Plant Tissue Culture and Its Bio-technological Application

3 – Turnover and Degradation of Secondary (Natural) Products

This chapter describes some recent aspects of the complex metabolism and degradation of phenolic constituents in higher plants. The concept behind this field of research is the assumption that phenolic plant constituents are not metabolically inactive storage products simply accumulating during the whole life of a plant, but are subject to relatively rapid turnover and degradation. This assumption is in contrast to previous ideas63, 76, 94and is based on a wide variety of experimental data3, 5, 6, 7, 26, 77, 94. These involve observations that various phenols or flavonoids found in seeds or young seedlings disappear completely upon germination3, 5, 36, 94. Certain phases of intensive growth are accompanied by substantial qualitative and/or quantitative changes in the spectrum of phenolics and these can only usually be interpreted by assuming catabolic processes to be involved. Various tracer studies5, 26have established the fact that both synthesis and turnover occur and that they occur simultaneously in the same plant. Phenol metabolism must be regarded, therefore, as a dynamic system involving steady-state concentrations of the various end products.

Metabolism and Degradation of Phenolic Compounds in Plants

Chickpea (Cicer arietinum L), commonly called gram, Bengal gram, or garbanzo bean, is the most important food grain legume of South Asia and the third most important in the world after common bean (Phaseolus vulgaris L) and field pea (Pisum sativum L) Chickpea is a diploid with 2n=2x=l6 chromosomes and a genome size of approximately 750 Mbp (Arumuganathan and Earle, 1991) Chickpea is one of the first grain crops cultivated by man and has been uncovered in Middle Eastern archaeological sites dated to the eighth millennium BC (Zohary and Hopf, 2000) Two distinct market type classes, desi and kabuli, are recognized in chickpea (Pundir, Rao, and van der Maesen, 1985) The desi types that account for about 85% of chickpea area usually have small, angularshaped, dark-colored seeds with a rough surface, pink flowers, anthocyanin pigmentation on the stems, and either semi-erect or semi-spreading growth habit The kabuli type, which cover the remaining 15% area, usually have large “rams head”-shaped smooth surface seeds, lack of anthocyanin pigmentation, and semi-spreading growth habit It has become increasingly clear during the last few decades that meeting the food needs of the world’s growing population depends, to a large extent, on the conservation and use of the world’s remaining plant genetic resources Conservation without use has little point and use will not come without evaluation Genetic resources encompass all forms of the cultivated species, as well as their related wild species (Harlan, 1984) That is a general concept to which chickpea is no exception In reviewing genetic resources and their multifaceted applications in chickpea genetic improvement, we have placed more emphasis on the wild genetic resources of the cultivated chickpea, while providing a brief overview of resources available in the cultivated species

Chickpea (Cicer arietinum L.)

Flavonol-deficient petunia pollen [conditionally male fertile (CMF) pollen] is unable to germinate but application of nanomolar concentrations of flavonol aglycones completely restores function (Mo et al. 1992). In this study a chemically synthesized radioactive flavonol, [4′-O-14C]kaempferide, was used as a model compound to study the metabolism of flavonols during the first few hours of pollen germination. [4′-O-14C] Kaempferide was as efficient at inducing CMF pollen germination as kaempferol and quercetin, the aglycone form of the endogenous flavonols in petunia pollen. Analysis by high-performance liquid chromatography (HPLC) of extracts from both in-vitro-germinated pollen and the germination medium showed that more than 95% of the applied radioactivity was recovered as three kaempferide 3-O-glycosides and unmetabolized kaempferide; no flavonol catabolites were detected. Only HPLC fractions that contained the aglycone, or produced it upon acid hydrolysis, could induce CMF pollen germination in vitro. Structurally diverse flavonols could be classified according to how efficiently the aglycone was internalized and glycosylated during pollen germination. The ability of an individual flavonol to restore germination correlated with the total uptake of flavonols but not with the amount of glycoside formed in the pollen. Thus this study reinforces the conclusion that flavonol aglycones are the active compound for inducing pollen germination.

Elfriede Muhle

Papers

Catabolism of flavonol glucosides in plant cell suspension cultures