The input costs of pesticides to control biotic constraints are often prohibitive to the subsistence farmers of Africa and seed based solutions to biotic stresses are more appropriate. Plant breeding has been highly successful in dealing with many pest problems in Africa, especially diseases, but is limited to the genes available within the crop genome. Years of breeding and studying cultural practices have not always been successful in alleviating many problems that biotechnology may be able to solve. We pinpoint the major intractable regional problems as: (1) weeds: parasitic weeds (Striga and Orobanche spp.) throughout Africa; grass weeds of wheat (Bromus and Lolium) intractable to herbicides in North Africa; (2) insect and diseases: stem borers and post-harvest grain weevils in sub-Saharan Africa; Bemesia tabaci (white fly) as the vector of the tomato leaf curl virus complex on vegetable crops in North Africa; and (3) the mycotoxins: fumonisins and aflatoxins in stored grains. Abiotic stresses may exacerbate many of these problems, and biotechnological alleviations of abiotic stress could partially allay some predicaments. Some of these constraints are already under study using biotechnological procedures, but others may require longer-term research and development to alleviate the problems. Despite the huge impacts of post-harvest weevils and of mycotoxins in grains, these issues had not been given high priority in national biotechnological programs, possibly due to a lack of knowledge of their immensity. The need for public sector involvement is accentuated for cases where immediate profits are not perceived (e.g. lowering mycotoxin levels in farmer utilized grain, which does not increase yield) but where the public weal will gain, and will be invaluable, especially where the private sector supplies genes already isolated.

Rhizobacteria strains were characterized for ability to synthesize hydrogen cyanide and for effects on seedling root growth of various plants. Approximately 32% of bacteria from a collection of over 2000 isolates were cyanogenic, evolving HCN from trace concentrations to > 30 nmoles/mg cellular protein. Cyanogenesis was predominantly associated with pseudomonads and was enhanced when glycine was provided in the culture medium. Concentrations of HCN produced by rhizobacteria were similar to exogenous concentrations inhibiting seedling growth in bioassays, suggesting that cyanogenesis by rhizobacteria in the rhizosphere can adversely affect plant growth. Growth inhibition of lettuce and barnyardgrass by volatile metabolites of the cyanogenic rhizobacteria confirmed that HCN was the major inhibitory compound produced. Our results suggest that HCN produced in the rhizospheres of seedlings by selected rhizobacteria is a potential and environmentally compatible mechanism for biological control of weeds.

Aim: To develop an appropriate formulation of the deleterious rhizobacterium Pseudomonas trivialis X33d and to evaluate its effectiveness to reduce brome growth. Methods and Results: Two formulations of Ps. trivialis X33d, a semolinakaolin granular formulation (Pesta) and talc-kaolin powder, were prepared and their effectiveness in reducing brome growth was evaluated. Both brome suppression and cell viability of X33d were higher in Pesta granular formulation than in talc-kaolin powder one. The impact of storage temperature and the addition of adjuvants (sucrose and oil) to the granular formulation of X33d were assessed in order to improve the shelf life of the formulation. The longest viability was found in formulated product supplemented with adjuvants and stored at 4°C. The effect of Pesta granules supplemented with adjuvants and stored for 6 months at 4°C on brome and wheat growth under controlled and greenhouse conditions was evaluated. The X33d formulation in Pesta increased the growth of wheat and reduced brome growth. Conclusion: Our results indicate that Ps. trivialis X33d formulated in Pesta has potential as a bioherbicide to control brome. Significance and Impact of the Study: Because of the impracticality of applying bacterial cell suspension on a large scale, the use of Pesta granules of X33d against brome could help in achieving a sustainable agriculture application of a bioherbicide.

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Bioassays using cell cultures and callus tissues of leafy spurge were devised to evaluate the potential of rhizobacteria as biocontrol agents. Rhizobacteria isolated from roots of leafy spurge seedlings were screened in suspension-cultured leafy spurge cells. Cell viability was assessed using the Evan's blue bioassay 48 h after bacterial inoculation. Among the 30 isolates tested, LS102 and LS105 consistently caused intensive cell death determined by measuring the A630 of the inoculated cell cultures. Cell death was 2.5 to 3 times higher in cultures inoculated with LS105 and LS102, respectively, than in the control. Population levels of the two isolates within cell cultures and callus tissues of leafy spurge increased during the first 48 h. Leafy spurge callus tissues were inoculated with rhizobacteria either directly or by using the Host Pathogen Interaction System (HPIS). The latter exposes calli to bacteria without any physical contact. LS102 caused cellular leakage and eventually death of the callus tissue. Callus growth was reduced by about 30 to 70% when exposed to LS102 and LS105, respectively. Results suggest that these two isolates may affect leafy spurge at the cellular level by different mechanisms. A screening method based on cell cultures and callus tissues offers a good and rapid technique for detecting deleterious rhizobacteria with potential as biocontrol agents for leafy spurge.

The rhizobacterial strain X33d was previously shown to suppress the growth of the weed great brome (Bromus diandrus Roth.). The aim of this work was to identify X33d, characterize its physiological activities, assess its specificity on different non-target crops, and its impact on the growth and the root architecture of great brome and durum wheat (Triticum durum Desf.) grown alone and together. Based on 16S rDNA sequencing, X33d was identified as Pseudomonas trivialis. The specificity assay, performed on a mixture of soil/sand/peat, highlighted the suppressive activity of P. trivialis X33d against great brome and the promoting effect on most of the considered crops, especially durum wheat. Although the growth of durum wheat on quartz sand was unaffected, P. trivialis X33d suppressed the growth and affected the root architecture of great brome, especially when co-seeded with durum wheat. Great brome plants inoculated with X33d and co-seeded with durum wheat showed low root biomass, short root systems and low surface area, volume and number of tips. Moreover, P. trivialis X33d synthesized indole-acetic acid (IAA) that could be involved both in great brome growth suppression and durum wheat growth promotion. Our results indicate that P. trivialis X33d could be exploited as a potential biocontrol agent against great brome without affecting the durum wheat growth. These results are discussed in relation to the competitive capability of great brome towards durum wheat.

Resistance to acetolactate synthase (ALS) inhibiting herbicides has recently been reported in Glebionis coronaria from wheat fields in northern Tunisia, where the weed is widespread. However, potential resistance mechanisms conferring resistance in these populations are unknown. The aim of this research was to study target-site resistance (TSR) and non-target-site resistance (NTSR) mechanisms present in two putative resistant (R) populations. Dose–response experiments, ALS enzyme activity assays, ALS gene sequencing, absorption and translocation experiments with radiolabeled herbicides, and metabolism experiments were carried out for this purpose. Whole plant trials confirmed high resistance levels to tribenuron and cross-resistance to florasulam and imazamox. ALS enzyme activity further confirmed cross-resistance to these three herbicides and also to bispyribac, but not to flucarbazone. Sequence analysis revealed the presence of amino acid substitutions in positions 197, 376, and 574 of the target enzyme. Among the NTSR mechanisms investigated, absorption or translocation did not contribute to resistance, while evidences of the presence of enhanced metabolism were provided. A pretreatment with the cytochrome P450 monooxygenase (P450) inhibitor malathion partially synergized with imazamox in post-emergence but not with tribenuron in dose–response experiments. Additionally, an imazamox hydroxyl metabolite was detected in both R populations in metabolism experiments, which disappeared with the pretreatment with malathion. This study confirms the evolution of cross-resistance to ALS inhibiting herbicides in G. coronaria from Tunisia through TSR and NTSR mechanisms. The presence of enhanced metabolism involving P450 is threatening the chemical management of this weed in Tunisian wheat fields, since it might confer cross-resistance to other sites of action.