Effect of diammonium phosphate application on strigolactone production and Striga hermonthica infection in three sorghum cultivars

Striga hermonthica infection poses a major constraint to sorghum production in sub-Saharan Africa, and low soil fertility aggravates the S. hermonthica problem. Under mineral nutrient deficiency, the sorghum host secretes large quantities of strigolactones, signalling molecules, into the rhizosphere. These induce S. hermonthica seed germination and subsequent infection of the host roots. In a combination of field and glasshouse experiments, we analysed the effect of microdose applied diammonium hydrogen phosphate (DAP) fertiliser on production of strigolactones, S. hermonthica infection and yield of three different African sorghum genotypes (CGM-19/1-1, Lina-3, DouaG). The sorghum cultivars all produced the strigolactones sorgomol and 5-deoxystrigol, albeit in different quantity and ratio. Without fertiliser, high S. hermonthica infection and emergence occurred under both glasshouse and field conditions. DAP application reduced secretion of sorgomol and 5-deoxystrigol and reduced S. hermonthica germination (66–70%), emergence (49–73%) and dry biomass (90–96%) under glasshouse conditions. Under field conditions, DAP microdosing reduced S. hermonthica emergence by 40–84% and increased sorghum grain yield by 47–142%. Thus DAP application reduced secretion of strigolactones into the rhizosphere and S. hermonthica parasitism both under controlled and field conditions. Microdosing of DAP may prove to be an efficient and cost effective option to reduce S. hermonthica damage in sorghum in sub-Saharan Africa, particularly in combination with other control options, such as intercropping, use of organic fertiliser and hand pulling of S. hermonthica at flowering to achieve integrated S. hermonthica management

Influence of Fertilizer Microdosing on Strigolactone Production and Striga hermonthica Parasitism in Pearl Millet

Parasitism by the root-parasitic plant, Striga (Striga hermonthica L.), is a main threat to pearl millet production in sub-Saharan Africa and nutrient deficiency aggravates this problem, often leading to complete failure of pearl millet crops. Like many other species, pearl millet secretes germination stimulants (strigolactones) into the soil in response to mineral nutrient deficiency, which triggers Striga seed germination resulting in infection. A greenhouse experiment was conducted to evaluate the influence of different doses of di-ammonium phosphate (DAP) fertilizer on strigolactone production and Striga infection in three different African pearl millet cultivars (KBH, Sadore Local and Striga resistance). All the pearl millet genotypes produced varying amounts of different strigolactones like orobanchol, epi-orobanchol, orobanchyl acetate and 5-deoxystrigol, the level of which decreases with increasing doses of DAP. The control treatment (no DAP) showed maximum Striga germination, emergence and dry biomass production in all cultivars of pearl millet. Supply of DAP fertilizer up to 4 g per hill suppressed Striga germination by 69, 64 and 59%; emergence by 87, 85 and 95% and dry biomass by 91, 98 and 83% in cvs KBH, Sadore Local and Striga Resistance, respectively. The present findings reveal that DAP fertilizer minimizes strigolactones production and, as a result, reduces Striga infection in pearl millet. Low doses of DAP fertilizer is a promising strategy to lower the destructive effect of Striga on pearl millet. The use of small doses of DAP fertilizer combined with resistant crop cultivars, intercropping with legumes and hand pulling of Striga at flowering in an integrated Striga control strategy should be developed to help African farmers control this noxious weed

Differential Activity of Striga hermonthica Seed Germination Stimulants and Gigaspora rosea Hyphal Branching Factors in Rice and Their Contribution to Underground Communication

Strigolactones (SLs) trigger germination of parasitic plant seeds and hyphal branching of symbiotic arbuscular mycorrhizal (AM) fungi. There is extensive structural variation in SLs and plants usually produce blends of different SLs. The structural variation among natural SLs has been shown to impact their biological activity as hyphal branching and parasitic plant seed germination stimulants. In this study, rice root exudates were fractioned by HPLC. The resulting fractions were analyzed by MRM-LC-MS to investigate the presence of SLs and tested using bioassays to assess their Striga hermonthica seed germination and Gigaspora rosea hyphal branching stimulatory activities. A substantial number of active fractions were revealed often with very different effect on seed germination and hyphal branching. Fractions containing (-)-orobanchol and ent-2'-epi-5-deoxystrigol contributed little to the induction of S. hermonthica seed germination but strongly stimulated AM fungal hyphal branching. Three SLs in one fraction, putative methoxy-5-deoxystrigol isomers, had moderate seed germination and hyphal branching inducing activity. Two fractions contained strong germination stimulants but displayed only modest hyphal branching activity. We provide evidence that these stimulants are likely SLs although no SL-representative masses could be detected using MRM-LC-MS. Our results show that seed germination and hyphal branching are induced to very different extents by the various SLs (or other stimulants) present in rice root exudates. We propose that the development of rice varieties with different SL composition is a promising strategy to reduce parasitic plant infestation while maintaining symbiosis with AM fungi

Differential Activity of Striga hermonthica Seed Germination Stimulants and Gigaspora rosea Hyphal Branching Factors in Rice and Their Contribution to Underground Communication

Strigolactones (SLs) trigger germination of parasitic plant seeds and hyphal branching of symbiotic arbuscular mycorrhizal (AM) fungi. There is extensive structural variation in SLs and plants usually produce blends of different SLs. The structural variation among natural SLs has been shown to impact their biological activity as hyphal branching and parasitic plant seed germination stimulants. In this study, rice root exudates were fractioned by HPLC. The resulting fractions were analyzed by MRM-LC-MS to investigate the presence of SLs and tested using bioassays to assess their Striga hermonthica seed germination and Gigaspora rosea hyphal branching stimulatory activities. A substantial number of active fractions were revealed often with very different effect on seed germination and hyphal branching. Fractions containing (-)-orobanchol and ent-2'-epi-5-deoxystrigol contributed little to the induction of S. hermonthica seed germination but strongly stimulated AM fungal hyphal branching. Three SLs in one fraction, putative methoxy-5-deoxystrigol isomers, had moderate seed germination and hyphal branching inducing activity. Two fractions contained strong germination stimulants but displayed only modest hyphal branching activity. We provide evidence that these stimulants are likely SLs although no SL-representative masses could be detected using MRM-LC-MS. Our results show that seed germination and hyphal branching are induced to very different extents by the various SLs (or other stimulants) present in rice root exudates. We propose that the development of rice varieties with different SL composition is a promising strategy to reduce parasitic plant infestation while maintaining symbiosis with AM fungi

The interaction of strigolactones with abscisic acid during the drought response in rice

Published online: 10 March 2018; Open Access ArticleBoth strigolactones (SLs) and abscisic acid (ABA) biosynthetically originate from carotenoids. Considering their common origin, the interaction of these two hormones at the biosynthetic and/or regulatory level may be anticipated. Here we show that, in rice, drought simultaneously induces SL production in the root, and ABA production and the expression of SL biosynthetic genes in the shoot. Under control conditions, the ABA concentration was higher in shoots of the SL biosynthetic rice mutants dwarf10 (d10) and d17 than in wild-type plants, while a similar trend was observed for the SL perception mutant d3. These differences were enhanced under drought. However, drought did not result in an increase in leaf ABA content in the rice mutant line d27, carrying a mutation in the gene encoding the first committed enzyme in SL biosynthesis, to the same extent as in the other SL mutants and the wild type. Accordingly, d10, d17, and d3 lines were more drought tolerant than wild-type plants, whereas d27 displayed decreased tolerance. Finally, overexpression of OsD27 in rice resulted in increased levels of ABA when compared with wild-type plants. We conclude that the SL and ABA pathways are connected with each other through D27, which plays a crucial role in determining ABA and SL content in rice

Reconstitution of the Costunolide Biosynthetic Pathway in Yeast and Nicotiana benthamiana

The sesquiterpene costunolide has a broad range of biological activities and is the parent compound for many other biologically active sesquiterpenes such as parthenolide. Two enzymes of the pathway leading to costunolide have been previously characterized: germacrene A synthase (GAS) and germacrene A oxidase (GAO), which together catalyse the biosynthesis of germacra-1(10),4,11(13)-trien-12-oic acid. However, the gene responsible for the last step toward costunolide has not been characterized until now. Here we show that chicory costunolide synthase (CiCOS), CYP71BL3, can catalyse the oxidation of germacra-1(10),4,11(13)-trien-12-oic acid to yield costunolide. Co-expression of feverfew GAS (TpGAS), chicory GAO (CiGAO), and chicory COS (CiCOS) in yeast resulted in the biosynthesis of costunolide. The catalytic activity of TpGAS, CiGAO and CiCOS was also verified in planta by transient expression in Nicotiana benthamiana. Mitochondrial targeting of TpGAS resulted in a significant increase in the production of germacrene A compared with the native cytosolic targeting. When the N. benthamiana leaves were co-infiltrated with TpGAS and CiGAO, germacrene A almost completely disappeared as a result of the presence of CiGAO. Transient expression of TpGAS, CiGAO and CiCOS in N. benthamiana leaves resulted in costunolide production of up to 60 ng.g−1 FW. In addition, two new compounds were formed that were identified as costunolide-glutathione and costunolide-cysteine conjugates

New strigolactone mimics: structure-activity relationship and mode of action as germinating stimulants for parasitic weeds.

Strigolactones (SLs) are new plant hormones with varies important bio-functions. This Letter deals with germination of seeds of parasitic weeds. Natural SLs have a too complex structure for synthesis. Therefore, there is an active search for SL analogues and mimics with a simpler structure with retention of activity. SL analogues all contain the D-ring connected with an enone moiety through an enol ether unit. A new mechanism for the hydrolysis SL analogues involving bidentate bound water and an a,ß-hydrolase with a Ser-His-Asp catalytic triad has been proposed. Newly discovered SL mimics only have the D-ring with an appropriate leaving group at C-5. A mode of action for SL mimics was proposed for which now supporting evidence is provided. As predicted an extra methyl group at C-4 of the D-ring blocks the germination of seeds of parasitic weeds

Carotenoid inhibitors reduce strigolactone production and Striga hermonthica infection in rice

The strigolactones are internal and rhizosphere signalling molecules in plants that are biosynthesised through carotenoid cleavage. They are secreted by host roots into the rhizosphere where they signal host-presence to the symbiotic arbuscular mycrorrhizal (AM) fungi and the parasitic plants of the Orobanche, Phelipanche and Striga genera. The seeds of these parasitic plants germinate after perceiving these signalling molecules. After attachment to the host root, the parasite negatively affects the host plant by withdrawing water, nutrients and assimilates through a direct connection with the host xylem. In many areas of the world these parasites are a threat to agriculture but so far very limited success has been achieved to minimize losses due to these parasitic weeds. Considering the carotenoid origin of the strigolactones, in the present study we investigated the possibilities to reduce strigolactone production in the roots of plants by blocking carotenoid biosynthesis using carotenoid inhibitors. Hereto the carotenoid inhibitors fluridone, norflurazon, clomazone and amitrole were applied to rice either through irrigation or through foliar spray. Irrigation application of all carotenoid inhibitors and spray application of amitrole significantly decreased strigolactone production, Striga hermonthica germination and Striga infection, also in concentrations too low to affect growth and development of the host plant. Hence, we demonstrate that the application of carotenoid inhibitors to plants can affect S. hermonthica germination and attachment indirectly by reducing the strigolactone concentration in the rhizosphere. This finding is useful for further studies on the relevance of the strigolactones in rhizosphere signalling. Since these inhibitors are available and accessible, they may represent an efficient technology for farmers, including poor subsistence farmers in the African continent, to control these harmful parasitic weed

Effect of phosphate-based seed priming on strigolactone production and Striga hermonthica infection in cereals

Strigolactones, plant-secreted underground signalling molecules, play an important role in agricultural ecosystems, because they mediate the interaction of crops with symbiotic AM fungi and parasitic weeds like Striga hermonthica. Cereal host plants secret these signalling molecules particularly under nutrient-deficient conditions and especially when phosphate (P) is limiting. The objective of the present study was to see the potential of P seed priming for S. hermonthica management in cereals in relation to strigolactone production. It has been demonstrated that P fertiliser application down-regulates the production of these signalling molecules in the rhizosphere, which results in lower S. hermonthica infection of cereals. The laboratory study showed maximum production of strigolactones from dry and water-soaked seeds, while seed soaking in P solution reduced their production. Similarly, maximum S. hermonthica infection was observed under control treatments with dry sowing or water soaking, while P seed soaking decreased S. hermonthica germination, emergence and dry biomass in all cereal crops. Our study shows that P seed priming resulted in lower exudation of strigolactones, which induced less S. hermonthica seeds germination and hence may lead to lower S. hermonthica infection. P-based seed priming could prove to be an effective and affordable strategy to reduce S. hermonthica infection in cereals. Further research for practical field application is neede