Hermonthica Infection Research Articles

The Structural Integrity of Lignin Is Crucial for Resistance against Striga hermonthica Parasitism in Rice.

Striga species are parasitic weeds that seriously constrain the productivity of food staples, including cereals and legumes, in Sub-Saharan Africa and Asia. In eastern and central Africa, Striga spp. infest as much as 40 million hectares of smallholder farmland causing total crop failure during severe infestation. As the molecular mechanisms underlying resistance are yet to be elucidated, we undertook a comparative metabolome study using the Striga-resistant rice (Oryza sativa) cultivar 'Nipponbare' and the susceptible cultivar 'Koshihikari'. We found that a number of metabolites accumulated preferentially in the Striga-resistant cultivar upon Striga hermonthica infection. Most apparent was increased deposition of lignin, a phenylpropanoid polymer mainly composed of p-hydroxyphenyl (H), guaiacyl (G), and syringyl (S) aromatic units, around the site of interaction in Nipponbare. The increased deposition of lignin was accompanied by induction of the expression of corresponding enzyme-encoding genes in the phenylpropanoid pathway. In addition, perturbing normal lignin composition by knocking down or overexpressing the genes that regulate lignin composition, i.e. p-COUMARATE 3-HYDROXYLASE or FERULATE 5-HYDROXYLASE, enhanced susceptibility of Nipponbare to S hermonthica infection. These results demonstrate that enhanced lignin deposition and maintenance of the structural integrity of lignin polymers deposited at the infection site are crucial for postattachment resistance against S hermonthica.

Genetic diversity and virulence of Striga hermonthica from Kenya and Uganda on selected sorghum varieties

Joel KA, Runo S, Muchugi A. 2018. Genetic diversity and virulence study of Striga hermonthica from Kenya and Uganda on selected sorghum varieties. Nusantara Bioscience 10: 111-120. Parasitic weeds pose a severe problem in agricultural production, causing massive crop losses in many regions of the world and especially Africa. One example to be considered the most significant biological constraint to food production in sub-Saharan Africa (SSA) is Striga hermonthica, the most widespread among the Striga species in the semi-arid tropical African zones. The wide geographical distribution set conditions for genetically structured populations. The genetic variations among the weed populations allow for the quick breakdown of resistance in crops hence making control of the weed difficult. Efficient and effective control of S. hermonthica demands knowledge on inherent genetic variability within local and regional races of the weed. However, the genetic diversity and virulence of S. hermonthica ecotypes in Kenya and Uganda on selected sorghum varieties remain unknown. This study aimed at evaluating the genetic diversity among seven S. hermonthica populations from locations in Kenya and Uganda using 5 primer sets of Expressed Sequence Tags-Simple Sequence Repeats (EST-SSR). The genetic diversity was moderate as shown by the Nei’s genetic distance values. AMOVA measured low genetic differentiation among the populations. This study also investigated twelve sorghum varieties for their response to S. hermonthica infection. The result demonstrated that the varieties resistance responses to S. hermonthica varied widely. The phenotype of resistant interaction was characterized by the inability of the weeds haustoria to penetrate the sorghums root endodermis due to severe necrosis and in rare cases the parasites radicle growing away from the host root. The resistant sorghum varieties were the Asareca W2, Asareca AG3, N13 and the Wild-type which had low mean number of S. hermonthica plantlets growing on their roots, while the most susceptible varieties were Sap 027, Epurpur which had the highest mean number of S. hermonthica plantlets growing on their roots. There was a highly significant difference in the means of the number of Striga growing on the roots of sorghum varieties, Striga dry biomass and S. hermonthica length between the susceptible and resistant ones. This knowledge holds unique potentiality since resistant sorghum germplasm tested will be sourced and targeted to the seven specific geographical areas where virulence of the particular S. hermonthica populations was characterized.

Density‐independent reproductive success of the hemiparasitic plant Striga hermonthica, despite positive and negative density‐dependent phases

AbstractThe hemiparasite Striga hermonthica is a major constraint to smallholder farmer livelihoods and food security in sub‐Saharan Africa. A better understanding of its life‐cycle can help developing more effective management strategies. Here, we studied density dependence in S. hermonthica on Sorghum bicolor. We exposed two genotypes of S. bicolor that differed in the level of tolerance and resistance to S. hermonthica to a range of seed densities of the parasite. We evaluated multiple host and parasite performance parameters through periodic, destructive harvests and related these to the initial seed density using model selection. Initially, the probability for attachment was positively density‐dependent, suggesting facilitation of new infections. However, at host maturity, S. hermonthica infection probability showed strong negative density dependence, indicating severe competition, in particular in the early developmental stages. Although parasite shoot dry weight showed a strong negative density dependence at host maturity, flower production per parasite exhibited positive density dependence again, suggesting compensation. The two host genotypes had similar responses to increased parasite densities, indicating differences between the genotypes in tolerance but not resistance. Consequently, despite density dependence in life‐cycle components, the per capita reproductive output of S. hermonthica, R0 (flowers seed−1) was density‐independent. Apparently, management of the hemiparasite can neither benefit from a negatively density‐dependent bottleneck, nor from a positively density‐dependent Allee effect. The most promising suggestion to obtain S. hermonthica population decline (R0 < 1) and long‐term control is to increase host shading by growing a vigorous, competitive crop.

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

SummaryStrigolactones, 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 needed.

Gas exchange of root hemi-parasite Striga hermonthica and its host Sorghum bicolor under short-term soil water stress

The gas exchange of the upper fully expanded leaf of the root parasite Striga hermonthica and of its host Sorghum bicolor was measured under wet and dry conditions to identify the mechanisms of the devastating effects of the parasite on its hosts under drought. The short-term water stress severely reduced photosynthetic rate in infected sorghum, but less in S. hermonthica. Soil water stress did not affect leaf respiration rate in either S. hermonthica or infected sorghum. This suggests that under dry conditions both infected sorghum and S. hermonthica decreased autotrophic carbon gain. The transpiration rate of S. hermonthica, a major driving force for assimilate uptake from the host, was higher and less affected by water stress than that of infected sorghum. Stomatal density on the abaxial surfaces of the leaves was higher in S. hermonthica than in sorghum. Both S. hermonthica infection and water stress decreased stomatal conductance of the sorghum leaves. S. hermonthica, irrespective of soil water status, had greater stomatal aperture on the adaxial and abaxial surfaces of its leaves than infected sorghum. These results indicate that the higher transpiration rate of S. hermonthica even under water stress, achieved through higher stomatal density on the abaxial surfaces of the leaves and greater stomatal aperture on both surfaces of the leaves, may induce the maintenance of water and solute transfers from the host to the parasite leading to severe damage to the host under drought.

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

Jamil M, Van Mourik TA, Charnikhova T & Bouwmeester HJ (2012). Effect of diammonium phosphate application on strigolactone production and Striga hermonthica infection in three sorghum cultivars. Weed Research.SummaryStriga 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.

Genetic variation in strigolactone production and tillering in rice and its effect on Striga hermonthica infection

Tillering in cereals is a complex process in the regulation of which also signals from the roots in the form of strigolactones play an important role. The strigolactones are signalling molecules that are secreted into the rhizosphere where they act as germination stimulants for root parasitic plants and hyphal branching factors for arbuscular mycorrhizal fungi. On the other hand, they are also transported from the roots to the shoot where they inhibit tillering or branching. In the present study, the genetic variation in strigolactone production and tillering phenotype was studied in twenty rice varieties collected from all over the world and correlated with S. hermonthica infection. Rice cultivars like IAC 165, IAC 1246, Gangweondo and Kinko produced high amounts of the strigolactones orobanchol, 2′-epi-5-deoxystrigol and three methoxy-5-deoxystrigol isomers and displayed low amounts of tillers. These varieties induced high S. hermonthica germination, attachment, emergence as well as dry biomass. In contrast, rice cultivars such as Super Basmati, TN 1, Anakila and Agee displayed high tillering in combination with low production of the aforementioned strigolactones. These varieties induced only low S. hermonthica germination, attachment, emergence and dry biomass. Statistical analysis across all the varieties confirmed a positive correlation between strigolactone production and S. hermonthica infection and a negative relationship with tillering. These results show that genetic variation in tillering capacity is the result of genetic variation in strigolactone production and hence could be a helpful tool in selecting rice cultivars that are less susceptible to S. hermonthica infection.Electronic supplementary materialThe online version of this article (doi:10.1007/s00425-011-1520-y) contains supplementary material, which is available to authorized users.

Host-parasite dynamics of Sorghum bicolor and Striga hermonthica – The influence of soil organic matter amendments of different C:N ratio

The effect of organic amendments on the interactions between Striga hermonthica and a sorghum host was studied in a field experiment during three cropping seasons, following a three-factorial design with (i) bare fallow versus continuous cropping, (ii) two Striga hermonthica infection levels and (iii) five organic matter levels, a single inorganic fertiliser treatment of 120 kg N ha −1 and a control. The effects of two different cotton by-products and their mixtures on sorghum yield were well described by their N-mineralisation pattern. The impact of organic amendments in the sorghum production system was directly related to N-mineralisation in the three cropping seasons. There was an increasing negative effect of organic matter on S. hermonthica as the quality of the applied material increased. The emerged numbers of S. hermonthica were well described by N-release after one month, while S. hermonthica biomass and sorghum biomass were well described by N-release after three months. As a stand-alone measure, addition of low-quality organic matter is disadvantageous in cropping systems with high S. hermonthica seed densities, as it does not improve sorghum performance compared to no addition of organic matter, while S. hermonthica numbers increase. Implications for integrated soil fertility and S. hermonthica management under different infection levels of S. hermonthica are discussed.

Quantification of the relationship between strigolactones and Striga hermonthica infection in rice under varying levels of nitrogen and phosphorus

Jamil M, Charnikhova T, Cardoso C, Jamil T, Ueno K, Verstappen F, Asami T & Bouwmeester HJ (2011). Quantification of the relationship between strigolactones and Striga hermonthica infection in rice under varying levels of nitrogen and phosphorus. Weed Research51, 373–385.SummaryStrigolactone exudation, as well as Striga hermonthica germination and attachment, was studied under different levels of nitrogen (N) and phosphorus (P) in two cultivars of rice (IAC 165 and TN 1). Exudation of strigolactones by rice was the highest under mineral‐deficient conditions, whereas increasing N and P dose reduced the amount of strigolactones in the exudates. Deficiency of P led to the highest strigolactone exudation, when compared with N or NP deficiency. Production of strigolactones differed strongly between the two cultivars. IAC 165 produced about 100‐fold higher amounts than TN 1 of 2′‐epi‐5‐deoxystrigol, orobanchol and three new strigolactones. Across all N and P treatments, a positive relationship was found between the amount of strigolactones in the exudates of both cultivars and in vitro S. hermonthica germination. These results show that the positive effect of fertiliser application in S. hermonthica control is, at least partly, because of the suppression of strigolactone production and hence of S. hermonthica germination and subsequent attachment. This warrants further research into practical application. Maintaining suitable N and P nutrient status of soil through fertiliser use might be a promising strategy to reduce damage in cereals by this notorious weed.

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 weeds.

Multiple layers of incompatibility to the parasitic witchweed, Striga hermonthica

* Witchweeds (Striga spp.) are major agricultural pests that infest important crops in sub-Saharan Africa. Striga hermonthica parasitizes gramineous plants including sorghum, maize and rice, but not dicots. To understand host recognition mechanisms of S. hermonthica, we investigated its interaction with nonhost dicots including Arabidopsis, cowpea, Lotus japonicus and Phtheirospermum japonicum, a hemiparasite. * Striga hermonthica seeds were pretreated with strigol, a germination stimulant, and allowed to germinate next to a potential host root. We characterized the histological phenotype of the interactions. Moreover, we monitored the infection of a host rice and the nonhost P. japonicum by S. hermonthica using time-lapse photography. * All nonhost dicots tested did not support S. hermonthica shoot growth beyond the six leaf-pair stage; however, the arrest of parasite development occurred at different stages. Striga hermonthica haustoria were able to reach the steles of Arabidopsis and cowpea, while L. japonicus blocked S. hermonthica infection in the root cortex. Striga hermonthica often failed to penetrate P. japonicum roots. * Our analysis indicates that there are at least four types of incompatible interaction to S. hermonthica. Combinations of these different incompatibility mechanisms contribute to the total resistance to S. hermonthica.

Novel sources of resistance to Striga hermonthica in Tripsacum dactyloides, a wild relative of maize.

• The parasitic weed Striga hermonthica lowers cereal yield in small-holder farms in Africa. Complete resistance in maize to S. hermonthica infection has not been identified. A valuable source of resistance to S. hermonthica may lie in the genetic potential of wild germplasm. • The susceptibility of a wild relative of maize, Tripsacum dactyloides and a Zea mays-T. dactyloides hybrid to S. hermonthica infection was determined. Striga hermonthica development was arrested after attachment to T. dactyloides. Vascular continuity was established between parasite and host but there was poor primary haustorial tissue differentiation on T. dactyloides compared with Z. mays. Partial resistance was inherited in the hybrid. • Striga hermonthica attached to Z. mays was manipulated such that different secondary haustoria could attach to different hosts. Secondary haustoria formation was inhibited on T. dactyloides, moreover, subsequent haustoria formation on Z. mays was also impaired. • Results suggest that T. dactyloides produces a signal that inhibits haustorial development: this signal may be mobile within the parasite haustorial root system.

Effects of soil pasteurisation and soil N status on severity of Striga hermonthica (Del.) Benth. in maize

The nature of interrelationships between soil pasteurisation, and increased soil N content, on the severity of Striga hermonthica infection in maize was studied in pot experiments in the screen-house. An initial experiment revealed that there was no significant difference in amount of S. hermonthica infection in maize grown in potted soil collected from Bida (9°05′N, 6°01′E) and Ibadan (7°17′N, 3°30′E). However, there was 57% increase in amount of S. hermonthica infection and a 68% reduction in maize shoot dry matter when maize was grown in pasteurised soil compared with natural soil. In another experiment, soil steaming significantly influenced the effects of increased soil N fertility (from preceding soybean cv. SAMSOY-2 or application of 90 kg N ha −1) on severity of S. hermonthica in maize. In natural soil, application of N to maize reduced (by 53%) the number of emerged S. hermonthica plants and increased (by 154%) maize dry matter compared with no fertilizer application. In pasteurised soil, application of N to maize increased S. hermonthica severity by 26% and also significantly increased maize dry matter compared with no fertilizer application. The same effects were observed when soybean was used to increase soil N content. Analysis of natural and pasteurised soil revealed only minor differences in composition of K +, Na +, Cu, Mn, and Fe, and none of these changes was directly related to S. hermonthica infection in maize. Results of this study indicate that the differences in S. hermonthica infection in pasteurised and natural soil could be attributed to soil biotic factors that reduce S. hermonthica infection in natural soil. The results partly provide an explanation for the wide variation in reports on the effects of N fertilization on severity of S. hermonthica infection and stress the need for understanding the mechanisms of natural reduction in S. hermonthica infection and interactions of these natural mechanisms with other control techniques.

The effect ofStriga hermonthica(Del.) Benth. infection on gas-exchange characteristics and yield of a sorghum host, measured in the field in Mali

The effect of the parasitic weed Striga hermonthica (Del.) Benth. on the gas-exchange characteristics and yield of a Sorghum bicolor (L.) Moench host was studied in the field in Mali in 1990. Infection by S. hermonthica significantly reduced sorghum grain and straw yield at harvest 109 d alter sowing (DAS). In contrast with results from previous laboratory studies, S. hermonthica infection did not decrease host photosynthesis, transpiration and water use efficiency 67 or 79 DAS

Nitrogen relations of the sorghum‐Sfriga hermonthica hostparasite association: growth and photosynthesis

ABSTRACTThe extent to which the parasitic angiosperm Striga hermonthica reduces the growth of its sorghum host is dependent on the concentration of nitrogen (as NH4NO3 in 40% Long Ashton Solution) supplied to the plants. The biomass of 0.5,1 and 2 mol m−3 N‐grown infected plants was 22,30 and 66%, respectively, of uninfected plants after 140d growth. The biomass of 3 and 4 mol m−3 N‐grown infected plants differed little from uninfected plants. No grain was set in 0.5 and 1 mol m−3 N‐grown infected plants, grain yield reached 42 and 73% of controls in 2 and 3 mol m−3 N‐grown plants, and was unaffected in 4 mol m−3 N‐grown plants. Striga hermonthica also altered the allometry and architecture of the host, at all but the highest N concentration. Higher N concentration (3 and 4 mol m −3 N) reduced the growth of S. hermonthica. Foliar N concentrations in sorghum ranged from 11 mg g−1 dwt. in 0.5 mol m−3 N‐grown plants, to 28 mg g−1 dwt. in 4 mol m−3 N‐grown plants, and were not affected by S. hermonthica. Higher N concentrations were measured in S. hermonthica, and ranged from 18 to 45 mg g−1 dwt. in 0.5 and 3 mol m−3 N‐grown plants, respectively. The relationship between photosynthesis (CO2 flux) and N concentration differed between uninfected and infected sorghum. This was most apparent in 0.5 mol m−3 N‐grown plants, with rates of 16 and 11 μmol m−2 s−1 in uninfected and infected plants, respectively (at 1500–1800 μmol m−2 s−1 photosynthetic photon flux density). At higher N concentrations, this difference was smaller, with both sets of plants reaching 26 μmol m−2 s−1 at 4 mol m−3 N. Varying the level of S. hermonthica infection showed that the effect of N on host photosynthesis cannot be explained by differences in the mass or number of parasites supported by the host. At low levels of infection in 1 mol m−3 N‐grown plants, the negative effect of the parasite was reversed, and photosynthesis in infected plants exceeded that in uninfected plants by 20%. Photosynthesis in S. hermonthica at 3 mol m−3 N (8 μmol m−2 s−1) was double that in 0.5 mol m−3 N‐grown plants. Stable carbon isotope and gas exchange measurements data demonstrated that this higher level of autotrophic carbon fixation was accompanied by a lower dependency on hetero trophic carbon. The latter ranged from 27 to 6% in 0 5 mol m−3 and 3 mol m−3 N‐grown plants, respectively.