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Le projet majeur africain de la Grande Muraille Verte

Abdoulaye Dia
Robin Duponnois

Partie I. Espèces végétales de la Grande Muraille Verte

An idea for the selection of species for the Great Green Wall; from the views of plant nutrition and a possible contribution of plant molecular biology

Ken Matsuoka


The great green wall should be of the sustainable system with low input and maximum benefit for the local people. Based on such criteria, possible species of trees for the GGW should include legume trees as these species have root nodules with symbiotic bacteria, which can fix nitrogen from the air. Some of the legume species that are native in Sahel region are used not only for fuel or timber but also other applications. Seeds or other parts including secretory gums from these species are used for foods and industrial raw materials. Among them one of the ideal species on such criteria is Acacia senegal, a tree that extrude gum arabic. Compounds in gum arabic affect the quality. Biochemical process for the production of such compounds can be presumed from the knowledge of the biochemistry and molecular biology of other plant species. Using A. senegal as a model species the possible strategy for the contribution of molecular biology to select good trees (trees that can produce gum arabic with high protein content and low tannin content) just after seedling is discussed.

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1In a classical agroforest system, farmers have for many generations maintained a traditional land-use system. In the semi-arid and sub-humid zones of west Africa such system is known as the ‘agroforestry parklands’ system (Pullan, 1974). The system is characterized by the deliberate retention of several-to-divergent species of trees on cultivated or recently fallowed land. Trees are an integral part of the system, providing food, fuel, fodder, medicinal products, building materials and saleable commodities, as well as contributing to the maintenance of soil fertility, water conservation and environmental protection. In contrast, the windy parts of the flat lands faced to sea, farmers developed windbreaker forest to protect farmlands from strong winds as well as salts from seawater. In this system, the cropping lands and the forests are clearly separated. The windbreaker is consisted with one or limited numbers of species causing the vegetation of such forest fairly uniform. Even in this case, the forests provide fuel for farmers and protect erosion.

2In both cases the systems are sustainable when people keep living in the traditional household system. However, in the rapidly developing world people tend to gain well sophisticated commercial products for the improvement of living status. To fulfill such demands it is essential to get cash income form the use of lands. In contrast, minimum request for the forests of great green wall (GGW) is to protect the expansion of Sahara Desert in the Sahel region and to maintain the cultivated filed in order to keep supplying foods for people. Thus it is better to keep the cultivated lands for the supply of foods and to use the tree zone not only the border of the Sahara Desert but also the area where products for the cash income can be harvested.

3Based on such ideas and on knowledge of biotechnology and plant nutrition, the idea of selecting species and individuals plants for the constitution of GGW is discussed in this report.

Supply of nitrogen

4Plant growth requires carbon dioxide, water and macro and micronutrients from soils. The nutrients include minerals, phosphate, sulfur and nitrogen. Among them supply of nitrogen is critical for plant growth as this element is one of the major constituents in all living organisms. Because most plants cannot convert nitrogen gas in the air into bioactive form of nitrogen, such as ammonia, nitrite and nitrate, supply of active form of nitrogen is the limited factor to grow plants including trees. The exception is the legume plants. This family of plants tend to have nitrogen-fixing nodules that contain nitrogen fixing bacteria. In such nodules organic compounds, which are the products of photosynthesis, are supplied to bacteria from plants. Using such organic compounds as the energy source, bacteria fix nitrogen (i.e. convert nitrogen gas into ammonia or nitrate) and the fixed nitrogen is supplied to host plants. This nutritional exchange is the basis of the symbiosis between legumes and nitrogen-fixing bacteria.

5There are many species of bacteria in soils that can fix nitrogen under an appropriate condition. However, nitrogen fixing activity in such soil bacteria is not usually high. In contrast, symbiotic nitrogen fixation is quite efficient. The bacteria that can make nitrogen-fixing nodules in legumes are classified into limited numbers of genus, including Rhizobium and related genus. Some of such bacteria have narrow host specificity. For example, soybean cannot grow efficiently in Africa as bacteria that can make nodules in soybean are not rich in soils in Africa. As soybean is originated from East Asia, this bad adaptation is not surprising. Moreover, the interaction of symbiotic bacteria and soybean is very complex. Some isolate of Bradyrhizobium japonicum can only make functional nodules in limited variety of soybean. Therefore, local adaptation of Bradyrhizobium population and variety of soybean in crop field can be found in some region.

6The soybean case suggests that the legume species that will be used for GGW should be at least native to Africa and ideally the local species where the GGW is made. Once such legume trees are grown in GGW, the detached leaves and dead branches on the ground is digested by soil bacteria. This digestion allows an accumulation of functional nitrogen to support the growth of other species. Therefore, the leaves and dead branches that fell off from the legume trees will not be harvested completely.

Legume trees and its use

7In savanna area in West Africa, many species of Leguminosae trees are known to be valuable for people. Tamarind (Amarindus indica) is one of such trees. It is a native to tropical Africa but was introduced into India long ago, thus the name of the species is written as “indica”. The tree can grow up to 20m in height. Tamarind is used for both timber and foods. Timber consists of dark red heartwood yellowish sapwood. The fruit is a brown pod-like legume, which contains a soft acidic pulp and many hard-coated seeds. The hard green pulp of a young fruit is very sour and acidic and is often used as a component of savory dishes. The ripened fruit is less sour and somewhat sweeter. It is used in desserts and sweetened drinks, or as a snack.

8Another legume trees useful for many purposes is Parkia biglobosa, which also grow up 20m in height. Seeds of this tree are used as protein-rich foodstuff in West Africa. After fermentation of its coked seed a seasoning called Netetu is generated. The fermentation process alters the physical state of the cooked and dehulled beans. The condiment is also an excellent preservative against mould. The spore-forming bacteria classified as Bacillus were the major bacteria use for fermentation.

9Pterocarpus mildbraedii is also use for both the foodstuff and for timbers. This tree grows up to 35m in height. In Nigeria the leaves are used as vegetables. The timber is very hard and is used to make mortars. However, these trees are not so useful for the cash-income. A more useful tree legume native in West Africa is Acacia senegal, which is the source of gum arabic. The detail of this species, its use and possible future contribution of molecular biological techniques for the selection of good trees of this species is discussed from the next section.

Acacia senegal, a multipurpose tree

10Acacia senegal is a small deciduous acacia tree. It is native from semi-desert regions of Sub-Saharan Africa, as well as Oman, Pakistan, and northwestern India. It grows to a height of 5-12m, with a trunk up to 30cm in diameter. This specie is very drought resistant. It grows on sites with annual rainfall between 100-950 mm and 5-11 month dry periods. It tolerates high daily temperatures (mean maximum temperatures of up to 45°C or more), dry wind, and sandstorms. It prefers coarsetextured soils such as fossil dunes, but it will also grow on slightly loamy sands and skeletal soils such as Lithosols. Although generally soils are well-drained, there are exceptions: in the Kayers region, South-Kordofan, East-Sudan, A. senegal grows on heavy clay soils with approximately 800 mm annual precipitation. The best sites have pH of 5 to 8.

11The gum produced from this species is known as gum arabic. The seeds are edible for humans and are rich in protein. The foliage and pods are browsed by sheep, goats, camels, impala, and giraffe. Leaves contain 10%-13% digestible protein and 0.12%-0.15% phosphorus, while the pods contain 15% digestible protein and 0.12%-0.14% phosphorus. Acacia senegal wood is locally valued for fuel wood and charcoal although biomass yield per unit land area is not sufficient to plant A. senegal purely for fuel wood. Wood is used in local construction for poles and fence posts, the light-colored wood for tool handles and dark heartwood for weaver’s shuttles. Strong ropes are made from the bark fibers of the tree’s long surface roots. In Sudan, this species is grown in agroforestry systems in “gum gardens’ for gum as well as to restore soil fertility. After five years of growth from seedling, the tree can be used to harvest gum arabic for 7-15 years. When the rainy season ends the trunk of the acacia begins to ooze gum. Usually trunks are tapped to enhance the production. This gum is collected from December to June and is used to make commercial gum Arabic products. The primary export agent is a French company founded during the colonial period to process the gum, which maintains large processing facilities in Nigeria and Rouen, France. Total world gum arabic exports in 2008 are estimated at 60,000 tonnes, having recovered from crises caused by the destruction of trees by Desert locust sevrel years ago.

Gum arabic, its use, composition and biosynthesis of major components

12Gum arabic is mixture of saccharides and glycoproteins edible by humans. It is widely used in food industry and historically in other industries including paint production, printing and glues. Its solution is colorless, orderless and low viscosity even up to 20-40%. It is an acid-stable polymers consisted with polysaccharides and proteins. Its emulsification, adhesive and binding properties and good mouth feel characteristics directed this gum in many areas of food industry which include confectionary, beverages and emulsions, flavor encapsulation, bakery products and brewing. Increasing the economical status in these decades in China, India, Russia and other countries and introduction of the Americanized way of foods and living in these countries, the net demands of gum arabic in food industry had increased. Although the recent world-wide crisis of economy will slow down the increase in consumption of such processed foods, the regain of economy in the near future will causes to increase the consumption.

13The quality of gum arabic is classified with the content of purities. The gum tears picked from trees and dried before selection. Gums tears with less impurities showed light amber color and mainly transparent whereas low quality gum shows dark amber color. Such impurities mainly consisted with the presence of tannin, a class of polymerized phenolic compounds. The major components of the gum arabic are complex mixture of arabinogalactan, and arabinogalactosylated glycoproteins. In gum arabic, the content of proteins is one of the important criteria for the emulsification property.

14Arabinogalactan is a complex carbohydrate polymer mainly consisted with or D-galactose and L-arabinose. The minor sugars in arabinogalactan are rhamnose, galacturonic acid and fucose, and sometimes the galacturonic acid makes a complex with pectin polysaccharide. Biosynthesis of arabinogalactan is predicted from the analysis of the arabinogalactan proteins. The synthesis of arabinogalactan starts from the synthesis of small oligo peptide that function the platform of the oligosaccharide assembly. Such peptide is called arabinogalactan peptide. The precursor of the arabinogalactan peptide contains amino-terminal signal peptide and carboxyterminal peptide for the attachment of GPI-anchor; the former is removed cotranslationally during the synthesis and translocation of and the latter is converted to GPI-anchor structure which attach to the inner leaflet of the membrane of the endoplasmic reticulum. Proline residues in the GPI-anchored arabinogalactan peptide are converted to hydroxyproline ressidue during the transport from the endoplasmic reticulum to the Golgi apparatus. At the same time the hydroxyproline residues are glycosylated with galactose. Thereafter the galactose chain is elongated branched. Then the branched chains are arabinosylated and sometimes other sugars, such as rhamnose, fucose, N-acetylglucosamine and glucuronic acids are transferred to the partially assembled oligosaccharides. Finally, after transport of the glycosylated peptide to the plasma membrane, GPI-anchor is cleaved by the action of phospholipase. This process causes the release of arabinogalactan to the intracellular space.

15The protein-rich fraction from gum arabic is rich in hydroxoproline-rich glycoproteins known as gum arabic glycoprotein, which contains highly glycosylated hydorxyprolinerich region. Many of the hydroxyprolines in this region are glycosyated with arabinogalactan although some of the hydroxyprolines are arabinosylated with some extent. The biosynthesis of hydroxyproline-rich glycoproteins in plants so far analyzed resembles with the synthesis of arabinogalactan assembled on peptides. However, some of the precursors to glycoproteins do not have GPI-anchor.

16The biosynthetic pathways of arabinogalactan described above indicated that the biosyntheis require many gene products, not only common in protein secretion but also unique to the biosynthesis of arabinogalactan and arabinogalactan proteins. These include peptidyl prolyl hydroxylase, peptidyl hydroxyproline galactosyltransferase and many glycosyltransferases that are required for both the elongation of galactan chain in arabinogalactan and for the synthesis and assembly of the side chains consisted with galactose, arabinose, rhamnose, N-acetylglucosamine and glucuronic acid. Among these enzymes that are proposed to be involved in the biosynthesis, only peptidyl prolyl hydroxylases are characterized by our hands and with others at the molecular level (Yuasa et al., 2005: Tiainen et al., 2005). Most of the other enzymes are still yet characterized but we had recently found the activity of the core enzyme (Oka et al., 2008) and now we are purifying the enzyme. Although this scheme is proposed in the biosynthesis of arabinogaralctan, little molecular mechanism for the biosynthesis of gum arabic has been investigated.

17To date only partial amino acid sequence of the glycoproteinin arbic gum were reported (Goodrum et al., 2000) and characterization of arabinogalactan proteins in this gum is just started (Mahendran et al., 2008). However, no precise structure of the arabinogalactan oligosaccharides from gum arabic was solved. Therefore it is not clear how many genes are involved in the biosynthesis of gum arabic and how many protein backbone or platform are involved in the gum arabic biosynthesis.

Phenolic compounds biosynthesis and induction

18Tannins are the major pigment of gum arabic and other gums from trees that prevent the use of for food industry. The tannins originated from the bark of the tree and are the common compounds for the protection from the injury. As the gum production usually takes place by the topping of trunks, this process can induce the wound response of trees to produce protective substances including gum and tannin. The contaminated tannin causes problems for industrial use and for human health. Therefore, the FAO suggestion of gum arabic for food additive is free of tannin (FAO, 1997).

19The wound induction of the genes for the biosynthesis of phenolic compounds is well studied in many model plants. The primary products for the source of tannin are catechins, gallic acid and cyanidins, collectively called polyphenols. Such compounds are also known phenylpropanoids because of the biosynthetic pathway and precursor compounds. The key enzymes for the biosynthesis of phenylpropanoides are phenylalanine ammonia lyase, cinnamic acid dehydrogenase, chalcon synthase and so on. Many of these genes are induced by wounding and wound-related hormones such as jasmonate. Another enzyme that is related to the defense process that is induced by wounding is the anionic peroxidase which is involved in the formation of lignin, a polymerized polyphenols rich in hardwoods and wounded part of many woody plants. Therefore, the same sets of the genes in Acacia senegal will also be induced by topping at least with some extent.

Possible contribution of molecular biological technique

20Once a legume species for GGW is selected, it is better to identify individual young plants to be planted to the place where the wall is to be made. For example, if Acacia senegal is chosen, the following criteria should be considered. 1) High growth rate. 2) High yield of gum arabic. 3) High content of proteins in the gum. 4) Low content of tannin in the gum. If these traits can be monitored before the planting of young trees the benefit for the farmers should be high. In contrast, if such trait can be monitored after planted trees are grown to enough size, i. e. five years of growth in the case of plantations in Sudan, the benefit is not so high.

21One of the methods to produce many young trees that will be of highly valuable because these trees will have the traits listed above is to propagate the species using cutting of branches and rooting of these cut branches to obtain young trees. In this method, once a tree that has showed such phenotypes is identified, the clones of the same trees can be produced with very high number. Unfortunately however, such methods are not developed in many valuable trees in Africa, thus such methods should be developed for each species. Another method is the collect seeds from trees with such traits for propagation. However, this method can not be always applied because inheritance ways of such traits are unknown. The phenotype with low tannin in arabic gum can be considered to link multiple genes for the biosynthesis. Thus the offspring of the tree which produce gum with light brown color might not always produce high quality gums. Therefore, using seedling as the source of young trees it is required to establish such phenotypes before growing up the trees. One of the methods for this is to analyze genes that are involved in such traits. The first step for the identification of such genes is to identify mRNAs that is linked to the phenotype of interest. Until recently the screening of such mRNA species are time consuming and was not easy to carry out. However, recent development of high throughput DNA sequencers allows one to identify such mRNA species with only a short period of time.

22The first step is to isolate mRNA from the trees under the condition for the production. From here the proposed case of Acacia senegal is discussed as a model. In order to assess genes that is linked to the production of high quality arabic gum, mRNAs from the barks of healthy and topped trees be collected. One of the trees used to recover mRNAs would produce light color of gum and the other should produce well pigmented gum. After preparation of cDNA from the mRNAs, nucleotide sequenced of the cDNAs will be determined. The DNA sequencer that can be use for this purpose is the 454 sequencer with Titanium kit. The combination of this machine with the kit allows one to analyze the sequence of about a million of clones within a day or two. As the average length of reading is roughly 400 bases, encoding protein from the mRNA can be easily predicted. Since most of the tissues express no more than twenty thousands of genes, analysis of millions of genes are enough to quantify the level of expression of each gene.

23The comparison of the gene expression frequency will allow identifying genes that are linked to the trait. Then whole gene sequence will be determined from two individuals one of which will yield high quality gum and the other produce low quality one. After the comparison of the sequence of the genes polymerase chain reaction (PCR) primers for the amplification of good and bad genes will be generated. Thereafter, analyzing the genotype of young offspring from the good tree will be analyzed by PCR. The requirement of the samples for PCR is usually one leaves or less. Thus without disturbance of the young tress genotypes of each tree will be determined.

24This is a proposed method for the identification of individual plants with higher production of valuable stuff. This approach will not only be applied to legume trees but also other species of trees with other important traits, such as the content of medicinal compounds. The minimum requirement to take this method is to have a method to identify the individuals with the phenotype and one should have at least two individuals showing good and bad phenotypes. The biggest problem for this method is the high cost of the initial investment for the characterization of genes (can be over ten thousand US dollars) as well as the characterization of genotype by PCR is a little bit costly (about a US dollar or less). Therefore proposed benefit to take this method should be higher than a dollar and supports of developed countries for this method should be essential.


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Laboratory of Plant Nutrition, Department of Plant Resources, Faculty of Agriculture,
Kyushu University, Fukuoka 812-8581, Japan.

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