Enzymes lignifying

Phenylalanine Ammonia-Lyase. The building units of lignin are formed from carbohydrate via the shikimic acid pathway to give aromatic amino acids. Once the aromatic amino acids are formed, a key enzyme for the control of lignin precursor synthesis is phenylalanine ammonia-lyase (PAL) (1). This enzyme catalyzes the production of cinnamic acid from phenylalanine. It is very active in those tissues of the plant that become lignified and it is also a central enzyme for the production of other phenylpropanoid-derived compounds such as flavonoids and coumarins, which can occur in many parts of the plant and in many different organs (35). Radioactive phenylalanine and cinnamic acid are directly incorporated into lignin in vascular tissue (36). 

The induction of PAL activity at the onset of vascular differentiation can be shown by the use of plant tissue cultures (37-39). Xylem cells with secondary and lignified walls are differentiated over a time course of 3-14 days by the application of the plant growth factors naphthylene acetic acid (NAA) and kinetin in the ratio 5 1 (1.0 mg/liter NAA, 0.2 mg/liter kinetin) to tissue cultures of bean cells (Phaseolus vulgaris) (37,40). The time for differentiation varies with the type of culture, solid or suspension, and with the frequency and duration of subculture, but for any one culture it is relatively constant (37,41,42). At the time of differentiation when the xylem vessels form, the activity of PAL rises to a maximum. The rising phase of the enzyme activity was inhibited by actinomycin D and by D-2,4-(4-methyl-2,6-dinitroanilino)-N-methylpropionamide (MDMP) applied under carefully controlled conditions (42). This indicated that both transcription and translation were necessary for the response to the hormones. Experiments using an antibody for PAL and a cDNA probe for the PAL-mRNA have also shown that there is an increase in the amount of transcript for PAL during the formation of lignin when Zinnia mesophyll cells are induced to form xylem elements in culture (Lin and Northcote, unpublished work). 

Thus ferulic acid, which is not in vivo a natural substrate for peroxidases involved in lignification processes, can be oxidized not only in vitro but also in situ, i.e., in the normal, biological environment of the enzyme. Furthermore, the oxidation seems to be limited to the walls of lignifying cells. This restricted localization has been described only in the case of syringaldazine, a synthetic substrate closely related to cinnamic compounds... 

The reduction of />coumaroyl-CoA (3.31) to />coumaryl aldehyde (3.69) is catalyzed by the enzyme cinnamoyl-CoA NADP oxidoreductase (CCR). This enzyme was initially purified from soybean cultures (Wegenmayer et al., 1976), and was later on efficiently isolated from lignifying cambium of eucalyps (Eucalyptus gunnii) (Gofifiier et al., 1994). A CCR cDNA was identified in a cDNA library that was screened with oligonucleotiede derived from the peptide sequence of the CCR protein. CCR is considered the first enzyme committed towards the biosynthesis of monolignols and shows... 

Most heavy metal resistant plants belong to the so-called excluders , they prevent the accumulation of heavy metals inside their tissues [5]. Metal exclusion can function in several different ways. The probably most simple way is a reduction of the unselective permeability of cells. This is typically reached by lignification of plant cell walls, and the enhancement of lignifying enzymes is a well-known response to cadmium toxicity [6]. In addition, exclusion can occur by precipitation... 

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