Striga seed-germination activity of root exudates and compounds present in stems of Striga host and nonhost (trap crop) plants is reduced due to root colonization by arbuscular mycorrhizal fungi

Root colonization by arbuscular mycorrhizal (AM) fungi reduces stimulation of seed germination of the plant parasite Striga (Orobanchaceae). This reduction can affect not only host plants for Striga, resulting in a lower parasite incidence, but also false hosts or trap crops, which induce suicidal Striga seed germination, thereby diminishing their effectiveness. In order to better understand these AM-induced effects, we tested the influence of root colonization by different AM fungi on the seed-germination activity of root exudates of the Striga hermonthica nonhost plants cowpea and cotton on S. hermonthica. We also tested the effect of AM fungi on the seed-germination activity of the Striga gesnerioides host plant cowpea on S. gesnerioides. Moreover, we studied whether mycorrhization affects the transport of seed-germination activity to above-ground plant parts. Mycorrhization not only resulted in a lower seed germination of S. gesnerioides in the presence of root exudates of the S. gesnerioides host cowpea but also seed germination of S. hermonthica was also lower in the presence of root exudates of the S. hermonthica nonhosts cowpea and cotton. Downregulation of the Striga seed-germination activity occurs not only in root exudates upon root colonization by different AM fungi but also in the compounds produced by stems. The lowered seed-germination activity does not appear to depend on the presence of seed germination inhibitors in the root exudates of mycorrhizal plants. The implication for Striga control in the field is discussed.     

Growth and carbon allocation in Pennisetum typhoides infected with the parasitic angiosperm Striga hermonthica

Abstract Growth and gas exchange measurements are used in conjunction with a carbon balance model to describe the millet (Pennisetum typhoides)–witchweed (Striga hermonthica) host—parasite association. Striga hermonthica reduces the growth of millet by 28% and radically alters the architecture of infected plants. Whilst grain yield and stem dry weight are reduced (by 80 and 53%, respectively), leaf and root growth are stimulated (by 41 and 86%, respectively). The difference in production between infected and uninfected millet plants can be accounted for by two processes: first, export of carbon to the parasite (accounting for 16% of the dry weight not gained); and second, parasite-induced reductions in host photosynthesis (accounting for 84% of the dry weight not gained). Striga hermonthica is dependent on carbon exported from the host, since the plant has low rates of photosynthesis coupled with high rates of respiration. The carbon balance model suggests that in mature S. hermonthica plants parasitic on millet, 85% of the carbon is host-derived. Carbon fluxes are also estimated using δ¹³C measurements, since S. hermonthica is a C₃ plant parasitizing a C₄ host. In conjunction with gas exchange measurements, these suggest that in root, stem and leaf of S. hermonthica, 87, 70 and 49% of carbon is hostderived, respectively.     

A carbon balance model of the sorghum-Striga hermonthica host-parasite association

Abstract. Growth and gas exchange measurements are used to formulate a carbon balance model to describe the sorghum- Striga hermonthica host-Parasite association. S. hermonthica reduces the growth and radically alters the architecture of infected sorghum plants. Grain and stem weight are reduced, whilst leaf and root biomass are maintained. Losses in host productivity result from two processes: export of carbon to the parasite and Parasite-induced reductions in host photosynthesis. The latter occurs before the emergence of the Parasite above ground and accounts for 80% of the Predicted loss in production over the lifecycle of the association. S. hermonthica is dependent on carbon exported from the host, since the plant has low rates of photosynthesis coupled with high rates of respiration. Host-derived carbon accounts for approximately one-third of the total parasite carbon requirement.     

Striga gesnerioidesParasitising Cowpea: Development of Infection Structures and Mechanisms of Penetration

Striga gesnerioides(Scrophulariaceae) is an obligate hemiparasiticangiosperm, that infects the roots of cowpea (Vigna unguiculata).The development ofStrigaseedlings was characterized after transferto the surface of cowpea roots using cryo-scanning electronmicroscopy. Contact with the surface of the root caused distinctmorphological changes in theStrigaradicles. Radicle elongationceased 8 h after contact with host roots and radicle hairs developed.These hairs adhered to the surface of the host root. Early stagesof root penetration, i.e. prior to connection to the host vascularsystem were examined by light microscopy and transmission electronmicroscopy to determine the mechanisms by whichStrigainvadesroot tissue. Cowpea roots were penetrated byS. gesnerioidesradiclesafter 48 h. Host cells immediately surrounding the invadingStrigawerecompressed, but other host cells were not affected. Immunocytochemistry,using monoclonal antibodies JIM 5 and JIM 7, showed thatStrigaadvancedintercellularly through the root tissues without extensive removalof pectins at the site of penetration.Copyright 1998 Annalsof Botany Company Striga gesnerioides,Vigna unguiculata, mechanisms of penetration, host-parasite interactions, electron microscopy, immunocytochemistry.     

Structural requirements of strigolactones for germination induction and inhibition of Striga gesnerioides seeds

Key message

Structure–activity relationship studies of strigolactones and Striga gesnerioides seed germination revealed strict structural requirements for germination induction and a new function of the plant hormones as germination inhibitors.

Abstract

Stereoisomers of the naturally occurring strigolactones, strigol, sorgolactone, orobanchol, sorgomol and 5-deoxystrigol, 36 in total, were prepared and screened for the ability to induce and/or inhibit the germination of Striga hermonthica and Striga gesnerioides seeds collected from mature plants that parasitized on sorghum and cowpea, respectively. All of the compounds induced S. hermonthica seed germination, albeit displayed differential activities. On the other hand, only a limited number of the compounds induced significant germination in S. gesnerioides, thus indicating strict structural requirements. Strigolactones inducing high germination in S. gesnerioides induced low germination in S. hermonthica. Strigolactones with the same configuration at C3a, C8b and C2′ as that in 5-deoxystrigol (9a) induced high germination of S. hermonthica seeds, but most of them inhibited the germination of S. gesnerioides. The differential response of S. gesnerioides to strigolactones may play an important role in the survival of the species. However, the compounds could be used as means of control if mixed cropping of cowpea and sorghum is adopted.     

Gas exchange characteristics of the sorghum-striga host-parasite association

Gas exchange characteristics are reported for both members of the sorghum-Striga host-parasite association. Both Striga hermonthica (Del.) Benth and Striga asiatica (L.) Kuntze had transpiration rates considerably in excess of those of sorghum (Sorghum bicolor (L.) Moench, cv CSH1). Stomatal conductance in both Striga spp. showed little response to periods of darkness and moderate water stress. Low rates of net CO₂ fixation and high rates of dark respiration led to no net daily (24 hours) C gain, and Striga would appear to be reliant on its host for photosynthate. Infection of sorghum plants with either S. hermonthica or S. asiatica reduced host photosynthetic capacity. Infected sorghum plants were also more prone to water stress, but reduced rates of CO₂ fixation could not be accounted for in terms of lower stomatal conductance. Lower stomatal conductances were associated with an increase in water use efficiency (WUE) in uninfected sorghum; however, Striga-infected sorghum plants had lower WUE than those of uninfected plants. We suggest that Striga exerts a specific effect on processes affecting C acquisition in sorghum leaves. The water relations of S. hermonthica and S. asiatica are not characteristic of plants growing in semiarid environments and are more likely to reflect the nature of the parasitic life-style. Despite transfer of water and solutes from host to parasite, the reduction in C fixation observed in infected sorghum plants appears to be the major determinant of growth reductions observed in sorghum supporting Striga.     

Glutamine Synthetase Isoenzymes of Striga hermonthica and other Angiosperm Root Parasites

The occurrence of multiple forms of glutamine synthetase inStriga hermonthica and other angiosperm root parasites was investigated.The facultative chlorophyllous parasite Melampyrum arvense exhibitedtwo isoenzymes in leaf tissue, the cytosolic component (GS₁)comprised less than 30% of total glutamine synthetase. In contrastGS₁ was the major component (<70%) in photosynthetic tissueof Striga hermonthica and S. gesnerioides. Only a single isoenzyme(GS₁) was detectable in the achlorophyllous root parasites Orobancheand Lathraea and in non-photosynthetic tissue of S. gesnerioides.The kinetic and physical properties of GS₁ and GS₂ of theseangiosperm parasites were similar to those of the isoenzymesin other non-parasitic angiosperms. Key words: Glutamine synthetase, Angiosperms, Root parasites     

STUDIES ON THE GERMINATION OF SEEDS OF THE ROOT PARASITES,ALECTRA VOGELII AND STRIGA GESNERIOIDES. I. ANATOMICAL CHANGES IN THE EMBRYOS

Anatomical changes in the radicle and shoot meristems of embryos of germinating seeds of the obligate root parasites, Alectra vogelii and Striga gesnerioides were studied. When germination of seeds was stimulated by cowpea (Vigna unguiculata) root exudate, growth occurred mainly in the radicular pole of embryos and minimally in the plumular pole, resulting in seedlings with elongated radicles. Maximum radicle elongations of about 3 mm in A. vogelii and 2 mm in S. gesnerioides were recorded during a period of 8 and 11 days, respectively. Analysis of the radicular tip during the course of seed germination revealed that the activity of the meristematic tissue progressively decreased until it completely disappeared. When germinated seeds were cultured on nutrient agar media, the radicle meristem of A. vogelii continued to grow producing a normal root with a root cap. On the other hand, the radicles of cultured S. gesnerioides seeds elongated only slightly before meristematic activity ceased. During continued growth of seedlings of both species on agar media, lateral roots whose tips had typical angiosperm root topography, were initiated from the radicle.     

Initiation, Development and Structure of the Primary Haustorim in Striga gesnerioides (Scrophulariaceae)

A light-microscopic study is reported on the initiation, establishmentand structure of the primary haustorium of Striga gesnerioideson the host, cowpea (Vigna unguiculata). The radicular apexof the germinated parasite seed dissolves its way through thehost root cortex to the stele. Thus, it is converted into aprimary haustorium. Some of the haustorial front-line cellsin contact with the host endodermis penetrate into the steleand make contact with the xylem vessels. Differentiation ofthese haustorial cells into xylem vessels occurs and extendsbackwards through the median axial region of the haustorialtract in the host cortex to connect with the conductive xylemof the radicle outside the host root. Subsequently the parasite'splumule develops into a leafy shoot. On penetrating the steleof the host, the haustorium stimulates cell division in thehost pericycle whose triggered proliferation together with expansionof the parasite haustorial tissues result in the formation ofa large, tuberous primary haustorium. At various points of thehost-parasite interface, differentiation of xylem elements occurs,presumably maximizing nutrient transfer from host to parasite.In spite of this, many proliferated host cells at the interfaceremain apparently meristematic showing densely-stained cytoplasmand prominent nuclei.     

Germination stimulants produced by Vigna unguiculata Walp cv Saunders Upright

A germination stimulant, alectrol, for the seeds of the angiospermous root parasites Alectra vogelii Benth. and Striga gesnerioides (Willd.) Vatke has been isolated from root exudates of its genuine host plant Vigna unguiculata Walp cv Saunders Upright. Its spectroscopic data lead to a chemical structure closely related to (+)-strigol. The compound is more active than strigol in stimulating seeds of the respective parasites, Alectra vogelii and Striga gesnerioides.Abbreviations AV Alectra vogelii - OA Orobanche aegyptiaca - SA Striga asiatica - SG Striga gesnerioides - SH Striga hermonthica - TFA trifluoroacetic acid     

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.     

Exaggerated root respiration accounts for growth retardation in a starchless mutant of Arabidopsis thaliana

The knock‐out mutation of plastidial phosphoglucomutase (pgm) causes a starchless phenotype in Arabidopsis thaliana, and results in a severe growth reduction of plants cultivated under diurnal conditions. It has been speculated that high soluble sugar levels accumulating during the light phase in leaf mesophyll might cause a reduction of photosynthetic activity or that shortage of reduced carbon during the night is the reason for the slow biomass gain of pgm. Separate simultaneous measurements of leaf net photosynthesis and root respiration demonstrate that photosynthetic activity per unit fresh weight is not reduced in pgm, whereas root respiration is strongly elevated. Comparison with a mutant defective in the dominating vacuolar invertase (AtβFruct4) revealed that high sucrose concentration in the cytosol, but not in the vacuole, of leaf cells is responsible for elevated assimilate transport to the root. Increased sugar supply to the root, as observed in pgm mutants, forces substantial respiratory losses. Because root respiration accounts for 80% of total plant respiration under long‐day conditions, this gives rise to retarded biomass formation. In contrast, reduced vacuolar invertase activity leads to reduced net photosynthesis in the shoot and lowered root respiration, and affords an increased root/shoot ratio. The results demonstrate that roots have very limited capacity for carbon storage but exert rigid control of supply for their maintenance metabolism.     

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

The 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.     

Photosynthesis and stomatal characteristics of Striga hermonthica in relation to its parasitic habit

Photosynthesis and stomatal characteristics of the angiosperm parasite Striga her-monthica (Del.) Benth. have been compared with those of Antirrhinum majus L. cv. Suttons Yellow Rust Resistant, a related, non-parasitic species. The concentration of photosynthetic pigments in S. hermonthica leaves was less than 40% of those of A. majus leaves. Light saturated CO₂ assimilation rate of S. hermonthica was less than 40% that of A. majus on a chlorophyll basis and under 20% than of A. majus on a leaf area basis.
Stomata of Striga showed only partial closure in darkness, remained open in water stressed leaves and showed little response to exogenously applied abscisic acid. Stomatal conductance and transpiration were considerably higher in Striga compared with Antirrhinum . The high transpiration and low photosynthetic rates of S. hermonthica resulted in a low water use efficiency. The water relations of Striga leaves, while seemingly inappropriate for growth in drought prone environments, do appear to maximise nutrient and water acquisition from the host and as such may be an adaptation to the parasitic habit.     

Genome-wide association mapping and agronomic impact of cowpea root architecture

Key message

Genetic analysis of data produced by novel root phenotyping tools was used to establish relationships between cowpea root traits and performance indicators as well between root traits and Striga tolerance.

Abstract

Selection and breeding for better root phenotypes can improve acquisition of soil resources and hence crop production in marginal environments. We hypothesized that biologically relevant variation is measurable in cowpea root architecture. This study implemented manual phenotyping (shovelomics) and automated image phenotyping (DIRT) on a 189-entry diversity panel of cowpea to reveal biologically important variation and genome regions affecting root architecture phenes. Significant variation in root phenes was found and relatively high heritabilities were detected for root traits assessed manually (0.4 for nodulation and 0.8 for number of larger laterals) as well as repeatability traits phenotyped via DIRT (0.5 for a measure of root width and 0.3 for a measure of root tips). Genome-wide association study identified 11 significant quantitative trait loci (QTL) from manually scored root architecture traits and 21 QTL from root architecture traits phenotyped by DIRT image analysis. Subsequent comparisons of results from this root study with other field studies revealed QTL co-localizations between root traits and performance indicators including seed weight per plant, pod number, and Striga (Striga gesnerioides) tolerance. The data suggest selection for root phenotypes could be employed by breeding programs to improve production in multiple constraint environments.

     

Respiration and photosynthesis in cones of Norway spruce (Picea abies (L.) Karst.)

Summary Dark respiration and photosynthetic carbon dioxide refixation in purple and green Picea abies cones were investigated from budbreak to cone maturity. The rate of dark respiration per unit dry weight and CO₂ refixation capacity decreased during cone maturation. At the beginning of the growing season, photosynthetic CO₂ refixation could reduce the amount of CO₂ released by respiration in green and purple cones by 50% and 40%, respectively. The seasonal performance of the components of the cone carbon balance was calculated using information on the seasonal course of respiration, refixation capacity and the light response curves of cone photosynthesis, as well as the actual light and temperature regime in the field. The daily gain of CO₂ refixation reached 28%–34% of respiration in green and 22%–26% in purple cones during the first month of their growth, but decreased later in the season. Over the entire growth period refixation reduced carbon costs of cone production in both cone colour polymorphs by 16%–17%.     

Germination stimulant from root exudates of Vigna unguiculata

Root exudate of Vigna unguiculata was extracted from a soil system consisting of charcoal and vermiculite. Germination stimulating activity for Striga gesnerioides was found in extracts of the soil system, and an active compound was isolated. The chemical structure of the active ingredient was determined to be (+)-4-O-acetylorobanchol, based on analysis of the spectral data of 1-D and 2-D NMR together with nuclear Overhauser effect (NOE) experiments. Application of the active compound to the seeds of S. gesnerioides at a concentration of 0.35 × 10⁻⁹ mol/disk led to 69% germination. The germination observed with application of GR-24, a positive control, at 0.57 × 10⁻¹⁰ mol/disk was 80%.     

Arbuscular Mycorrhizal Symbiosis with Arundo donax Decreases Root Respiration and Increases Both Photosynthesis and Plant Biomass Accumulation

The effect of arbuscular mycorrhiza (AM) symbiosis on plant growth is associated with the balance between costs and benefits. A feedback regulation loop has been described in which the higher carbohydrate cost to plants for AM symbiosis is compensated by increases in their photosynthetic rates. Nevertheless, plant carbon balance depends both on photosynthetic carbon uptake and respiratory carbon consumption. The hypothesis behind this research was that the role of respiration in plant growth under AM symbiosis may be as important as that of photosynthesis. This hypothesis was tested in Arundo donax L. plantlets inoculated with Rhizophagus irregularis and Funneliformis mosseae. We tested the effects of AM inoculation on both photosynthetic capacity and in vivo leaf and root respiration. Additionally, analyses of the primary metabolism and ion content were performed in both leaves and roots. AM inoculation increased photosynthesis through increased CO₂ diffusion and electron transport in the chloroplast. Moreover, respiration decreased only in AM roots via the cytochrome oxidase pathway (COP) as measured by the oxygen isotope technique. This decline in the COP can be related to the reduced respiratory metabolism and substrates (sugars and tricarboxylic acid cycle intermediates) observed in roots.     

Solute fluxes from tobacco to the parasitic angiosperm Orobanche cernua and the influence of infection on host carbon and nitrogen relations

Orobanche species are holoparasites which are very efficient sinks for host-derived solutes. Here, we report the use of direct measurements of xylem sap solute concentrations and water fluxes, together with a modelling procedure to calculate element fluxes within an association between Orobanche cernua and its tobacco host. Infection of tobacco by the parasite markedly influenced carbon acquisition and partitioning; net fixation of carbon was 20% higher in infected tobacco compared with controls. Orobanche cernua caused a 84% increase in net carbon flux moving downward from the tobacco shoot and 73% of this carbon was intercepted by the parasite, almost entirely through the phloem (>99%). Further, the parasite also exerted a large impact on the nitrogen relations of the plant, notably nitrate uptake was stimulated and the amino acid content of xylem sap was lower. The parasite also relied heavily on host phloem for the supply of other resources, with only 5 to 15% of N, and 16% of K, 23% of Na, 63% of Mg and 13% of S being derived from the xylem. Thus, we provide quantitative information on the phloem dependency of the parasite and show that host carbon and nitrogen metabolism is stimulated as a consequence of infection.     

Carbon isotope ratios demonstrate carbon flux from c(4) host to c(3) parasite

Carbon isotope ratios of mature leaves from the C₃ angiosperm root hemiparasites Striga hermonthica (Del.) Benth (−26.7‰) and S. asiatica (L.) Kuntze (−25.6‰) were more negative than their C₄ host, sorghum (Sorghum bicolor [L.] Moench cv CSH1), (−13.5‰). However, in young photosynthetically incompetent plants of S. hermonthica this difference was reduced to less than 1‰. Differences between the carbon isotope ratios of two C₃-C₃ associations, S. gesnerioides (Willd.) Vatke—Vigna unguiculata (L.) Walp. and Oryza sativa L.—Rhamphicarpa fistulosa (Hochst.) Benth differed by less than 1‰. Theoretical carbon isotope ratios for mature leaves of S. hermonthica and S. asiatica, calculated from foliar gas exchange measurements, were −31.8 and −32.0‰, respectively. This difference between the measured and theoretical δ¹³C-values of 5 to 6‰ suggests that even in mature, photosynthetically active plants, there is substantial input of carbon from the C₄ host. We estimate this to be approximately 28% of the total carbon in S. hermonthica and 35% in S. asiatica. This level of carbon transfer contributes to the host's growth reductions observed in Striga-infected sorghum.