A.J. van der Westhuizen

Antioxidative enzymes and the Russian wheat aphid (Diuraphis noxia) resistance response in wheat (Triticum aestivum).

A crucial function of antioxidative enzymes is to remove excess reactive oxygen species (ROS), which can be toxic to plant cells. The effect of Russian wheat aphid (RWA), Diuraphis noxia (Mordvilko), infestation on the activities of antioxidative enzymes was investigated in the resistant (cv. Tugela DN) and the near-isogenic susceptible (cv. Tugela) wheat (Triticum aestivum L.). RWA infestation significantly induced the activity of superoxide dismutase, glutathione reductase and ascorbate peroxidase to higher levels in the resistant than in susceptible plants. These findings suggest the involvement of antioxidative enzymes in the RWA-wheat resistance response, which was accompanied by an early oxidative burst. The results are consistent with the role of ROS in the resistance response and the control of their levels to minimise toxic effects.

Test of semiochemicals and a resistant wheat variety for Russian wheat aphid management in South Africa

Abstract:  Aphid behaviour‐modifying semiochemicals were tested against Russian wheat aphid Diuraphis noxia in South African wheat. Volatile substances from plant essential oils, methyl salicylate, 1,8‐cineole and menthol were tested in the laboratory and field in combination with the D. noxia‐susceptible wheat variety Betta and the resistant variety Elands. All three substances were repellent to D. noxia in olfactometric tests. Diuraphis noxia settled less on Elands plants that had been exposed to the volatiles, whereas the effect of the volatiles on D. noxia settling on Betta was less obvious. A slow‐release pellet formulation was used to apply semiochemicals in wheat in replicated plot field trials in 2004 and 2005. In 2004, semiochemicals reduced aphid populations in Elands, but led to increased aphid populations in Betta. Further, the impact of the chemicals on aphid numbers and grain quality (thousand grain weight) varied according to plant variety, indicating an interaction between semiochemicals and plant resistance/variety.

Effect of culture age and pH of the culture medium on the composition of the toxin of the cyanobacterium Microcystis aeruginosa (UV-006)

Axenic cells of Microcystis aeruginosa, originally collected from the Hartbeespoort Dam, South Africa, were cultured under controlled laboratory conditions. It was shown that culture age and pH (CO2 concentration) did influence the relative peptide composition of the toxin. The amino acid composition of the major constituent toxic peptides, as determined by GLC, remained unchanged. Changes in toxicity thus are related to concentration and probably are not due to structural changes of the toxic peptides.

Root formation and attempts to establish morphogenesis in callus tissues of beans (Phaseolus vulgaris L.)

Callus growth and subsequent root formation were obtained from radicle starting material using kinetin or [2-isopentenyl] adenine and 2,4-D. Substituting NAA for 2,4-D resulted in poor callus growth and no root formation. Gibberellic acid, c-AMP and c-GMP as supplements tended to reduce callus growth. When KNO3 and NH4NO3 of the Murashige and Skoog medium were replaced by ammonium citrate as nitrogen source no callus growth was observed.

The Effect of Various Lipids on Flowering of Pharbitis nil in in vitro Culture

The effect of applied arachidonic acid, prostaglandin (PGE1) and various sterols and combinations of arachidonic acid + sterols, on flowering of Pharbitis nil were ascertained by using a tissue culture technique. It was found that arachidonic acid, PGE1 stigmasterol, testosterone, cholesterol, stigmasterol + arachidonic acid, β-sitosterol + arachidonic acid and cholesterol + arachidonic acid all caused earlier flowering. Four inhibitors of prostaglandin biosynthesis (gentisic acid, acetylsalicylic acid, salicylic acid and oleic acid), inhibited flowering completely. The results confirm that the compounds tested could possibly play a role in the flowering of P. nil.

The metabolism of inhibitor of flowering and prostaglandin biosynthesis, acetylsalicylic acid, in Pharbitis nil cotyledons

Acetylsalicylic acid, which applied to cotyledons of the short day plant Pharbitis nil prior to an inductive 16-h dark period inhibits flowering by 90 %, is converted to salicylic acid and to a lesser extent to gentisic acid in the cotyledons during this 16-h dark period. Our results confirmed that salicylic acid and gentisic acid are responsible for the inhibition of flowering. They also inhibit prostaglandin biosynthesis.

Plant defence mechanisms: relevance to agriculture in Africa

A vital priority for Africa today is to feed its burgeoning population while sustaining agricultural production and safeguarding the natural environment. It is anticipated that agricultural biotechnology will suit the African farming community excellently in terms of acceptability and affordability, and therefore make a major contribution towards the improvement of food production and alleviation of poverty. A major hurdle towards food production in Africa is the huge crop losses caused by pests and diseases. In addition to the fact that control by means of pesticides is often harmful to the environment, a great proportion of the African farmers, especially subsistence farmers, do not have access to modern pesticides. Inherent resistance provided via the seed, thus harnessing the plant’s own defence mechanisms, would be one of the simplest solutions to pest and disease control in Africa. It boils down to the fact that plant biologists have to incorporate the technology into the seed. Otherwise, other methods of harnessing the plant’s defence mechanisms have to be devised. Key events of the plant’s defence mechanisms are discussed in relation to resistance manipulation. They include eliciting, with special reference to resistance (R) genes, and signalling events, the hypersensitive response (HR) and systemic acquired resistance (SAR). In this regard, recent insights and potential applications are accentuated. It is concluded that plant biologists have a major task to continue studies on plant defence mechanisms in an effort to provide fresh insights for the design of new and effective disease control strategies. The importance of such research in Africa, especially in the public sector, is stressed. Also, it is emphasised that the technological outcomes of the research should be protected in patents and made available to African farmers at affordable prices. Another challenge is the application of modern biotechnology in Africa; however, when packaged in the seed, the application of this cutting-edge concept would become natural, even to the most traditional farmer.

The effect of applied arachidonic acid on the formation of prostaglandins in plantlets from excised apices of the short-day plant, Pharbitis nil

The effect of arachidonic acid (AA) application to excised apices of Pharbitis nil and the subsequent production of prostaglandins (PGs) were examined. The detection of the PGs was done by means of a radioimmunoassay (RIA) and tritium labelling techniques. Both PGE 2 and PGF 2α were detected and quantified by RIA after AA application. This suggests the synthesis of PGs from AA. These results were confirmed using tritium labelled AA. This experiment was done in connection with the fact that Pharbitis nil is a short-day plant. Arachidonic acid, a possible precursor of PGs, possibly promotes flowering of this plant. Arachidonic acid might be converted by cyclo-oxygenases to PGs which are involved in flowering.

Establisbment of embryogenic suspension cultures of Pinus radiata Don.

Synopsis The development of embryonal suspensor mass (ESM) from immature embryos of Pinus radiata on a solidified growth medium containing 0,5 mgl−1 benzyladenine, 3,0 mgl−1 2,4-dichlorophenoxyacetic acid, 500 mgl−1 casein hydrolysate and 250 mgl-1 L-glutamine was used as inoculum to establish cell suspension cultures. Regular transplants of the ESM established on the solidified medium onto a hormone-free medium ensured sustainable growth and hence a source of ESM. Conditions for optimal growth in liquid media were investigated. The cell suspension turbidity was measured and used as a growth parameter. Optimal growth and normal embryo development were obtained in a liquid medium containing maltose as the carbohydrate source at a concentration of 3% (m/v) and supplemented with 0,5 mgl−1 2,4- dichlorophenoxyacetic acid. The results obtained from this study present an opportunity for large-scale propagation of Pinus radiata.