In Design And Function During Evolution

According to Polhill et al. [38], there is an evolutionary trend in the family Leguminosae towards accumulation of reserves in the cotyledons. In general, the seeds from species belonging to the subfamily Caesalpinioideae, which is considered to be the most primitive, tend to accumulate large amounts of galactomannan in their endosperm cell walls. Within this subfamily, it is also possible to observe that some tribes (Amherstieae and Detariae) tend not to have endosperms (normally consumed during maturation), and to accumulate large deposits of xyloglucan in their cotyledon cell walls (Copaifera, Hymenaea) (see Figure 1). Although germination in these two genera is epigeal, there is little expansion of the cotyledons. The subfamily Caesalpinioideae is thought to have originated in the warm and moist tropical regions, but after the Cretaceous there was vast drying and elevation of the lands so that cooler and drier regions and even deserts evolved, restricting the members of this subfamily to the tropics [135], The other two subfamilies of the Leguminosae, the Mimosoideae and Faboideae are thought to have developed from the Caesalpinioideae. Species in the Mimosoideae have remained strictly tropical but the more ubiquitous

Faboideae (or Papilionoideae) contains species well adapted to tropical regions and species equally well adapted to temperate regions and even the edges of the desert [136].

The main diversity in growth form and systematic composition of Faboideae occur in the plateau of Brazil, the Mexican region, eastern Africa, Madagascar and Sino-Himalayan region. The Mediterranean, Cape and Australia have some species which appear to have radiated from a few basic stocks [136], In this subfamily it is possible to find species, or entire tribes with high amounts of galactomannan stored in the cell walls of very thick endosperms (several species in Crotalarieae and Robinieae for example). Nevertheless, closely related species store starch in granules which are located in the cytoplasm of the cotyledonary cells (e.g. Vicia faba, Pisum sativum and Glycine max).

Thus, the storage function in legume seeds seems to have been transferred during evolution from the endosperm (galactomannan) to the cotyledons (xyloglucan and arabinogalactan), but maintaining the storage substance in the cell walls. Stebbins [137] suggested that cycles of transference of function have probably occurred during evolution. He proposed a succession of transfers of the function of protection of the embryo sac. This function is believed to have been transferred successively from the megasporangial wall to the ovule integument and then to the cupule wall. With the origin of the Angiosperms, protection of the ovules became the function of the carpel of ovary wall. Each above structure, which possibly had a protective function for a certain period, finally adapted to other functions i.e. the megasporangial wall became the nucellus and the cupule wall became the ovular integument.

Buckeridge and Reid [6] and Buckeridge et al. [4] proposed that similar cycles of transfer occurred in the case of the seed CWSP. They proposed that the function of carbohydrate reserve for plantlet growth in the Leguminosae was originally performed by the endosperm (galactomannan in Caesalpinioideae, primitive Mimosoideae and some tribes of Faboideae) and then was transferred to the cotyledons in some Caesalpinioideae (xyloglucan, still in the cell wall) and some tribes of Faboideae (galactan in storage cell wall of lupins and starch in the cytoplasm of some species of Phaseoleae).

Buckeridge and Reid [6] mentioned that transfers probably occurred not only from one tissue to another, but also from one type of cell wall polysaccharide molecule to another i.e. the reserve function has been transferred from the galactomannans to the xyloglucans or galactans. With an increase of galactose branching, the CWSP became more involved with the control of water relations in seeds.

The concept of cycles of transference of function could explain why some storage cell wall polysaccharides are considered multifunctional molecules. It may be that some of the cell wall polysaccharides are at intermediate stages or have maintained earlier functions during cycles of transference, so that they have more than one function. Nevertheless, one can not discard the possibility that secondary functions, such as the imbibing and water-buffering function of galactomannans during germination [138] might be purely accidental, being simply a consequence of the fact that galactomannans are there, and that they are viscous substances with unique hydrodynamic properties [139],

Analysing the data available in the literature, concerning galactomannan yield and mannose:galactose ratios (Table 1), the tendencies indicated quite clearly that as yield decreases from Caesalpinioideae and Mimosoideae towards Faboideae, the degree of galactose branching increases (Figures 1 and 3). It is therefore reasonable to suppose that selective pressure existed to preserve water solubility of seed galactomannans. To understand such a process one can contrast the ecological situation in which the mainly tropical species belonging to Caesalpinioideae and Mimosoideae occur (i.e. hot and humid forest soil), with the environmental conditions in which some species of Faboideae developed (i.e. near the border of deserts). In the tropical forests, galactomannan containing seeds (higher yields and lower galactose branching) would be harder (avoiding attack of herbivors), imbibe water slowly with the final amount of water imbibed being lower in relation to those with highly branched galactomannans. In this case the storage function appears to be important since biodiversity is greater and so is the competition between plant species, especially during early growth. On the other hand, seeds from species of Faboideae, containing either high or low galactomannan yields but much higher galactose branching degrees, would imbibe water relatively faster and retain it more strongly. This is possibly an advantage for seeds germinating in dry environments in which water may be available for a relatively short time.

As shown above, although seed anatomy varies from species to species, in all cases studied, the endosperm completely surrounds the embryo, suggesting that it is also a Afunctional organ since it serves to distribute water uniformly during germination and after radicle protrusion it delivers galactomannan degradation products (mannose and galactose) in a uniform fashion as well.

Taking these data into consideration, it can be assumed that the galactomannan structure and distribution within the Leguminosae, represent two different stages of transference of functions. After transference from the primary cell walls, there was a great increase in yield in some tribes of Caesalpinioideae and Mimosoideae, together with a discrete increase in galactose branching. This first step of transfer, characterises a transference from the structural to the storage/defence function. The second step may have been from the storage to the water relations function, i.e., in some tribes of Faboideae galactomannan yield is relatively low and the galactose branching reached a maximum of 100%.

Probably many factors acted in concert in order to proceed the complex changes that were necessary to promote the transference of function of galactomannan from the primary wall to the storage one. Although this phenomenon may have been very complex, it is not unreasonable to speculate that a progressive increase in the degree of galactosylation was somehow involved in it. In fact, the decrease in expression of the gene that encodes for the a-galactosidase that edits galactomannan after biosynthesis of a highly branched polymer would be a reasonably simple explanation for the higher galactose branching observed in legumes as a whole and mainly in some tribes of Faboideae. It is important to remember that at the same time, the proportion of galactomannan greatly increased as cellulose and pectin decreased. Although this remains to be explained, turnover and cell wall polysaccharide biosynthesis mechanisms are very likely to be involved.

7. APPLICATIONS OF GALACTOMANNANS: PERSPECTIVES FOR FUTURE USE OF NEW LEGUME SPECIES

Galactomannans have been reported to be useful for industrial applications and are usually classified as seed gums [1]. A large range of industrial sectors use galactomannan due to their unique rheological properties. In foods, they are normally used as thickeners, emulsifiers and stabilisers and also because they interact with other polysaccharides, changing the rheological features of gels such as xanthan, carragennan and agar [1].

According to Cerda et al. [140], when ingested in small amounts, galactomannan is capable to form an unstirred water layer within the large intestine, which has been suggested to inhibit absorption of cholesterol and glucose by humans. Some authors report that cholesterol absorption can be decreased by 10-15% if the diet is complemented with galactomannan and on this basis propose that galactomannan could be used therapeutically in patients who are not dependent on insulin [141],

Apart from food, galactomannans are also used in cosmetic, paper, textile, explosives, oil prospection to name but a few. In the textile and paper industry, galactomannans are used mainly for its capacity to increase colour-printing quality. In this case, guar polymer is mixed with cellulose, to which it is thought to be adsorbed [33],

There are two plant species from which galactomannan is extracted and exploited industrially: guar and carob. Because guar is more branched with galactose, it is used in applications that need high viscosity, whereas carob galactomannan, being less branched with galactose is normally used in applications that need rheological modifications on gels.

Because different applications need different degrees of galactosylation, it would be useful to find means of changing this property and/or obtain galactomannans with the desired degree of branching. Bulpin et al. [142] used an a-galactosidase produced by genetically modified cells of Saccharomyces cerevisiae, to modify guar galactomannan on an industrial scale. Sherbukin and Anulov [33] reported that Russian inventors obtained transgenic guar plants which were able to produce a galactomannan with lower degree of galactosylation (M/G 1.75 and 2.0 against 1.65 in the untransformed plants).

The information now available can be used to devise some strategies to obtain different types of galactomannans for human use. Some possible steps are depicted in Figure 5. This figure considers the plant's life cycle (from plant to seed and backwards) and the processes that lead to each other. Concerning the plants, one can use some different strategies to obtain modified sources of galactomannan: 1) using the chemotaxonomy information one can search, with a reasonable degree of precision, for species with the desired M/G ratio and yield. This is possibly a relatively cheaper way to find new materials with desired properties, but for uses in medical, food and cosmetic industries, for instance, thorough toxicity studies have to be performed; 2) utilisation of galactomannan containing species already widely used, for programmes of genetic improvement of the plant (possible for guar, both too time consuming for carob). Only one study of populational variation has been performed [143] and although yield can vary, M/G ratios were shown to be constant in distinct population of Leucaena leucocephala. On the other hand, several different values for M/G ratios of guar have been presented in the literature, but it is not known to what extent this is related to the methods of extraction; 3) a more sophisticated form to change M/G ratio in a given species is the use of techniques of molecular biology to genetically modify the biosynthetic machinery to obtain changes in yield and M/G ratios; 4) since galactomannans are polydisperse and there are reports that different M/G ratios can be found in galactomannans extracted from a single species, industrial extraction can be controlled to obtain more or less branched polysaccharides and 5) enzymatic modification of galactomannan using hydrolytic enzymes (a-galactosidase and/or endo-(3-mannanase) can be used for either to obtain debranched polymers [142] or decrease in molecular weight in order to decrease viscosity.

Starch Synthesis Plants

Figure 5. Possible strategies to obtain legume species containing galactomannan with altered properties (mainly the mannose:galactose ratio) for industrial applications. With grey background, the natural process to obtain galactomannan (polysaccharide) is depicted through the life cycle of a legume (from plant to seed and backwards) and the processes to reach each stage (germination-growth and seed maturation. With transparent backgrounds are the possible actions which should be performed in order to obtain galactomannans with varying compositions and properties.

Figure 5. Possible strategies to obtain legume species containing galactomannan with altered properties (mainly the mannose:galactose ratio) for industrial applications. With grey background, the natural process to obtain galactomannan (polysaccharide) is depicted through the life cycle of a legume (from plant to seed and backwards) and the processes to reach each stage (germination-growth and seed maturation. With transparent backgrounds are the possible actions which should be performed in order to obtain galactomannans with varying compositions and properties.

In conjunction or separately, these five ways offer approaches for improved and/or sustainable forms to obtain galactomannans with desired structure and properties. These choices will be probably very important in the beginning of the 21st century, when techniques of molecular biology are available for genetic transformation of plants and at the same time preservation and sustainable use of tropical areas in the planet are key issues for economic development.

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