. This organ penetrates into the host stem or root and connects to its vasculature, allowing exchange of materials such as water, nutrients, proteins, nucleotides, pathogens, and retrotransposons between the host and the parasite. In this review, we focus on the formation and function of the haustorium in parasitic plants, with a specific emphasis on recent advances in molecular studies of root parasites in the Orobanchaceae and stem parasites in the Convolvulaceae.
The root hemiparasite witchweed (Striga spp.) is a devastating agricultural pest that causes losses of up to $1 billion US annually in sub-Saharan Africa. Development of resistant crops is one of the cost-effective ways to address this problem. However, the molecular mechanisms underlying resistance are not well understood. To understand molecular events upon Striga spp. infection, we conducted genome-scale RNA sequencing expression analysis using Striga hermonthica-infected rice (Oryza sativa) roots. We found that transcripts grouped under the Gene Ontology term defense response were significantly enriched in up-regulated differentially expressed genes. In particular, we found that both jasmonic acid (JA) and salicylic acid (SA) pathways were induced, but the induction of the JA pathway preceded that of the SA pathway. Foliar application of JA resulted in higher resistance. The hebiba mutant plants, which lack the JA biosynthesis gene ALLENE OXIDE CYCLASE, exhibited severe S. hermonthica susceptibility. The resistant phenotype was recovered by application of JA. By contrast, the SA-deficient NahG rice plants were resistant against S. hermonthica, indicating that endogenous SA is not required for resistance. However, knocking down WRKY45, a regulator of the SA/benzothiadiazole pathway, resulted in enhanced susceptibility. Interestingly, NahG plants induced the JA pathway, which was down-regulated in WRKY45-knockdown plants, linking the resistant and susceptible phenotypes to the JA pathway. Consistently, the susceptibility phenotype in the WRKY45-knockdown plants was recovered by foliar JA application. These results point to a model in which WRKY45 modulates a cross talk in resistance against S. hermonthica by positively regulating both SA/benzothiadiazole and JA pathways.
Dar es Salaam — Thousands of farmers in western Kenya are successfully battling the invasion of a deadly parasitic weed called Striga, dubbed the 'violet vampire' because of its beautiful violet flowers.
Genomic Characterization for Parasitic Weeds of the Genus Striga by Sample Sequence Analysis
Generation of ∼2200 Sanger sequence reads or ∼10,000 454 reads for seven Striga Lour. DNA samples (five species) allowed identification of the highly repetitive DNA content in these genomes. The 14 most abundant repeats in these Striga species were identified and partially assembled. Annotation indicated that they represent nine long terminal repeat (LTR) retrotransposon families, three tandem satellite repeats, one long interspersed element (LINE) retroelement, and one DNA transposon. All of these repeats are most closely related to repetitive elements in other closely related plants and are not products of horizontal transfer from their host species. These repeats were differentially abundant in each species, with the LTR retrotransposons and satellite repeats most responsible for variation in genome size. Each species had some repetitive elements that were more abundant and some less abundant than the other Striga species examined, indicating that no single element or any unilateral growth or decrease trend in genome behavior was responsible for variation in genome size and composition. Genome sizes were determined by flow sorting, and the values of 615 Mb [S. asiatica (L.) Kuntze], 1330 Mb [S. gesnerioides (Willd.) Vatke], 1425 Mb [S. hermonthica (Delile) Benth.] and 2460 Mb (S. forbesii Benth.) suggest a ploidy series, a prediction supported by repetitive DNA sequence analysis. Phylogenetic analysis using six chloroplast loci indicated the ancestral relationships of the five most agriculturally important Striga species, with the unexpected result that the one parasite of dicotyledonous plants (S. gesnerioides) was found to be more closely related to some of the grass parasites than many of the grass parasites are to each other.
Strigolactones, phytohormones with diverse signaling activities, have a common structure consisting of two lactones connected by an enol-ether bridge. Strigolactones derive from carotenoids via a pathway involving the carotenoid cleavage dioxygenases 7 and 8 (CCD7 and CCD8) and the iron-binding protein D27. We show that D27 is a β-carotene isomerase that converts all-trans-β-carotene into 9-cis-β-carotene, which is cleaved by CCD7 into a 9-cis–configured aldehyde. CCD8 incorporates three oxygens into 9-cis-β-apo-10′-carotenal and performs molecular rearrangement, linking carotenoids with strigolactones and producing carlactone, a compound with strigolactone-like biological activities. Knowledge of the structure of carlactone will be crucial for understanding the biology of strigolactones and may have applications in combating parasitic weeds.
The Push-Pull technology involves use of behaviour-modifying stimuli to manipulate the distribution and abundance of stemborers and beneficial insects for management of stemborer pests (Figure above). It is based on in-depth understanding of chemical ecology, agrobiodiversity, plant-plant and insect-plant interactions, and involves intercropping a cereal crop with a repellent intercrop such as desmodium (push), with an attractive trap plant such as Napier grass (pull) planted as a border crop around this intercrop.
Desmodium also controls striga, resulting in significant yield increases of about 2 t/ha per cropping season. In the elucidation of the mechanisms of striga suppression by D. uncinatum, it was found that, in addition to benefits derived from increased availability of nitrogen and soil shading, an allelopathic effect of the root exudates of the legume, produced independently of the presence of striga, is responsible for this dramatic reduction in an intercrop with maize. Presence of blends of secondary metabolites with Striga seed germination stimulatory, 4′′,5′′,-dihydro-5,2′,4′-trihydroxy-5′′,-isopropenylfurano-(2′′,3′′;7,6)-isoflavanone, and post-germination inhibitory, 4′′,5′′-dihydro-2′-methoxy-5,4′-dihydroxy-5′′-isopropenylfurano- (2′′,3′′;7,6)-isoflavanone, activities in the root exudates of D. uncinatum which directly interferes with parasitism was observed. This combination thus provides a novel means of in situ reduction of the striga seed bank in the soil through efficient suicidal germination even in the presence of graminaceous host plants in the proximity. Other Desmodium spp. have also been evaluated and have similar effects on stemborers and striga weed and are currently being used as intercrops in maize, sorghum and millets.
Obligate parasitic plants in the Orobanchaceae germinate after sensing plant hormones, strigolactones, exuded from host roots. In Arabidopsis thaliana, the α/β-hydrolase D14 acts as a strigolactone receptor that controls shoot branching, whereas its ancestral paralog, KAI2, mediates karrikin-specific germination responses. We observed that KAI2, but not D14, is present at higher copy numbers in parasitic species than in nonparasitic relatives. KAI2 paralogs in parasites are distributed into three phylogenetic clades. The fastest-evolving clade, KAI2d, contains the majority of KAI2 paralogs. Homology models predict that the ligand-binding pockets of KAI2d resemble D14. KAI2d transgenes confer strigolactone-specific germination responses to Arabidopsis thaliana. Thus, the KAI2 paralogs D14 and KAI2d underwent convergent evolution of strigolactone recognition, respectively enabling developmental responses to strigolactones in angiosperms and host detection in parasites.
Researchers using maize inbred lines have developed two early maturing hybrid maize varieties that have been released by the Malian government to boost maize production. The improved hybrids, which are locally named sanu and mata, are resistant toStriga, a parasitic weed, and possess genes that enable them to withstand drought which occurs at the flowering and grain filling periods. They are also tolerant of low soil nitrogen, and are high yielding with good cooking qualities.
Dar es Salaam-March 25, 2013.Thousands of farmers in western Kenya are successfully battling the invasion in their farms by a deadly parasitic weed called Striga, dubbed the violet vampire because of...
Recent studies have suggested that plant genomes have undergone potentially rampant horizontal gene transfer (HGT), especially in the mitochondrial genome. Parasitic plants have provided the strongest evidence of HGT, which appears to be facilitated by the intimate physical association between the parasites and their hosts. A recent phylogenomic study demonstrated that in the holoparasite Rafflesia cantleyi (Rafflesiaceae), whose close relatives possess the world's largest flowers, about 2.1% of nuclear gene transcripts were likely acquired from its obligate host. Here, we used next-generation sequencing to obtain the 38 protein-coding and ribosomal RNA genes common to the mitochondrial genomes of angiosperms from R. cantleyi and five additional species, including two of its closest relatives and two host species. Strikingly, our phylogenetic analyses conservatively indicate that 24%–41% of these gene sequences show evidence of HGT in Rafflesiaceae, depending on the species. Most of these transgenic sequences possess intact reading frames and are actively transcribed, indicating that they are potentially functional. Additionally, some of these transgenes maintain synteny with their donor and recipient lineages, suggesting that native genes have likely been displaced via homologous recombination. Our study is the first to comprehensively assess the magnitude of HGT in plants involving a genome (i.e., mitochondria) and a species interaction (i.e., parasitism) where it has been hypothesized to be potentially rampant. Our results establish for the first time that, although the magnitude of HGT involving nuclear genes is appreciable in these parasitic plants, HGT involving mitochondrial genes is substantially higher. This may represent a more general pattern for other parasitic plant clades and perhaps more broadly for angiosperms.
Strigolactones were originally identified as stimulators of the germination of root-parasitic weeds1 that pose a serious threat to resource-limited agriculture2. They are mostly exuded from roots and function as signalling compounds in the initiation of arbuscular mycorrhizae3, which are plant–fungus symbionts with a global effect on carbon and phosphate cycling4. Recently, strigolactones were established to be phytohormones that regulate plant shoot architecture by inhibiting the outgrowth of axillary buds5, 6. Despite their importance, it is not known how strigolactones are transported. ATP-binding cassette (ABC) transporters, however, are known to have functions in phytohormone translocation7, 8, 9. Here we show that the Petunia hybrida ABC transporter PDR1 has a key role in regulating the development of arbuscular mycorrhizae and axillary branches, by functioning as a cellular strigolactone exporter. P. hybrida pdr1 mutants are defective in strigolactone exudation from their roots, resulting in reduced symbiotic interactions. Above ground, pdr1 mutants have an enhanced branching phenotype, which is indicative of impaired strigolactone allocation. Overexpression of Petunia axillaris PDR1 in Arabidopsis thaliana results in increased tolerance to high concentrations of a synthetic strigolactone, consistent with increased export of strigolactones from the roots. PDR1 is the first known component in strigolactone transport, providing new opportunities for investigating and manipulating strigolactone-dependent processes.
8 February 2012, Juba/Rome - Millions of people in South Sudan will face hunger this year if urgent action is not taken, according to a joint report issued by the Food and Agriculture Organization of the United Nations (FAO) and the World Food Programme (WFP).
The FAO-WFP report, Crop and Food Security Assessment Mission to South Sudan, is based on a joint mission conducted in the world’s newest nation between October and November 2011 at the request of the government of South Sudan’s Ministry of Agriculture and Forestry.
The report finds that the level of food insecurity in the country has risen sharply. The number of food-insecure people has jumped from 3.3 million in 2011 to 4.7 million in 2012. Of those, about one million people are severely food insecure, compared to 900,000 in 2011.
Farmers in Nyanza and Western provinces lose maize worth more than Sh1.8 billion to the deadly striga weed annually.
Experts on Thursday warned that the weed was posing a threat to food security in the region, as it affects more than 540,000 acres of crops every season.
Prof George Odhiambo of Maseno University said striga was a major problem in the two regions where most families depend on maize as staple food.
Striga, known in Nyanza as kayongo and Oluyongo in Western, affects maize, millet, sorghum and rice production leading to loses of 180,000 tonnes.
Odhiambo said effects of the weed were so immense that in a one-hectare farm a farmer can only harvest 500kg of maize instead of 5,000.
"But under heavy infestation of a farm by striga, a farmer could get zero yield," he added.
He was speaking during a striga weed demonstration site at Aboke in Ugenya constituency where farmers were trained on modern techniques to fight the wild plant.
The demonstration was organised by the implementing agency in Kenya, the Organisation for Transforming Initiated Technologies, which was represented by its director Mr Emmanuel Opil and programme officer Ms Joan Mwanga.
Odhiambo said the demonstration sites spread across Nyanza and Western were being implemented as a programme to fight striga under the Integrated Striga Management in Africa focusing on Kenya and Nigeria and funded by the Bill and Melinda Gates Foundation.
The sites are in Siaya, Teso, Busia, Kisumu, Rachuonyo and Migori.
He said the programme had also introduced the Desmodium plant that is grown between rows alternate with maize to help fight striga weed.
"We are also encouraging farmers to use seeds that are striga resistance or striga tolerance such as legume crops that act as a trap for the weed," he added.
He explained that striga can only be controlled through integrating several techniques to reduce the weed in the farms.
Odhiambo who was accompanied by other experts from Icipe, said a striga tolerance sorghum variety was also being introduced into the market and would soon be released by the Kenya Seed Company.
He advised farmers who uproot the weed from their farms not to throw them along the way as they would be washed back by rains.
"The only alternative is to heap them together and burn them," he said.
Highlights Functional categories of ESTs are broadly similar in all three parasitic plants Photosynthetic gene expression is lost in the nonphotosynthetic P. aegyptiaca Chlorophyll synthesis genes are expressed and selectively maintained in P. aegyptiaca Analyses suggest a history of genome duplication in Orobanchaceae Summary
Parasitism in flowering plants has evolved at least 11 times . Only one family, Orobanchaceae, comprises all major nutritional types of parasites: facultative, hemiparasitic (partially photosynthetic), and holoparasitic (nonphotosynthetic) . Additionally, the family includes Lindenbergia, a nonparasitic genus sister to all parasitic Orobanchaceae [3,4,5,6]. Parasitic Orobanchaceae include species with severe economic impacts: Striga (witchweed), for example, affects over 50 million hectares of crops in sub-Saharan Africa, causing more than $3 billion in damage annually . Although gene losses and increased substitution rates have been characterized for parasitic plant plastid genomes [5,8,9,10,11], the nuclear genome and transcriptome remain largely unexplored. The Parasitic Plant Genome Project (PPGP; http://ppgp.huck.psu.edu/)  is leveraging the natural variation in Orobanchaceae to explore the evolution and genomic consequences of parasitism in plants through a massive transcriptome and gene discovery project involving Triphysaria versicolor (facultative hemiparasite), Striga hermonthica (obligate hemiparasite), and Phelipanche aegyptiaca (Orobanche ; holoparasite). Here we present the first set of large-scale genomic resources for parasitic plant comparative biology. Transcriptomes of above-ground tissues reveal that, in addition to the predictable loss of photosynthesis-related gene expression in P. aegyptiaca, the nonphotosynthetic parasite retains an intact, expressed, and selectively constrained chlorophyll synthesis pathway.
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