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Calliandra selloi Macbride

[56]

C. bracteosa Benth.

[56]

Enterolobium s

[56]

Inga marginata Willd.

[56]

I. uraguensis Hook. & Arn.

[56]

Pithecellobium incurialli Benth.

[56]

P. langsdorfii Benth.

[56]

* contain xyloglucan as cell wall storage polysaccharide

observed (40% of the species with yields between 10-20% and 30% of the species with yields between 25-35%) indicating a shift of the mode towards a lower yield (Figure 3B). In Faboideae, a much stronger shift is observed toward lower yields, almost 50% of the species in Faboideae presenting from 0.1-10% of their dry weight as galactomannan.

Whereas almost 50% of the species in Caesalpinioideae present M/G ratios between 2.5-3.5, in Mimosoideae more than 50% of the species present M/G ratios between 1-2 and in Faboideae more than 50% of the species present M/G ratios of 1-1.5. This clearly indicates an increase in galactosylation of galactomannans in the more advanced species (Figure 3D-F). It can be concluded from the data available that higher yields of less branched galactomannans were preserved in Caesalpinioideae whereas in Faboideae an increasing branching was associated with decrease in yield. In Mimosoideae galactomannans virtually disappeared in the most advanced tribes. The ecological implications of these features will be discussed below.

4.3. Seed anatomy: location of galactomannan in the seed and in the cells

Anatomy of fenugreek seeds was first studied by Nadelman in the late 19th century [28] and later by Reid in 1971 [108], This author found that the endosperm cells of seeds of Trigonella foenum-graecum are totally filled with galactomannan and although a primary cell wall is apparently present between the storage walls (Figure 2D), the endosperm was found to be non living.

In 1972, Reid and Meier found that the aleurone layer present in seeds of T. foenum-graecum, Trifolium incarnatum (crismon clover) and Medicago sativa (lucerne) is the cell layer responsible to produce galactomannan hydrolases. They first showed that galactomannan breakdown is inhibited when protein synthesis was halted. They performed ultrastructural studies and observed that polysomes were formed and became associated with vesicles formed from the endoplasmic reticulum. As galactomannan breakdown progressed, corrosion of the cell walls of aleurone cells was observed and the aleurone grains disappeared. They concluded that aleurone layer was responsible for the production and secretion of the hydrolytic enzymes responsible for galactomannan hydrolysis.

In seeds of guar, the endosperm is also non-living and the cells are almost totally filled with galactomannan (Figure 2B)[109, 108], On the other hand, in seeds of Ceratonia siliqua (carob), endosperm cells are living and galactomannan can be viewed as wall thickenings. In this case, there is no clear distinction between endosperm and aleurone layer and the enzymes are probably produced and liberated into the cell walls by the endospermic cells. The cytoplasm of endosperm cells is filled with protein bodies, which also serve as a reserve during germination. Although a one cell aleurone layer is present between seed coat and endosperm, there is no direct proof that it produces galactomannan hydrolases.

Differently from carob, in seeds of Sesbania marginata (Figures 2E-F) an aleurone layer is clearly seen between seed coat and endosperm (Figure 2E), but at the same time most endosperm cells contain protein bodies (Figures 2F and G). Based on this observation, Buckeridge and Dietrich [6] proposed that galactomannan hydrolytic enzymes might be produced both in the aleurone and endosperm cells. However, the possibility that in the endosperm, the protein bodies are rather storage organelles can not be discarded.

A completely different anatomy occurs in seeds of Schyzolobium parahybum (Figure 2A). Instead of an aleurone layer, long haustorium-like cells can be observed, which seem to be immersed in a galactomannan vitreous endosperm. In fact, Nadelman [28] showed

drawings of a seed of the same genera and also found that the seeds of Gymnocladus canadensis display the same anatomic features. Although no experiments have been performed yet, it is likely the haustorium like cells are responsible for the production of enzymes, since no cytoplasm appears to be left in endosperm cells during maturation. An interesting observation has been made by professor Sawadzki-Baggio (personal communication) that although seeds of S. parahybum present a galactomannan with average mannose:galactose ratio 3:1, pure mannan molecules can be found in appreciable proportion.

In seeds of Senna occidentalis, very large spaces with unknown filling can be observed (Figure 2C). A tentative hypothesis is that these might be filled with gases that will be used for early development of the embryo in a flooded soil. This is a very common situation during tropical summer in the rain forests and might also be an important adaptive feature for species of gallery forests.

Thus, although a reasonably clear picture can be obtained from the analysis of yield and mannose:galactose ratios in seeds of different legume species, a very wide range of patterns can be appreciated after analysis of the anatomy of some of their endosperms. It is clear from the patterns shown in Figure 2 that further attempts of comparison of the anatomies of galactomannan containing seeds will be very instructive for understanding the importance of galactomannan for adaptation of species to different environmental conditions.

4.4 Metabolism

4.4.1. Deposition and Biosynthesis

Nadelman [28] observed in 1890 that the mucilages present in seed endosperms of Colutea breviata, Indigofera hirsuta, Tetragonolobus purpureas and Trigonella foenum graecum were first formed in vacuoles and later deposited into the endosperm cell wall.

In 1970, Reid and Meier [110] studied the deposition of galactomannan in seeds of T. foenum-graecum and the fact that deposition during maturation of raffinose series oligosaccharides (mainly stachyose) was observed at the same time as galactomannan, myoinositol, galactose and galactinol in the endosperms, led the authors to suggest that galactomannan could be synthesised by a mechanism similar to oligosaccharide synthesis.

During deposition of galactomannan in seeds of fenugreek, the mannose:galactose ratio was constant [110]. However, Dey [111] found that the M/G ratio increased in seeds of different legumes species. In Gleditsia triacanthos, a small change in galactose substitution from 41% to 35% was observed during galactomannan deposition [112], whereas it remained constant in Gleditsia ferox [45, 46]. Furthermore, Mallet et al. [112] observed that the molecular-size distribution of the accumulating galactomannan became more disperse as deposition proceeded.

Figure 2. Cross sections of endospermic legume seeds from the subfamilies Caesalpinioideae (A-C) and Faboideae (D-G). (A) Schizolobium parahybum; (B) Cyamopsis tetragonolobus (guar); (C) Senna occidentalis', (D) Trigonella foenum graecum (fenugreek) and (E, F and G) Sesbania marginata, F and G are higher magnifications of E. The seed structures and cell components shown are: /11-aleurone layer; CO-cotyledon; £<V-endosperm; //-haustorium; GM-galactomannan; PB-protein bodies; SC-seed coat. Bars correspond to 300|im. Cross sections were stained with methylene blue and B, C and D were supplied by Professor Grant Reid from Stirling University.

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