The shikimate pathway has been recently reviewed by Herrmann and Weaver (1999). The cDNAs of the higher plant shikimate pathway encode proteins with amino terminal signal sequences for plastid import, suggesting that plastids are the exclusive locale for chorismate biosynthesis. Whereas in microorganisms, the shikimate pathway is regulated by feedback inhibition and by repression of the first enzyme, no physiological feedback inhibitor has been identified in higher plants (Herrmann and Weaver, 1999). This suggests that pathway regulation may occur exclusively at the genetic level in plants (Herrmann and Weaver, 1999). For more details of this pathway see Herrmann (1995a) and Herrmann and Weaver (1999) [and references cited therein].

DHAP synthase (also referred to as DAHP synthase (Herrmann, 1995ab) or 2-dehydro-3-deoxyphosphoheptonate aldolase) [EC 4.1.2.15] is the first enzyme of the shikimate pathway that leads to the biosynthesis of the aromatic amino acids and secondary metabolites derived therefrom. The enzyme is induced by glyphosate, wounding and pathogens (Herrmann, 1995ab). DHAP synthase may exist as different isoforms, dependent on Mn²⁺ or Co²⁺. In Petroselinum crispum the Mn²⁺-dependent enzyme appears to be plastidic and inducible by fungal elicitors, while the Co²⁺ appears to be cytosolic and unaffected by elicitors (McCue and Conn, 1989).

Arabidopsis contains two genes encoding DHAP synthase, only one of which (DHS1) is induced by physical wounding or by infiltration with pathogenic Pseudomonas syringae strains (Keith et al, 1991). The presence of amino-terminal extensions characteristic of chloroplast transit peptides on DHS1 and DHS2 suggests that both proteins may be targeted to the chloroplast (Keith et al, 1991). Similarly, tomato (Lycopersicon esculentum) contains two distinct DHAP synthase genes that are differentially expressed (Gorlach et al, 1993). Both tomato genes code for putative plastidic DHAP synthase isoforms (Gorlach et al, 1993).

Note that gene symbols shown in the above figure (and subsequent figures on the shikimate pathway) are those recommended by Herrmann (1995a), and do not correspond to those employed in the ExPASy database.

Gorlach J, Schmid J, Amrhein N 1993 Differential expression of tomato (Lycopersicon esculentum L.) genes encoding shikimate pathway isoenzymes. II. Chorismate synthase. Plant. Mol. Biol. 23: 707-716.

Herrmann KM 1995a The shikimate pathway: early steps in the biosynthesis of aromatic compounds. Plant Cell 7: 907-919.

Herrmann KM 1995b The shikimate pathway as an entry to aromatic secondary metabolism. Plant Physiol. 107: 7-12.

Herrmann KM, Weaver LM 1999 The shikimate pathway. Annu. Rev. Plant Physiol. Plant Mol. Biol. 50: 473-503.

Keith B, Dong XN, Ausubel FM, Fink GR 1991 Differential induction of 3-deoxy-D-arabino-heptulosonate 7-phosphate synthase genes in Arabidopsis thaliana by wounding and pathogenic attack. Proc. Natl. Acad. Sci. U.S.A. 88: 8821-8825.

McCue KF, Conn EE 1989 Induction of 3-deoxy-D-arabino-heptulosonate-7-phosphate synthase activity by fungal elicitor in cultures of Petroselinum crispum. Proc. Natl. Acad. Sci. U.S.A. 86: 7374-7377.