Role of Gibberellins in Dormancy and Germination

A hormone balance theory has been invoked in which ABA and GA act antagonistically to control both dormancy breakage and germination (Karssen and Lacka, 1986; Karssen, 1995 and references therein). Generally GAs are viewed as important for the promotion and maintenance of germination, while ABA controls seed developmental events including the inception of dormancy (Bewley, 1997). However, GA also appears to act as an antagonistic to ABA function during seed development (White et al., 2000). 

Diagnostic Tools for Plant Disease in Africa

Diagnostic tools, particularly for viruses in crops such as yams, cassava and plantain/banana, and for pathogen strain-fingerprinting were developed by the IITA (Ibadan, Nigeria) researchers through investments of unrestricted funds or small grants from a pool of development investors, e.g. the Bundesministerium fur Wirtschaftliche und Entwicklung Zusammenarbeit (BMZ, Germany), Danish International Development Agency (Danida), Gatsby Charitable Foundation, Rockefeller Foundation and USAID, among others. These tools include both protein-based diagnostics (including polyclonal and monoclonal antibody technology) as well as nucleic acid based diagnostics, either alone using polymerase chain reaction (PCR) tests or combined with protein-based tests in combined immunocapture PCRs (IC-RT-PCR). They are routinely used for indexing plant material (germplasm exchanges and production of cleansed planting material), monitoring distribution of biocontrol agents, developing control strategies for disease epidemics, and detecting pathogens and other pests studied by the institute. The links with laboratories in West Africa on the application of diagnostic methods are particularly effective (Ortiz, 2002b).

In Tunisia, researchers of the Faculty of Science Laboratory of Genetics and Molecular Biology, the National Institute for Applied Science (National Institute for Applied Sciences and Technology; INSAT) Laboratory of Biological Engineering and the National Institute for Agricultural Research (National Agricultural Research Institute of Tunisia; INRAT) Laboratory of Virology are working on grapevine viral diseases. One of these diseases, the rugose wood complex, is widely distributed and causes significant reduction in yield and quality of the crop. Phloem-limited filamentous virus particles are closely associated with these diseases. Two trichoviruses, grapevine virus A (GVA), and grapevine B (GVB) are thought to be involved in the kober stem grooving and corky bark diseases of the rugose wood. Instead of relying on the ELISA method, the Tunisian researchers could successfully detect GVA and GVB from infected grapevine tissue by standard reverse transcription-PCR (RT-PCR) or RT-PCR coupled with immunocapture. The efficiency of the latter method resides on a few rapid steps used to obtain the purified viral RNA preventing its degradation before the reverse transcription reaction. Immunocapture can, therefore, be commonly used in the molecular detection of the two trichoviruses in Tunisian vineyards.

Grapevine infectious degeneration disease affects both productivity and longevity of the plant. One causal agent is a nepovirus with bipartiteRNA-genome: the GFLV (grapevine fanleaf virus). All virus serotypes (fanleaf, yellow mosaic virus or vein banding) are transmitted by the nematode vector Xiphinema index. So far, the only control strategy against this disease has been to select and produce virus-free stocks and avoid the use of contaminated soil to eliminate virus reservoirs and to decrease nematode population. The Tunisian researchers were able to identify the virus in its nematode vector underground, in infected grapevines in the northern Tunisian vineyards, using molecular biology techniques, RT-PCR and IC-RT-PCR. The IC-RT-PCR is more efficient in detecting the GFLV in its nematode vector than other serological and molecular biology techniques.

Grapevine leaf roll is one of the widespread and economically important viral diseases of grapevine in the world. Seven serologically distinct types of grapevine leaf roll associated closteroviruses have been described, but the GLRaV3 is the most important and abundant closterovirus in Tunisian grapevine cultures. It is transmitted by mealy bug species, Pseudococcus and Planococcus. The Tunisian researchers described the implication of Pseudococcus citri in the transmission of the virus in Tunisian vineyards. GLRaV3 in mealy bugs was detected by the IC-RT-PCR technique most efficiently, and the latter is to be used on a large scale to detect the virus in grapevine cultures (Marzouki and Marrakchi, 1998).

Rice Research

From the 1950s to 1970s, in an effort to combat world hunger, plant breeders at the International Rice Research Institute (IRRI) in the Philippines developed new rice varieties that were, when fertilized, higher yielding than traditional varieties. The new varieties were shorter and less likely to fall over, which made them easier to harvest mechanically. They also ripened sooner, reducing the risk of poor weather affecting yield, and enabling farmers to harvest and replant several times during the growing season. While successful in many areas, the new varieties required more money for fertilizer and chemical pesticides, and in some cases, machines for sowing and harvesting—tools often too costly for peasant farmers. In some areas a single new rice variety replaced diverse, centuries-old varieties adapted to thrive in a particular climate and soil type and with some resistance to local insects and diseases. The new variety was not able to thrive in these areas, and the crop yields were not always greater.

Rice breeders at IRRI and other research facilities are now trying to increase yields through genetic engineering. They hope to create rice varieties that are genetically designed to require less fertilizer, resist insects and diseases, tolerate poor soil, require less irrigation, and photosynthesize more efficiently.

Scientific classification: Rice is an annual grass in the grass family, Poaceae (formerly Gramineae). Asian rice is classified as Oryza sativa and African rice as Oryza glaberrima.