Do NERICA rice cultivars express resistance to Striga hermonthica (Del.) Benth. and Striga asiatica (L.) Kuntze under field conditions?

The parasitic weeds Striga asiatica and Striga hermonthica cause high yield losses in rain-fed upland rice in Africa. Two resistance classes (pre- and post-attachment) and several resistant genotypes have been identified among NERICA (New Rice for Africa) cultivars under laboratory conditions (in vitro) previously. However, little is known about expression of this resistance under field conditions. Here we investigated (1) whether resistance exhibited under controlled conditions would express under representative Striga-infested field conditions, and (2) whether NERICA cultivars would achieve relatively good grain yields under Striga-infested conditions. Twenty-five rice cultivars, including all 18 upland NERICA cultivars, were screened in S. asiatica-infested (in Tanzania) and S. hermonthica-infested (in Kenya) fields during two seasons. Additionally, a selection of cultivars was tested in vitro, in mini-rhizotron systems. For the first time, resistance observed under controlled conditions was confirmed in the field for NERICA-2, -5, -10 and -17 (against S. asiatica) and NERICA-1 to -5, -10, -12, -13 and -17 (against S. hermonthica). Despite high Striga-infestation levels, yields of around 1.8 t ha−1 were obtained with NERICA-1, -9 and -10 (in the S. asiatica-infested field) and around 1.4 t ha−1 with NERICA-3, -4, -8, -12 and -13 (in the S. hermonthica-infested field). In addition, potential levels of tolerance were identified in vitro, in NERICA-1, -17 and -9 (S. asiatica) and in NERICA-1, -17 and -10 (S. hermonthica). These findings are highly relevant to rice agronomists and breeders and molecular geneticists working on Striga resistance. In addition, cultivars combining broad-spectrum resistance with good grain yields in Striga-infested fields can be recommended to rice farmers in Striga-prone areas

A physiological, genetic and transcriptomic study of rice-Striga interactions

In this thesis, physiological aspects of several hosts and Striga interaction were studied.  Striga­-resistant QTLs were mapped and host gene expression during Striga attachment was analysed in rice. Mapping Striga-resistance QTLs in Bala x Azucena F6 recombinant inbred lines identified a prominent QTL region at an average position 123.2 cM on the Rice Genome Research Program (RGP) map between marker RM246 and RM212 on chromosome 1.  This region contains QTLs for all traits tested with LOD scores ranging from 3.4-10.2 explaining 7.4-36.8% of the phenotypic variation. Database searching between position 121-124.8 cM revealed 47 interesting genes potentially responsible for the effect of Striga on rice. A microarray experiment was conducted to monitor gene expression in Azucena and Bala root upon Striga attachment.  Nine and 25 probe sets were differentially regulated 2 fold or more in Azucena and Bala during Striga attachment, respectively.  Many of these probe sets represent genes involved in stress response, signal transduction, amino acid metabolism and cell death.  There were 2,194 probe sets differentially regulated 2 fold or more between Azucena and Bala seedlings in the control treatment.  Among these probe sets, 11 represent genes in the QTL region for Striga resistance.  These genes are involved in endochitinase, toxin catabolism, signal transduction, starch biosynthesis and response to abscisic acid.  Endochitinase is proposed to be a notable candidate gene for Striga resistance as the gene is induced in both Azucena and Bala upon Striga attachment and is more highly expressed in Azucena compared with Bala in the control treatment.  Results generated from this project can be valuable for the study on host-Striga relationship and for the future breeding experiments to incorporate Striga-resistance genes into elite crop cultivars.EThOS - Electronic Theses Online ServiceGBUnited Kingdo

“Soaked in rainwater” effect of Ageratina adenophora on seedling growth and development of native tree species in Nepal

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Airborne and belowground phytotoxicity of invasive Ageratina adenophora on native species in Nepal

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Impacts of invasive Chromolaena odorata on species richness, composition and seedling recruitment of Shorea robusta in a tropical Sal forest, Nepal

Chromolaena odorata is an invasive species known to have its adverse impacts on native diversity. We studied its impacts on plant species richness, composition and Shorea robusta seedlings in a tropical Sal forest of Nepal. We conducted field experiment along transects established in invaded and uninvaded understory vegetation. Our results show differences in native species richness and S. robusta seedling density between invaded and uninvaded plots. The invaded plots are associated with fewer species than uninvaded plots. Plot type (invaded and uninvaded), C. odorata density and cover show an effect on vegetation composition. Moreover, some of the native species are found replaced from C. odorata invaded sites. Overall, the Sal forest protects native diversity and Sal seedling recruitment besides having its high economic value and beneficial impacts on people’s livelihood. Therefore, control and proper management of C. odorata is needed for conserving native vegetation and preventing future problems associated with invasion

Host-plant defence against Striga spp.: reconsidering the role of tolerance

Parasitic weeds of the genus Striga cause high yield losses in cereal crops across Africa. Host-plant defence against Striga spp. can be an effective control strategy. It ideally consists of resistance, to reduce infection, complemented with tolerance, to mitigate the effects of infection. As resistance against Striga spp. can both minimise yield losses and reduce future infestation levels in infested fields, current breeding efforts are mainly directed towards this trait. Because it is nearly impossible to screen for tolerance on highly resistant genetic lines, tolerance is often neglected. Here, we argue reconsidering the role of tolerance, as recent findings regarding the physiological expression of tolerance offer a promising track for identifying the genetic background of tolerance. Identification of quantitative trait loci for tolerance would facilitate the inclusion of this trait in adapted cultivars with high levels of resistance, where its main role would be to function as a safety net in case the genetically highly variable parasite populations overcome host-plant resistance. Because Striga spp. are mainly prevalent in subsistence farming systems, we consider this an important addition and it is for this reason that we make a plea for a more prominent role of tolerance in present-day integrated management of this weed

The genetics underlying natural variation of plant-plant interactions, a beloved but forgotten member of the family of biotic interactions

BGPI : équipe 4 Interactions Céréales Agents Pathogènes (ICAP)International audienceDespite the importance of plant-plant interactions on crop yield and plant community dynamics, our understanding of the genetic and molecular bases underlying natural variation of plant-plant interactions is largely limited in comparison with other types of biotic interactions. By listing 63 quantitative trait loci (QTL) mapping and global gene expression studies based on plants directly challenged by other plants, we explored whether the genetic architecture and the function of the candidate genes underlying natural plant-plant interactions depend on the type of interactions between two plants (competition versus commensalism versus reciprocal helping versus asymmetry). The 16 transcriptomic studies are unevenly distributed between competitive interactions (n = 12) and asymmetric interactions (n = 4, all focusing on response to parasitic plants). By contrast, 17 and 30 QTL studies were identified for competitive interactions and asymmetric interactions (either weed suppressive ability or response to parasitic plants), respectively. Surprisingly, no studies have been carried out on the identification of genetic and molecular bases underlying natural variation in positive interactions. The candidate genes underlying natural plant-plant interactions can be classified into seven categories of plant function that have been identified in artificial environments simulating plant-plant interactions either frequently (photosynthesis, hormones), only recently (cell wall modification and degradation, defense pathways against pathogens) or rarely (ABC transporters, histone modification and meristem identity/life history traits). Finally, we introduce several avenues that need to be explored in the future to obtain a thorough understanding of the genetic and molecular bases underlying plant-plant interactions within the context of realistic community complexity