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  AGRONOMY  -  Maize adapted to the Tropics

As the world’s third most cultivated cereal, maize is a vital ingredient in the Latin American diet. Yet it is highly sensitive to the acid soils that are found so frequently in tropical regions. German, Brazilian, Spanish and Mexican molecular biologists and physiologists worked for three years with geneticists and agronomists from France, Brazil, Cameroon and Colombia to improve methods for selecting maize varieties that are adapted to acid soils.

We knew why maize was sensitive to acid soils, but not the mechanisms by which some varieties were resistant to the problem. Research carried out by the Maize for sustainable cropping systems on tropical acid soils project, supported by the Union, is shedding new and promising light on the subject.
We knew why maize was sensitive to acid soils, but not the mechanisms by which some varieties were resistant to the problem. Research carried out by the Maize for sustainable cropping systems on tropical acid soils project, supported by the Union, is shedding new and promising light on the subject.
Although it originates in Central America, where it is thought to have been domesticated between 8 000 and 10 000 thousand years ago, maize is not a plant that is always easy to grow in tropical regions where acid soils represent 43% of the surface area, and as much as 64% in Latin America. This is why the past three decades have witnessed a major effort to select acid-resistant varieties. Within this ‘germplasm’ – the term used by plant improvement specialists – selectors are able to choose the most efficient varieties for crossing to produce new hybrids. This purely empirical task is nevertheless a long and very often ‘hit and miss’ affair. Hence the need to develop a better understanding of the mechanisms by which maize is able to resist soil acidity so as subsequently to localise the genes involved and speed up the development of new cultivars. This task, which combines agronomy, physiology and genetics, was at the heart of the Maize for sustainable cropping systems on tropical acid soils project that ran from 2000 to 2003 and received financing of €892 740 under the EU’s INCO programme.

The root of the problem 
The project’s subtitle – ‘from molecular biology to field cultivation’ – sums up the approach, that is, initially to characterise the mechanisms and genes involved in the acidity resistance and then to use this knowledge to develop new varieties for testing under real growing conditions. "Maize is sensitive to soil acidity as a result of two indirect effects: the solubilisation of aluminium ions, which are highly toxic for the plant, and the reduced availability of phosphate ions that are vital to growth,” explains Walter Horst of Hanover University’s Institute of Plant Nutrition, the project coordinator. “But we knew little about the mechanisms whereby certain maize varieties are able to resist this acidity.” 

As a result of the consortium’s research – carried out in particular in Barcelona (Autonomous University and the Instituto de Biologia Molecular), in Irapuato, Mexico (Centro de Investigación y Estudios Avanzados), in Brazil (University of Campinas) and in Hanover – we now have a clearer picture. It is all down to the root tip or apex. The root tips of maize varieties that are resistant to acidity have a cell membrane with a very specific composition that is enriched with saturated fatty acids, and a cell wall which is poor in pectin, thereby reducing the propensity to fix aluminium and the resulting inhibition of cell growth. In addition, the ability to excrete citrate makes it possible to detoxify the aluminium and, at the same time, improve phosphate uptake. A number of genes, expressed under the control of the same promoter (a region of a DNA molecule that, like a switch, controls a gene’s protein expression) are expressed specifically in the root tip in the event of a lack of phosphate or an excessive aluminium concentration. “It is very important to characterise this promoter because it makes it possible to conceive transgenic genes that express a gene introduced purely into this tissue – thus in the root tip, and not in the complete plant – thereby avoiding energy wastage and an expression in the grain that is consumed,” explains Marcelo Menossi of the University of Campinas (Brazil).

Hybrid performances
"Genetically modified organisms are no more than research tools in this case,” stresses Walter Horst. “Our programme was not about developing them for agriculture.” The real objective of this research was to use genes as markers (five of them have been validated) so as understand the adaptive mechanisms that must be sought in the genetic resources, and thereby guide the selection of new maize cultivars. Brazil’s EMBRAPA-MS is currently pursuing this research, in the framework of germplasm exchange between the project partners. Another project aim, in which the Colombia-based CIMMYT was also involved, was to propose a strategy for drawing maximum benefit from the genetic diversity of maize in adapting it to these very constraining environments. Among other things, the sharing of genetic resources originating in the Caribbean (Institut national de la recherche agronomique, in Guadeloupe, France), sub-Saharan Africa (Institute for agricultural research on development IRAD –Yaounde, Cameroon) and Latin America (EMBRAPA-MS and CIMMYT) made it possible to work on a much larger scale than had been possible previously. The selection of hybrids obtained by crossing varieties showing tolerance obtained from different gene pools proved a very promising means of improving crop yields and stability and thus farmers’ incomes. In this respect, the research showed the need to take into account both resistance to aluminium and the ability to optimise phosphate uptake in achieving a lasting improvement in crop yields. Finally, the group research made it possible to define an easy-to-apply aluminium resistance test: the roots of resistant varieties form much less callose, characteristic of cell stress, when they are immersed in a concentrated aluminium solution than the non-resistant cultivars. This test will also make it possible to save time when drawing up plans for the genetic improvement of maize.  

Two experimental sites
When these new cultivars become available, how should they be grown? And, in the meantime, what are the best agricultural practices for growing existing acid-resistant cultivars? These are the questions asked by the Cameroon (IRAD) and Colombian (Corporación Colomiana de Investigación Agropecuaria) partners. International co-operation on such a project brings clear benefits, making it possible to compare the effectiveness of agricultural practices on acid soils that are otherwise very different. Two sites were selected: the grassy plains of Villavicienco, in Colombia – former pastures now used for intensive arable farming – and the mid-altitude former forest zones of Yaounde in Cameroon. Research in both cases showed that spreading lime, which reduces the soil acidity chemically, made it possible to improve yield, as did the spreading of chicken manure. Green manure or rotating maize with other leguminous crops that enrich the soil did not yield useful results.  

The next stage is to convert this research data into simple recommendations on use to farmers. Work on modelling the rhizosphere, the all-important interface between the soil and the root, is being carried out in Montpellier (France) by the Institut national de recherche agronomique (INRA) and the Centre de coopération internationale en recherche agronomique pour le développement (CIRAD). This will make it possible to extrapolate these recommendations to any tropical soil for which the key parameters are known, such as aluminium and phosphate content and, of course, acidity. This model will also incorporate climatic parameters, in particular rainfall. In fact, maize is a very thirsty crop and one of the effects of acidity is to reduce the size of the root system, that is already limited, thus rendering it more sensitive to drought. At a time when some climatic models suggest that climate warming threatens to reduce rainfall in tropical regions, and the present increase in the atmosphere’s CO2 content is causing a mechanical increase in soil acidity, it is important to anticipate the consequences for maize crops.

Finally, this project offered many opportunities for young researchers from Latin America as well as from Europe, with no fewer than 22 PhD and 30 Masters students participating in the research.