Increasing interest in the stress physiology of higher plants has, in the last few years, greatly expanded our knowledge concerning the effects of stresses such as drought, freezing, nutrient deficiencies, disease and insect attack on plant metabolism. This work, however, has concentrated on the influences of stress on the basic processes of growth and development (primary metabolism). Very little attention has been paid to the changes in secondary metabolism that may be occurring. Information on the effects of stress conditions on secondary metabolites has come mainly from research efforts to maximize the yield of active constituents from herb, spice and drug plants and from attempts to reduce the levels of toxins in certain food and forage crops. Advances in our understanding of the functional roles of secondary metabolites in plants, however, and new information about how pest resistance is altered by stress should stimulate additional interest in the responses of secondary metabolites to stress.

Changes in the Levels of Plant Secondary Metabolites Under Water and Nutrient Stress

Phenylalanine ammonia lyase

Phenylalanine ammonia-lyase - regulation of its induction, and its role in plant development

In leaf derived callus cultures of Solanum laciniatum Ait. both phenolics and solasodine concentrations increased when medium phosphate or nitrogen concs. were reduced to one-eighth or when sucrose concentration was increased from 3 to 4–8 %. Under these conditions growth was reduced and final FW:DW fell. Growth was inhibited by sucrose depletion and nitrogen supple -mentation. On additional nitrogen the concentrations of phenolics and protein significantly increased, FW:DW was reduced and solasodine concentration was unaffected. In seedling derived cultures phosphate depletion resulted in a significant increase in phenolics concentration, an inhibition of growth and a rise in solasodine concentration.

The effect of phosphate, nitrogen and sucrose on the production of phenolics and solasodine in callus cultures of solanum laciniatum

Publisher Summary Brown-midrib (bmr) mutants differing in quality and quantity of lignin from normal genotypes offer an opportunity to increase the overall digestion of plant fiber. It is a simple recessive trait that phenotypically produces a reddish-brown pigmentation associated with lignified tissues. Several different genes have been identified in both maize and sorghum, which produce the characteristic bmr phenotype. Lignin concentrations in bmr genotypes are consistently lower than their normal counterparts. Besides differences between bmr and normal alkali-labile phenolics, the nitrobenzene oxidation of the two lignins yields different ratios of products. In vitro digestibility of bmr genotypes has been consistently higher than normal, but in vitro rate of digestion does not appear to be consistently affected by the mutation. Animal performance on bmr forage has not always produced better results than with normal genotypes but the tendency is for improved animal performance with bmr genotypes. Activities of several enzymes involved in lignification differed between bmr and normal genotypes. Differences in activities are not consistent across species, indicating that several different modifications of the lignifications pathway may result in a similar phenotypic bmr response. This chapter concludes that bmr provide an excellent system for examining and possibly modifying the lignifications process in plants.

Potential of Brown-Midrib, Low-Lignin Mutants for Improving Forage Quality

1.

In dark-grown cell suspension cultures of Paul's Scarlet rose (Rosa sp.), polyphenol accumulation is essentially restricted to late- and post-exponential phase cells. The range of polyphenols synthesised is extensive, involving at least fourteen different compounds which show a characteristic pattern of development over the growth cycle.




2.

The duration of polyphenol accumulation is largely determined by the availability of carbohydrate, while the initiation and initial rate of synthesis is largely influenced by a complex of factors of which auxin concentration and light are most important. High 2,4-D concentrations (5x10-5M) supress accumulation by delaying initiation and reducing the subsequent rate of synthesis. High intensity illumination (6000 Lux) partially reverses the inhibition of polyphenol synthesis at 5x10-5M 2,4-D, and enhances accumulation at lower auxin concentrations (5x10-7M 2,4-D).




3.

Illuminated cultures also accumulate anthocyanin. Initiation of pigment synthesis is insensitive to the auxin concentration of the medium, and higher hormonal levels (5x10-5M 2,4-D) increase net anthocyanin accumulation by prolonging the period over which it is synthesised.

Polyphenol synthesis in cell suspension cultures of Paul's Scarlet rose

1. Under a variety of cultural conditions, including variation in initial auxin and carbohydrate levels, there appears to be a close correlation between phenylalanine ammonia-lyase (PAL) activity and polyphenol synthesis, although there is some discrepancy in timing between the maximum rate of polyphenol accumulation and the period of maximum enzyme content. The diverse responses of the two parameters to cycloheximide and actinomycin-D inhibition suggest that at certain phases of development, PAL is not a rate-limiting step in polyphenol synthesis. 2. Based on the sensitivity to antibiotic inhibitors, it is concluded that PAL is synthesised de novo, that it is subject to rapid turnover at all stages of development and that continuous RNA synthesis is required to sustain the increase in PAL levels. 3. When added to lag- and early exponential-phase cells high 2,4-D concentrations retard the development of PAL activity with a concomitant delay in polyphenol accumulation. Once the increase in PAL activity has started, the early stage of development of the enzyme is less sensitive to high hormone levels than is polyphenol synthesis. It is suggested, therefore, that in addition to inhibiting the initiation of PAL synthesis, high auxin levels may also inhibit some other component(s) of the biosynthetic pathway. High 2,4-D levels (10(-3)M) do not inhibit the activity of PAL in vitro.

Effects of auxin on polyphenol accumulation and the development of phenylalanine ammonia-lyase activity in darkgrown suspension cultures of Paul's Scarlet rose.

M. E. Davies

Papers

Polyphenol synthesis in cell suspension cultures of Paul's Scarlet rose

Effects of auxin on polyphenol accumulation and the development of phenylalanine ammonia-lyase activity in darkgrown suspension cultures of Paul's Scarlet rose.