The Cell Wall Function In The Mannan Group

It seems quite clear from the information discussed above on the mannan group as a whole, that there is a structure-function relationship which has a central point in the degree and/or positioning of the galactose residues on the p-mannan backbone. As the galactose branching approaches zero (pure mannans) their biological function seems to be more related to hardness. This can be clearly seen in Palmae and other non-legume species. Conversely, with higher degrees of galactosylation (fenugreek being one of the extreme cases) the biological function of galactomannan includes water relations. In both cases, however, the storage function appears to remain. Nevertheless, it is not known which function is primary. Storage, water-relations or hardness? It is certain, however, that in any case, (galacto)mannans are multifunctional molecules, playing their functions during distinct phases of the plant's life-time. How this may have happened during evolution will be discussed later.

The emphasis on hardness instead of the storage function of galactomannan in non-legume dicots (see above) is probably due to the fact that they occur in smaller quantities in relation to many legumes and palms. However, it is important to remember that some legumes belonging to the subfamily Faboideae, as soybean for instance, store relatively small amounts of galactomannan in a thin endosperm (see [2] for other examples). Thus it is possible that a similar hardness function is still preserved in legumes, but no studies of this kind have been performed yet. Such experiments would be important for understanding the ecological functions of galactomannan for seed germination and plantlet growth as well as the mechanisms of selection present during evolution that led to these types of adaptations. In species of some tribes of the tropical subfamilies Caesalpinioideae and Mimosoideae, hardness of the seeds due to the lower galactose branching is a possible function which may play a role in avoiding the attack of herbivores for example. Therefore hardness may represent another function besides storage in species of these tribes.

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