Plant exudates

When a plant is wounded there often exudes from the opening a viscous, sticky fluid which tends to cover and seal the incision. On drying in air the fluid thickens and hardens to a brittle, translucent, glassy mass. These exu-udates, plant gums, are in deciduous plants, often composed of polysaccharides. They are practically always highly branched structures (D in Fig. 1) composed of two to five types of monosaccharides. Almost all exu- 

Gum arabic, one of the most important polysaccharide gums of commerce, exudes from thorny shrublike trees of the genus Acacia found in the semiarid regions of the Sudan and Senegal regions. It is harvested by hand picking. A possible structure for arabic acid 1 0) based on extensive chemical work is illustrated. 

Galp = D-Galactopyranose Ara/ = L-Arabofuranose Rhap = L-Rhamnopyranose G.A. = D-Glucuronic acid 

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Plant Gums. There are a large number of plant gums that find appUcation as fat replacers (Table 4) (see Gums, industrial). Many are dried plant exudates. A good review of plant gums exists, giving sources, uses, and an extensive Ust of references (50). There is also a weU-known text avaUable... 

A wide variety of plant exudates have been used ia foods and medicines for centuries, including acacia, karaya, and ghatti. Plant gums derived from seeds iaclude arabic, guar, locust bean, tamatind, and tara. AH play a role ia fat replacement either singly or ia mixtures. 

Resin and Resinoid. Natural resins are plant exudates formed by the oxidation of terpenes. Many are acids or acid anhydrides. Prepared resins are made from oleoresins from which the essential oil has been removed. A resinoid is prepared by hydrocarbon extraction of a natural resin. 

Natural resins are generally described as solid or semisolid amorphous, fusible, organic substances that are formed in plant secretions. They are usually transparent or translucent yeUow-to-brown colored, and are soluble in organic solvents but not in water. The principal uses for natural resins are in varnishes, printing inks, adhesives, paper size, and polymer compositions. The term natural resins includes tree and plant exudates, fossil resins, mined resins, and shellac. They often have been altered from their original state during isolation and processing. For some appHcations, the resins have been chemically modified to increase their industrial utiUty. 

The early literature on naturally occurring plant growth inhibitors and the influence one plant might exert on another by chemical means (allelopathy) is considered comprehensively in various reviews (12, 15, 36, 37, 48, 49, 61, 67, 94, 121, 162). Reviews of studies concerned with the influence of plant exudates on root-infecting fungi (130) and the effects of phytotoxins which arise as decomposition products (113) have appeared recently. Because of the excellent coverage of the topics by others, no attempt is made here to review the early literature exhaustively. Instead, consideration is restricted essentially to specific compounds and to some of the more recent literature. 

There has been considerable interest in the use of plants for bioremediation and this merits a rather extensive discussion. Plants can play an important role in bioremediation for several reasons (1) they can transport contaminants from the soil, (2) they can metabolize the contaminants after uptake, or (3) they can produce exudes that support microbial activity for degradation of the contaminants. In addition, bacteria can produce metabolites that counter the effect of toxins produced by fungi, and serve as biocontrol agents that diminish the need for the application of agrochemicals. Plant exudates play an important role in supporting the growth and activity of bacteria that carry out the degradation of contaminants in the rhizosphere and rhizoplane (the external surface of roots... 

The interaction of plants and bacteria in the root system plays an important role in the remediation of contaminants, and may involve the use of plant exudates to stimulate the growth of degradative bacteria as well as the penetration of the soil by plant roots (Kuiper et al. 2004) ... 

M. N. Schroth and D. C, Hildebrand, Influence of plant exudates on root-infecting fungi. Annual Review of Phytopathology 2 101 (1964). 

Seeds of lettuce and other species have frequently been used to bioassay for the allelopathic activity of plant exudates (17.18.19). As with the use of cell suspensions, there are certain advantages and disadvantages to this methodology. The experimental simplicity, small amounts of material required and short time frame are certainly attractive qualities. However, species used in such bioassays quite often do not represent the actual target species under consideration. This is especially true when terrestrial crop species are substituted for weeds of aquatic systems. Nevertheless, information obtained from such experiments are often valuable when used in conjunction with results of other assays. 

At the end of the 1950s allelopathy was already well known among botanists and plant physiologists, but it was considered rather a minor and rare phenomenon that had no great ecological importance. We have screened many plant exudates, and reached the conclusion that many species are allelopathically active indeed, practically any plant under certain conditions so affects otiier plants... 

Resinoids are prepared by extracting plant exudates (balsams, oleo gum resins, natural oleo resins, and resinous products) with solvents such as methanol, ethanol, or toluene. Yields range from 50 to 95%. The products are usually highly viscous and are sometimes diluted (e.g., with phthalates or benzyl benzoate) to improve their flow and processing properties. 

Possibly the most important, and least understood, aspect of spray-dried flavorings manufacture is the role the wall material plays in this process. The polymers utilized for this product are controlled by FDA constraints, cost, finished product labelling considerations and compatability, functionality and historical usage. Given these considerations, polymers selected for the retention and maintenance of labile flavors and aromas in industrial spray dried, food grade systems include both carbohydrate (hydrolyzed starches, "lipophilic starches, plant exudates) and protein. The importance of these wall materials should not be underestimated. 

Gum acacia, a natural plant exudate polysaccharide, has historically been used as the wall material of choice. Due to fluctuations in availability and increasing costs of this natural polymer, alternate choices have been examined (9), Worth noting at this point is the 1.5% to 3% protein content associated with this polysaccharide (20). 

Metal bioavailability is generally increased with decreasing pH. This is due to the presence of phosphoric, sulfuric and carbonic acids, which increasingly solubilize organic- and particulate-bound metals. Particulate-bound metals are considered those bound to secondary minerals, for example, clays, iron and aluminum oxides, carbonates and sulfidic and phosphoric minerals. Due to the heterogeneous nature of soils and sediments, wide fluctuations in pH can exist in a given environment. For instance, metals may be more soluble in surface layers where plant exudates, microbial activity, moisture and leaching lower pH. 

Several plant exudates (gums and mucilages) have been used for pharmaceutical purposes and they still find significant application. However, the production of gums is laborious and expensive and their use will probably continue to decline. 

Cultures are being found that can degrade both polychlorinated biphenyls and petroleum hydrocarbons. There is also interest in the role of rhizosphere organisms in polychlorinated biphenyl degradation, particularly since some plants exude phenolic compounds into the rhizosphere that can stimulate the aerobic degradation of the less chlorinated biphenyls. 

Mulligan, J.T. Long, S.R. (1985). Induction of Rhizobium meliloti nodC expression by plant exudate requires nodD. Proceedings of the National Academy of Sciences (USA) 82, 6609-13. 

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