Striga hermonthica

Striga hermonthica, commonly known as purple witchweed[1] or giant witchweed, is a hemiparasitic plant[1] that belongs to the family Orobanchaceae. It is devastating to major crops such as sorghum (Sorghum bicolor) and rice (Oryza sativa).[2] In sub-Saharan Africa, apart from sorghum and rice, it also infests maize (Zea mays), pearl millet (Pennisetum glaucum), and sugar cane (Saccharum officinarum).[3]

Purple witchweed
Striga hermonthica flowers
Scientific classification
Kingdom: Plantae
Clade: Tracheophytes
Clade: Angiosperms
Clade: Eudicots
Clade: Asterids
Order: Lamiales
Family: Orobanchaceae
Genus: Striga
Species:
S. hermonthica
Binomial name
Striga hermonthica
(Delile) Benth.

Striga hermonthica has undergone horizontal gene transfer from Sorghum to its nuclear genome. The S. hermonthica gene, ShContig9483, is most like a Sorghum bicolor gene, and additionally shows significant but lesser similarity to a gene from Oryza sativa. It shows no similarity to any known eudicot gene.[2]

Host and symptoms

Purple witchweed infects a variety of grasses, and legumes in sub-saharan Africa including rice, maize, millet, sugarcane, and cowpea. The symptoms mimic that of drought or nutrient-deficiency symptoms. Chlorosis, wilt, and stunting result from witchweed’s ability to extract nutrients from its host. Pre-emergence symptoms are difficult to diagnose secondary to their similarity to general lack of nutrients. Once emergence of the plant has taken place, damage has become too severe to mitigate.[4]

Parasitic cycle

Seeds of witchweed overwinter in the soil after they are dispersed by wind, water, animals, or human machinery.[5] When the environment is correct, and if the seed is within a few centimeters of the host root, it will begin to germinate. The germinating plant grows towards hormones, called strigolactones, released from the host root.[6][7] The plant grows up the concentration gradient of these strigolactones. In the absence of strigolactone, the striga will not germinate. Strigolactone knockout plants have been used in an attempt to prevent infection by avoiding germination.[7] Once in contact with the root, the witchweed produces a haustorium establishing a parasitic relationship with the plant. It remains underground for several weeks while extracting nutrients. The stem while underground is round and white. After this stage, it emerges from the ground and rapidly flowers and produces seeds. The flowers self pollinate before opening. After emergence, the plant can perform photosynthesis to augment its metabolic demands.[6]

Environment

Witchweed’s ideal temperature for germination is 30-35 °C. Below 20 °C, the seeds will not germinate. Seeds can survive freezing temperatures.[8] However, the longevity of the seed is debated. Most say that under ideal conditions, seeds can remain viable up to 14 years, but wet soils greatly decrease the resilience of the seeds. At most in one year, 74% of viable seeds were lost secondary to wet soil.[9]

Management

Witchweed is historically among the hardest parasitic plants to control. Fusarium oxysporum may be used as a possible biocontrol of witchweed and its host specificity makes it a good candidate. This fungus is thought to infect the early vasculature of the Striga plant.[10] Applying native strains of Fusarium oxysporum has not shown adequate crop restoration. However, using strains selected for their ability to over-produce specific amino acids[11][12] has shown highly effective results. Data on 500 Striga-infested farms were obtained in paired plot trials over two growing seasons in 2014-2015, using hybrid seed and fertilizer compared to hybrid seed, fertilizer and FoxyT14 (a trio of the virulence-enhanced strains for Fusarium). Most (99.6%) of the farmers had equal or greater yield in their Foxy T14 plots relative to yield in their comparable farmer-practice plots without Foxy T14. The average maize yield in the March–June rains season was increased by 56.5% in Foxy T14 plots relative to the farmer-practice plots (p < 0.0001, pair-wise t-test). Approximately one third of the farmers doubled their yield in this test.[13] This technology development is called The Toothpick Project[14] based on mechanism used to deliver the fungal strains to smallholder farmers via a toothpick, where the farmer can make a fresh, on-farm inoculum by growing the fungal strains on cooked rice. The project is being launched in Kenya and a team of scientists in eleven other countries is working on isolating local strains for development.

Another potential solution to purple witchweed for millet and sorghum crops is herbicide priming. When herbicide-resistant seeds were soaked in herbicidal chemicals before planting, up to an 80% decrease in infestation occurred.[15] The use of nitrogen-rich fertilizers reduces the witchweed infection rate. Although the mechanism behind this is not fully understood, the abundance of nitrogen is thought to disrupt nitrogen reductase activity. This has a ripple effect, resulting in the dysregulation of the plant's light and dark cycle, resulting in the striga's death.[16]

In 2018, an essential protein for witchweed germination was found to consistently bind to molecules of the detergent Triton X-100, which appears to inhibit the germination of the striga seeds, preventing the natural strigolactones from binding to their usual substrate.[17]

Impact

In the late 1990s, "21 million hectares of cereals in Africa were estimated to be infested by S. hermonthica, leading to an estimated annual grain loss of 4.1 million tons".[3]

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References
  1. "Striga hermonthica". Natural Resources Conservation Service PLANTS Database. USDA. Retrieved 4 December 2015.
  2. Yoshida, Satoko; Maruyama, Shinichiro; Nozaki, Hisayoshi; Shirasu, Ken (28 May 2010). "Horizontal Gene Transfer by the Parasitic Plant Stiga hermonthica". Science. 328 (5982): 1128. doi:10.1126/science.1187145. PMID 20508124.
  3. Abbasher, A. A.; Hess, D. E.; Sauerborn, J. (1998). "Fungal pathogens for biological control of Striga hermonthica on sorghum and pearl millet in West Africa". African Crop Science Journal. 6 (2): 179–188. doi:10.4314/acsj.v6i2.27814.
  4. Johnson, Annie. New South Wales. Witchweed. 2005. http://www.wyong.nsw.gov.au/environment/Weeds_category_one_Witchweed.pdf
  5. Sand, Paul, Robert Eplee, and Randy Westbrooks.Witchweed Research and Control in the United States. Champaign, IL: Weed Science Society of America, 1990.
  6. Agrios, George N. Plant Pathology. 5th ed. London: Elsevier Academic Press, 2005
  7. Matusova, Radoslava; Rani, Kumkum; Verstappen, Francel W.A.; Franssen, Maurice C.R.; Beale, Michael H.; Bouwmeester, Harro J. (2005). "The Strigolactone Germination Stimulants of the Plant-Parasitic Striga and Orobanche spp. Are Derived from the Carotenoid Pathway".
  8. Lane, J.A., Moore, T.H.M. and Child, D.V. 1996. Characterisation of virulence and geographic distribution of Striga gesnerioides on cowpea in West Africa. Plant Disease 80: 299-301
  9. Gbehounou, G., A. H. Pieterse, and JAC Verkleij. "Longevity of Striga Seeds Reconsidered: Results of a Field Study on Purple Witchweed (Striga Hermonthica) in Benin." Weed Science 51.6 (2003): 940-6. ProQuest. Web. 11 Nov. 2014.
  10. Abbasher, A. A.; Hess, D. E.; Sauerborn, J. (1998). "Fungal pathogens for biological control of Striga hermonthica on sorghum and pearl millet in West Africa". African Crop Science Journal 6 (2): 179–188.
  11. Pilgeram, Alice L.; Sands, David C. (2010-08-12), "Bioherbicides", Industrial Applications, Springer Berlin Heidelberg, pp. 395–405, doi:10.1007/978-3-642-11458-8_19, ISBN 9783642114571
  12. Sands, David C; Pilgeram, Alice L (May 2009). "Methods for selecting hypervirulent biocontrol agents of weeds: why and how". Pest Management Science. 65 (5): 581–587. doi:10.1002/ps.1739. PMID 19288472.
  13. Nzioki, Henry S.; Oyosi, Florence; Morris, Cindy E.; Kaya, Eylul; Pilgeram, Alice L.; Baker, Claire S.; Sands, David C. (2016-08-08). "Striga Biocontrol on a Toothpick: A Readily Deployable and Inexpensive Method for Smallholder Farmers". Frontiers in Plant Science. 7. doi:10.3389/fpls.2016.01121. ISSN 1664-462X. PMC 4976096. PMID 27551284.
  14. "toothpickproject". toothpickproject. Retrieved 2019-05-25.
  15. Dembélé, B., Dembélé, D., & Westwood, J. (n.d.). Herbicide Seed Treatments for Control of Purple Witchweed (Striga hermonthica) in Sorghum and Millet. Weed Technology, 629-635.
  16. Igbinnosa, I., and P. A. Thalouarn. "Nitrogen Assimilation Enzyme Activities in Witchweed (Striga) in Hosts Presence and Absence." Weed Science 44.2 (1996): 224-32. ProQuest. Web. 11 Nov. 2014.
  17. Umar Shahul Hameed, Imran Haider, Muhammad Jamil, Boubacar A Kountche, Xianrong Guo, Randa A Zarban, Dongjin Kim, Salim Al‐Babili, and Stefan T Arold. "Structural basis for specific inhibition of the highly sensitive ShHTL7 receptor" EMBO Reports (2018) e45619. DOI 10.15252/embr.201745619 http://embor.embopress.org/content/early/2018/07/18/embr.201745619
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