Phenylpropanoids pathway

Root flavonoids that may act as signals for the initiation and development of endomycorrhizal and ectomycorrhizal symbio.ses have been identified (see Chap. 7). Metabolites of the phenylpropanoid pathways apparently act as signaling molecules in endo- and ectomycorrhizal interactions (14). The role of flavonoids is still controversial, but a variety of flavanones, flavones, and isoflavones... 

Figure 3.3. General phenylpropanoid pathway. Each arrow represents one enzymatic reaction.

The key reaction that links primary and secondary metabolism is provided by the enzyme phenylalanine ammonia lyase (PAL) which catalyzes the deamination of l-phenylalanine to form iran.v-cinnamic acid with the release of NH3 (see Fig. 3.3). Tyrosine is similarly deaminated by tyrosine ammonia lyase (TAL) to produce 4-hydroxycinnamic acid and NH3. The released NH3 is probably fixed by the glutamine synthetase reaction. These deaminations initiate the main phenylpropanoid pathway. 

TEUTSCH, H.G., HASENFRATZ, M.P., LESOT, A., STOLTZ, C., GARNIER, J.M., JELTSCH, J.M., DURST, F., WERCK-REICHHART, D., Isolation and sequence of a cDNA encoding the Jerusalem artichoke cinnamate 4-hydroxylase, a major plant cytochrome P450 involved in the general phenylpropanoid pathway, Proc. Natl. Acad. Sci. USA, 1993,90,4102-4106. 

Figure 12.1 A brief summary of representative phenylpropanoid pathways.

Dixon RA, Achnine L, Kota P, Liu CJ, Reddy MSS, Wang LJ (2002) The phenylpropanoid pathway and plant defence - a genomics perspective. Mol Plant Pathol 3 371-390 Dixon RA, Paiva NL (1995) Stress-induced phenylpropanoid metabolism. Plant Cell 7 1085-1097... 

Schalk, M., Cabello-Hurtado, F., Pierrel, M.A., Atanossova, R., Saindrenan, P. and Werck-Reichhart, D. (1998) Piperonylic acid, a selective, mechanism-based inactivator of the trans-annarrate 4-hydroxylase a new tool to control the flux of metabolites in the phenylpropanoid pathway. Plant Physiology, 118 (1), 209-218. 

The 4-coumarate CoA ligase (4CL EC 6.2.1.12) enzyme activates 4-coumaric acid, caffeic acid, ferrulic acid, and (in some cases) sinapic acid by the formation of CoA esters that serve as branch-point metabolites between the phenylpropanoid pathway and the synthesis of secondary metabolites [46, 47]. The reaction has an absolute requirement for Mg " and ATP as cofactors. Multiple isozymes are present in all plants where it has been studied, some of which have variable substrate specificities consistent with a potential role in controlling accumulation of secondary metabolite end-products. Examination of a navel orange EST database (CitEST) for flavonoid biosynthetic genes resulted in the identification of 10 tentative consensus sequences that potentially represent a multi-enzyme family [29]. Eurther biochemical characterization will be necessary to establish whether these genes have 4CL activity and, if so, whether preferential substrate usage is observed. 

Capsaicinoids Are Products of the Phenylpropanoid Pathway and the Branched Chain Fatty Acid Pathway... 

Stored under continuous light, and placental extracts from non-pungent fruit could synthesize capsaicinoids if vanillylamine and isocapric acid are provided. Together, these results raise the possibility that the gene product at Pml is a regulatory gene or a structural gene upstream in either the phenylpropanoid pathway or the branched chain fatty acid pathway and not capsaicinoid synthase. 

Myristicin content of some carrot samples was increased two-to five-fold over nonirradiated controls (Table I). The increase in concentration of myristicin is presumed to be via the phenylpropanoid pathway phenylalanine ammonia-lyase, an enzyme of that system, is activated by light (9). Failure of some samples exposed to UV light to synthesize myristicin may be due to the absence or inhibition of a key enzyme needed for myristicin synthesis. 

The tightly regulated pathway specifying aromatic amino acid biosynthesis within the plastid compartment implies maintenance of an amino acid pool to mediate regulation. Thus, we have concluded that loss to the cytoplasm of aromatic amino acids synthesized in the chloroplast compartment is unlikely (13). Yet a source of aromatic amino acids is needed in the cytosol to support protein synthesis. Furthermore, since the enzyme systems of the general phenylpropanoid pathway and its specialized branches of secondary metabolism are located in the cytosol (17), aromatic amino acids (especially L-phenylalanine) are also required in the cytosol as initial substrates for secondary metabolism. The simplest possibility would be that a second, complete pathway of aromatic amino acid biosynthesis exists in the cytosol. Ample precedent has been established for duplicate, major biochemical pathways (glycolysis and oxidative pentose phosphate cycle) of higher plants that are separated from one another in the plastid and cytosolic compartments (18). Evidence to support the hypothesis for a cytosolic pathway (1,13) and the various approaches underway to prove or disprove the dual-pathway hypothesis are summarized in this paper. 

To what extent is the response of cytosolic and plastidic isozymes of the shikimate pathway coordinated or coupled with one another and to alterations in expression of enzymes of the flavonoid and phenylpropanoid-pathway segments Some of the emerging information is given in Figure 6. Thus, light induction, well known to induce PAL and enzymes of the flavonoid pathway, also induces both DS-Mn and DS-Co in parsley cell cultures (49). However, only the cytosolic CM-2 (and not the plastidic CM-1) was induced. Fungal elicitor was reported to induce only DS-Mn—not DS-Co or either of the chorismate mutase isozymes (49). Previous studies... 

In a recent study (54), we showed increased activities of two enzymes of the general phenylpropanoid pathway, PAL and 4-coumarate CoA lig-ase, as well as one enzyme of the specific pathway of lignin biosynthesis, cinnamy 1-alcohol dehydrogenase (CAD), in resistant plants at the time of the hypersensitive host cell death. On the other hand, decreased activities were observed at the same time with susceptible host plants (54). Furthermore, we showed that the well known increase in peroxidase activities, which is strong in resistant and only weak in susceptible plants (55-58), is at least partly due to the increased activity of the lignin biosynthetic pathway (54,59). 

Coumaroyl-CoA is produced from the amino acid phenylalanine by what has been termed the general phenylpropanoid pathway, through three enzymatic conversions catalyzed by phenylalanine ammonia-lyase (PAL), cinnamate 4-hydroxylase (C4H), and 4-coumarate CoA ligase (4CL). Malonyl-CoA is formed from acetyl-CoA by acetyl-CoA carboxylase (ACC) (Figure 3.2). Acetyl-CoA may be produced in mitochondria, plastids, peroxisomes, and the cytosol by a variety of routes. It is the cytosolic acetyl-CoA that is used for flavonoid biosynthesis, and it is produced by the multiple subunit enzyme ATP-citrate lyase that converts citrate, ATP, and Co-A to acetyl-CoA, oxaloacetate, ADP, and inorganic phosphate. ... 

Schoch, G. et al., CYP98A3 from Arabidopsis thaliana is a 3-hydroxylase of phenolic esters, a missing link in the phenylpropanoid pathway. J. Biol Chem., 276, 36566, 2001. 

Rasmussen, S. and Dixon, R.A., Transgene-mediated and elicitor-induced perturbation of metabolic channeling at the entry point into the phenylpropanoid pathway. Plant Cell, 11, 1537, 1999. 

Blount, J.W. et ah, Altering expression of cinnamic acid 4-hydroxylase in transgenic plants provides evidence for a feedback loop at the entry point into the phenylpropanoid pathway. 

Loake, G.J. et al., Phenylpropanoid pathway intermediates regulate transient expression of a chalcone synthase gene promoter, Plant Cell, 3, 829, 1991. 

Yao, K.N., de Luca, V., and Brisson, N., Creation of a metabolic sink for tryptophan alters the phenylpropanoid pathway and the susceptibility of potato to Phytophthora infestans. Plant Cell, 1, 1787, 1995. 

Matsuda, F., Morino, K., Ano, R., Kuzawa, M., Wakasa, K., Miyagawa, H. (2005). Metabolic flux analysis of the phenylpropanoid pathway in elicitor-treated potato tuber tissue. Plant Cell Physiol., 46,454 66. 

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