Years of Trouble in Food Security: Witchweed and Its Parasitic Talents

Years of Trouble in Food Security: Witchweed and Its Parasitic Talents

Earth’s population is increasing continuously, and in the meantime, multiple problems are increasing along with it. Due to the rapid growth of populations, crop quality and quantity demand is increasing nonstop1. This issue is making researches and studies about improving a crop’s yield more important than ever. For achieving an effective solution, multiple challenges also came to the surface for dealing with it. One of the most difficult impediments that are giving hard times to both researchers and farmers is a type of organism, parasitic plants. All over the world, many crops are distributed by parasitic plants and resulting in serious economical damage along with loss of food supplies. For preventing these losses, studying these plants and their relationships has become a priority.

Parasitic plants are classified into two different groups according to their target on host’s tissue, root parasites, and stem parasites2. In this article, a root parasite, Striga, also termed as witchweed, will be mentioned as an example. This plant is an annual, root hemiparasitic weed and ideally knowing for infecting tropical crops1. Witchweed needs a host to require its carbon need after a couple of days passed of its germination. Even though they are parasites, they can still perform the photosynthesis process, so these characteristics make them obligate hemiparasite2 3. Witchweed’s homeland is being thought of in South Africa. Although they do not result in serious damage to the crops outside of Africa, it is still commonly found and giving minor damages on places like Arabia, Egypt, Madagascar, China, some Indian Ocean islands and so on1 4. Witchweed causes a problem to several types of crops. Particularly maize, kaffir corn, paspalum, sugar cane, monkey nut, rice, and so forth1 4. Even though it is hard to measure the damage that witchweed does, it is assumed that they are responsible for affecting 300 million people in Africa besides the economic damage due to high yield loss5

Figure 1: igure 1, Striga infested field in Kenya (a)

Figure 1: Striga infested field in Kenya (a)

The spread of Striga has multiple reasons; purchasing of crop seeds that are corrupted and infected, effects of climate change due to certain parasitic specie’s adaptation ability and being able to tolerate the climatic changes during the season1. With their increased adaptability, currently, they are one of the hardest parasites to control5. Striga seed can remain at the dormant stage for a long time and only starting to germinate if hot conditions with the host-derived stimulant provided. These specific requirements are exposing a very important and specific adaptation. It means that they can only germinate with the existence of stimulants derived by the host itself but what is the catch behind it?

These stimulants are currently known as strigolactones, a plant hormone that is isolated for the first time from a cotton plant’s roots. The first discovered function of this hormone was inducing the germination of Striga species6. Of course, stimulating an organism for its harm could not be the real objective of a hormone. Eventually, it was found that they are mainly responsible for regulating branching plant roots or being a mediator between plant and arbuscular mycorrhizal fungi for a symbiotic interaction6.

This symbiotic relationship is older than 450 million years and is assumed it is one of the most critical steps throughout the plant evolution3. This interaction is very critical for the host in certain circumstances. The plant might not be able to find enough minerals or water for itself. As a result, they can release these stimulants for creating a symbiotic relationship with these fungi. This relationship works with a simple two-sided beneficial agreement. While the plant is getting the necessary water and essential minerals like nitrogen or phosphorus, the fungi are getting the nutrients like sugar from the plant3. This parasite successfully adapted itself by interrupting this symbiotic interaction for its beneficial purposes. By the meaning of this, they can only be able to germinate if they are very close to the host’s roots to interact with the stimulant. They are not going to start the germination while the seed is far away from the target root or if there is no available host to attach it. With that, they had increased their probability of surviving and success of reproductivity3. After germination, Striga aims for invading host roots by penetration. Just like it is all common for parasitic plants, Striga also has a special invasion weapon called haustoria, for penetrating the roots for taking the host’s water, minerals and nutrients5.

Figure 2, development of lateral and terminal haustorium (b)

Figure 2: Development of lateral and terminal haustorium (b)

There are two kinds of haustoria structures developed by parasitic plants. Obligate hemiparasite plants develop this structure on the radicle tip, which is termed terminal haustorium (Figure 2a). Meanwhile, facultative parasites (a parasite that showing optional parasitic activity) and a few obligate parasites are developing lateral haustorium structure (Figure 2b) as a lateral extension on their stem or roots2. Striga, a hemiparasite plant, terminal haustorium is developed at the tipping point of the parasite’s roots and responds when contact occurred with the host root7. When the vascular connection developed, this parasite will take advantage of the host within its life cycle. Usually, this reaction is not going to be lethal for the host, otherwise, the parasite cannot obtain its requirements. Separation of this vascular connection is difficult, and it is hard to destroy the parasite without giving some damage to the host itself in return4.

Multiple ways of solutions are currently studying and applying for controlling this parasite. Although years of studies and researches made on this issue, witchweed is still managing to be a huge trouble in the world by damaging both the economy and food storage. Their specific specialities about invading plants and adaptation of multiple changes making them a tough case to solve. Potentially, some concerns about other parasitic weeds genus, including among other Striga species as well, still exist. It is mostly because of the evolutionary pathway that they follow up. Therefore, they have a high amount of tendency to become more parasitic in the near future1. In the end, it seems that the journey of parasitic plants is not ended yet.

References:

  1. Rodenburg J, Riches CR, Kayeke JM. Addressing current and future problems of parasitic weeds in rice. Crop Prot. 2010;29(3):210-221. doi:10.1016/j.cropro.2009.10.015
  2. Goyet V, Wada S, Cui S, et al. Haustorium Inducing Factors for Parasitic Orobanchaceae. Front Plant Sci. 2019;10(July). doi:10.3389/fpls.2019.01056
  3. López-Ráez JA, Shirasu K, Foo E. Strigolactones in Plant Interactions with Beneficial and Detrimental Organisms: The Yin and Yang. Trends Plant Sci. 2017;22(6):527-537. doi:10.1016/j.tplants.2017.03.011
  4. Pearson HHW. The Problem of the Witchweed. South African Agric J. 1913;6(5):803-805.
  5. Jamil M, Kountche BA, Al-Babili S. Current progress in Striga management. Plant Physiol. 2021;185(4):1339-1352. doi:10.1093/plphys/kiab040
  6. Khosla A, Nelson DC. Strigolactones, super hormones in the fight against Striga. Curr Opin Plant Biol. 2016;33:57-63. doi:10.1016/j.pbi.2016.06.001
  7. Yoder JI, Scholes JD. Host plant resistance to parasitic weeds; recent progress and bottlenecks. Curr Opin Plant Biol. 2010;13(4):478-484. doi:10.1016/j.pbi.2010.04.011

Figure References:

  1. Spallek T, Mutuku M, Shirasu K. The genus Striga: A witch profile. Mol Plant Pathol. 2013;14(9):861-869. doi:10.1111/mpp.12058
  2. Yoshida S, Cui S, Ichihashi Y, Shirasu K. The Haustorium, a Specialized Invasive Organ in Parasitic Plants. Annu Rev Plant Biol. 2016;67(April):643-667. doi:10.1146/annurev-arplant-043015-111702

 

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