Keywords

Striga and Its Parasitic Relationship with Cereals

Through their evolutionary course, some plants have lost their autotrophic nature and depend on other plants to provide them with the water and nutrients they require to complete their life cycle. These parasitic plants account for approximately 1% of angiosperms, totalling over 4000 species distributed in over 30 families (Nickrent 2018). Most parasitic plants are curious members of natural ecologies rather than agricultural pests. A few, like sandalwood, are even valuable crops themselves. Some, however, have adapted to crop hosts and have therefore become weeds. Striga is a genus name in the family Orobancaceae which contains 1725 parasitic plant species (Nickrent 2018). Striga itself includes approximately 40 species, all hemiparasitic, meaning that they have retained some photosynthetic capacity, but all obligate parasites, requiring a host plant to survive through maturity. Most Striga species parasitize grass hosts with S. gesnerioides, a parasitic weed in cowpea, a notable exception. The name Striga is the Latin word for witch. Striga are therefore commonly known as witchweeds in English with similar common names in the languages of many sub-Saharan African peoples that inhabit their native range. The name is descriptive of the syndrome cereals display under Striga infestation. Even before the weed emerges and transforms a grain field into a sea of red or purple flowers, the crop appears to be under a spell that robs it of its verdancy. The most notorious Striga species are S. asiatica and S. hermonthica and it is to these which we refer throughout this book as Striga. Striga causes an estimated seven billion USD in crop productivity losses annually across sub-Saharan Africa (Yacoubou et al. 2021). Striga are in many agroecologies the primary biological constraints to maize, rice and sorghum production. There are few control options among these crops. These include host plant genetic components that mitigate the degree of Striga infestation (resistance) and impact of infestation on crop productivity (tolerance). Host plant resistance and tolerance are considered vital components of Striga management strategies, but unfortunately sources are rare and largely uncharacterized among these staple grain crops (Rich 2020). Yield losses to Striga are concentrated among the subsistence farms where agricultural inputs that can reduce their impact (e.g., irrigation, fertilizer and even improved cultivars) are rarely used. It is with view to these farmers that this mutagenesis project was undertaken.

In order for the parasite to succeed on its host, it must cue its development to a series of vulnerabilities manifested by the cereal during its growing season. These are described in more detail in Chapters “Physical Mutagenesis in Cereal Crops”–“An Agar-Based Method for Determining Mechanisms of Striga Resistance in Sorghum” and elsewhere (see Rich 2020), but briefly involve germinating in response to chemical stimulants (strigolactones) exuded by the host seedling root. Next, the Striga radicle tip must form a haustorium (organ of attachment and acquisition) at or very near an actively growing host in response to 2,6-dimethoxybenzoquinone (DMBQ) and other by-products of secondary cell wall formation. The haustorium must then attach to the host root and penetrate the epidermis, cortex, endodermis and xylem elements, all the while evading any host defences that could halt its progression. Further haustorial development must then occur until a functional vascular union is achieved with its host, ensuring a sustained flow of water and nutrients to support Striga shoot development and sufficient growth to the point of emergence, flowering and seed production. A healthy S. hermonthica plant can produce in excess of 100,000 seeds and these seeds can remain viable for a decade or longer.

Resistance traits can be anything that interferes with establishment of this parasitic relationship, rendering a host less compatible to Striga infection at any or multiple points of this process such that the number or vigor of successful parasites is reduced relative to that on a susceptible, or more compatible host. Reducing the number of successful parasites protects both the current and future crops. Beyond the effects of resource theft, Striga is notorious for further negative effects on host plant fitness, things like increased root growth at the expense of shoot growth, chlorosis and leaf senescence, which in crop plants translates to severe or even total reductions in grain yield. Tolerance traits affect a host’s ability to be less sensitive to these peculiar toxic effects of Striga infestation and thereby sustain grain production in comparable amounts to uninfested plots.

Striga on Maize

Although maize (Zea mays) is an introduction from the Americas, it is today the major cereal crop in both area under cultivation and grain production in Africa (Rich and Ejeta 2008). Striga is a major biological constraint to maize cultivation across most areas of sub-Saharan Africa (SSA) and thereby reduces food security of millions of farmers and those that depend on the grain they produce (Yacoubou et al. 2021). There are limited control options and most of these are not readily available to subsistence farmers, by far the most numerous in Striga endemic areas. Integrated approaches employing both resistant varieties along with Striga management practices are generally more effective than singular control methods, but again, too costly or unavailable to most maize farmers in SSA (Rich 2020; Yacoubou et al. 2021). Sources of genetic resistance to Striga are rare and come largely from maize’s wild relatives, Tripsicum dactyloides and Zea diploperennis (Gurney et al. 2003; Amusan et al. 2008). Other sources include landraces, inbred lines, open pollinated varieties and some hybrids (Yacoubou et al. 2021). Tolerance, (reduced impact of Striga infestation on yield) is used more in maize than genetic resistance (host plant factors that reduce Striga infestation) because the former is more widely available (Yacoubou et al. 2021). However, tolerance does little to control the spread of Striga among maize production areas and pest populations can quickly overwhelm these lands to the point where they are no longer suitable for maize cultivation (Yacoubou et al. 2021). Known resistance mechanisms in maize include low Striga germination stimulant activity of root exudates (Gurney et al. 2003; Adetimirin et al. 2000; Karaya et al. 2012), low haustorial induction (Gurney et al. 2003; Mutinda et al. 2018), reduced attachment due to reduced host root branching (Amusan et al. 2008), reduced number of successful penetrations (Gurney et al. 2003; Amusan et al. 2011) and escape through early maturity (Oswald and Ransom 2004). The genetic basis underlying these resistance mechanisms remains poorly understood. No known genes specifically controlling resistance/susceptibility have been identified and therefore markers for Striga incompatible alleles do not yet exist (Yacoubou et al. 2021).

Striga on Rice

Although one species of rice (Oryza glaberrima) was domesticated in West Africa nearly 3000 years ago, the predominant species cultivated across sub-Saharan Africa is now Asian rice (O. sativa) introduced through East Africa from India 500 years ago and is today the second most important cereal on the Continent (Zenna et al. 2017). A group of rice varieties called NERICA (New Rice for Africa) developed at the Africa Rice Centre in East Africa from crosses between the native and introduced rice species are promising sources of Striga resistance (Rodenburg et al. 2017). Genomic regions in rice associated with Striga resistance have been reported. One of the progenitors of the mapping population used to identify quantitative trait loci (QTL) for Striga resistance was a line that showed a strong incompatible reaction to S. hermonthica. Parasites were unable to establish vascular connections with the rice host (Gurney et al. 2006). From testing the mapping population under Striga infestation, four QTL with major effects on resistance to S. hermonthica were identified. Expression profiling was used to find three candidate genes coding for uncharacterized proteins within one of the major QTL associated with resistance (Swarbrick et al. 2008). Quantitative trait loci with major effects on tolerance to S. hermonthica have also been reported in rice (Kaewchumnong and Price 2008). Mechanisms of low Striga germination stimulant activity and incompatibility were characterized among rice cultivars with field resistance to both S. asiatica and S. hermonthica (Samejima et al. 2016; Rodenburg et al. 2017). In one of these, a gene involving regulation of salicylic acid and jasmonic acid defense signaling pathways was found to condition the resistance (Mutuku et al. 2015).

Striga on Sorghum

Sorghum was domesticated in the same parts of Africa where Striga is believed to have originated (Rich and Ejeta 2008). It is perhaps because the two species share a common geological origin that Striga resistance in sorghum is better defined than in maize and rice.

In sorghum, low Striga germination stimulant activity is a useful source of resistance. Inheritance of this trait is through recessive alleles (lgs1) at a locus named LOW GERMINATION STIMULANT1 (Gobena et al. 2017). Sorghum varieties homozygous for lgs1 generally support fewer Striga plants relative to susceptible varieties. The LGS1 gene codes for a sulfotransferase unique to sorghum that controls the stereochemistry of the strigolactones during the final step of their biosynthesis (Yoda et al. 2021). That stereochemistry determines the Striga germination stimulant activity of the strigolactones that dominate the sorghum root exudate (Gobena et al. 2017). LOW GERMINATION STIMULANT1 is currently the only known Striga resistance gene in any cereal.

Exudation of germination inhibitors is a possible Striga resistance trait in sorghum, although the chemical identity of these inhibitors remains largely unknown (Weerasuriya et al. 1993; Rich et al. 2004). Reduced haustorial inducing capacity of sorghum root exudates is another possible Striga resistance trait in sorghum. Complete lack of haustorial initiation factors released by growing roots is unlikely, given that DMBQ is a by-product of host root growth through elongation. Certain sorghum lignin mutations called brown midrib12 (bmr12) specifically reduce syringyl components of secondary cell wall lignin (Saluja et al., 2021). These syringyl lignin subunits are direct precursors to DMBQ (Cui et al. 2018). A preliminary test of haustorial inducing capacity of sorghum bmr12 mutants with S. hermonthica showed 20% fewer haustoria formed near the roots of these mutants in agar than occurred in the presence of wildtype sorghum roots (Rich 2018). Whether bmr12 sorghum is less parasitized in a soil environment remains to be tested. We have also observed lower haustorial initiation capacity among certain wild sorghum accessions (Rich et al. 2004).

Root characteristics like decreased branches may result in fewer Striga attachments through avoidance of potential parasites. We have observed in our various co-culture laboratory methods that Striga are more likely to attach to and successfully penetrate thinner root branches than on the primary roots of sorghum seedlings. The altered lignin mutation bmr12 also causes root architectural changes resulting in fewer branches in the upper soil profile with respect to wildtype (Saluja et al. 2021). Several other QTLs have been identified in sorghum that control root architecture (Parra-Londono et al. 2018) and certain alleles that specifically condition fewer root branches in the upper soil profiles may therefore contribute to Striga resistance through avoidance.

Sorghum has a reputation for producing allelopathic chemicals like sorgoleone produced in its root hairs (Głąb et al. 2017). Sorgoleone is a potent phytotoxin, inhibiting multiple vital processes impacting photosynthetic, root and mitochondrial functions (Dayan 2006). Phytotoxicity toward Striga has not been specifically studied. If sorgoleone or the other allelopathic compounds present in sorghum root exudates have an antibiotic effect on Striga, they would likely protect it at the pre-attachment phases of the life cycle. These and other components of sorghum root exudates might also act indirectly by influencing the microflora of the rhizosphere favoring Striga-suppressive rhizobacterial or mychorrhizal species (Schlemper et al. 2017).

A number of post-attachment resistance reactions have been described in sorghum that stops the parasite before vascular connections are established. One of these is an apparent hypersensitive response that shows reddening and necrosis in host root epidermal and cortical cells surrounding the attachment site, generally isolating the invading tissues and blocking parasite establishment. The response was described in derivatives from wild sorghum (S. bicolor × S. b. verticilliflorum) challenged with S. asiatica in laboratory co-culture and is inherited through dominant alleles at two loci named Hrs1 and Hrs2 (Mohamed et al. 2010). This defence response appears similar to the hypersensitive response characterized in cowpea against S. gesnerioides and may be triggered by as yet unidentified effectors from the parasite (Li and Timko 2009). Other reactions generally described as “mechanical barriers” have been reported in resistant sorghums upon attachment of S. asiatica expressed in the cortex and endodermis that prevent invading parasite tissue from reaching the vasculature (Maiti et al. 1984). We have used the term incompatibility to indicate the collective post-attachment resistance responses that do not have obvious host-tissue necrosis (Pérez-Vich et al. 2013). These may include several mechanisms controlled by multiple loci. The overall effect is to arrest or reduce the rate of successful parasitic events. In sorghum infected with Striga, these are usually not 100% protective, that is, some parasites on resistant varieties usually do manage to emerge and set seed, but the frequency of these events is reduced relative to the numbers of successful parasites on susceptible varieties. Incompatibility may be expressed in the cortex, at the endodermis or even after penetration of xylem vessels. Attached Striga in these instances are slow to develop and often die before reaching maturity. Incompatible reactions are expressed in host tissues as extra thickening of the endodermis and deposition of phenolic compounds at the interface with haustorial cells or even occlusion of vessels where the parasite initially breached its xylem elements (Maiti et al. 1984; Arnaud et al. 1999; Amusan et al. 2011; Mbuvi et al. 2017). The haustoria of Striga infecting incompatible sorghum often appears diminished relative to those of successful parasitic events. These symptoms may represent instances of active defence responses triggered by the parasite or simply a constitutively unsupportive cellular environment, perhaps lacking key metabolites preferred by the parasite to establish and grow. Unfortunately, none of these post-attachment reactions in sorghum to Striga have been so precisely characterized and exploited in resistance breeding as lgs1, and markers specific to only lgs1 currently exist.

Potential Contribution of Mutagenesis to Improved Striga Resistance

Multiple resistance traits, even those that render a plant slightly incompatible with Striga establishment, combined in a crop genotype are more sustainable than individual resistant traits since a parasite population would need to accumulate multiple virulence mutations to overcome them. Tolerance too is ideally used in combination with resistance for long-term durable protection. The challenge of deploying these host characters in a sustainable combination is that they are rare, almost never singularly effective (there are no credible reports of Striga immune varieties in these crops) and their genetic basis is poorly understood. Adding to these fundamental limitations for the use of host resistance/tolerance, access to Striga resistant varieties and inputs that extend their effectiveness by the neediest farmers is extremely limited.

It is in the context of these great challenges that this CRP offers a contribution. We wanted to create through mutagenesis more genetic variants that offer some degree of Striga resistance. With view specifically to farmer acceptance of any new resistant varieties that might result from this endeavour, each participating Member State chose a crop variety popular among subsistence farmers with several desirable qualities, but lacking Striga resistance, in which to conduct the mutation breeding. Since very few genes are known among the target crops that control Striga resistance, a process that caused genome wide changes seemed worth trying. The goal was to identify among the mutagenized lineages Striga resistant progeny in farmer-preferred varieties. Useful gained resistance should come as an attribute without yield costs or loss of other desirable characteristics. The resulting germplasm would then have added value as a cultivar itself, at least as a demonstration that a favourite genotype can be genetically protected against the ills of Striga. More appropriately in terms of sustainability, the germplasm would serve as starting material for further resistance improvements based on the introgression of other known Striga resistance alleles from crosses with donor sources or through gene editing. The fruit of this gained resistance would be even greater if the underlying mutations that caused the improved Striga resistance can be identified through genomic analysis and thereby become new targets for natural allele mining or gene editing.