We report the production of two very long chain polyunsaturated fatty acids, arachidonic acid (AA) and eicosapentaenoic acid (EPA), in substantial quantities in a higher plant. This was achieved using genes encoding enzymes participating in the omega3/6 Delta8 -desaturation biosynthetic pathways for the formation of C20 polyunsaturated fatty acids. Arabidopsis thaliana was transformed sequentially with genes encoding a Delta9 -specific elongating activity from Isochrysis galbana, a Delta8 -desaturase from Euglena gracilis and a Delta5 -desaturase from Mortierella alpina. Instrumental in the successful reconstitution of these C20 polyunsaturated fatty acid biosynthetic pathways was the I. galbana C18-Delta9 -elongating activity, which may bypass rate-limiting steps present in the conventional Delta6 -desaturase/elongase pathways. The accumulation of EPA and AA in transgenic plants is a breakthrough in the search for alternative sustainable sources of fish oils.

ABSTRACT␥-Linolenic acid (GLA; C18:3 ⌬ 6,9,12 ) is a component of the seed oils of evening primrose (Oenothera spp.), borage (Borago officinalis L.), and some other plants. It is widely used as a dietary supplement and for treatment of various medical conditions. GLA is synthesized by a ⌬ 6 -fatty acid desaturase using linoleic acid (C18:2 ⌬ 9,12 ) as a substrate. To enable the production of GLA in conventional oilseeds, we have isolated a cDNA encoding the ⌬ 6 -fatty acid desaturase from developing seeds of borage and confirmed its function by expression in transgenic tobacco plants. Analysis of leaf lipids from a transformed plant demonstrated the accumulation of GLA and octadecatetraenoic acid (C18:4 ⌬ 6,9,12,15 ) to levels of 13.2% and 9.6% of the total fatty acids, respectively. The borage ⌬ 6 -fatty acid desaturase differs from other desaturase enzymes, characterized from higher plants previously, by the presence of an N-terminal domain related to cytochrome b 5 .

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The effect of cadmium on the biosynthesis of chlorophyll has been investigated in the leaves of dark‐grown seedlings of barley (Hordeum vulture L. cv. Proctor). Cd2+ inhibited the production of chlorophyll by affecting 1) the synthesis of 5‐aminolacvulinic acid and 2) the protoehlorophyllide reductase ternary complex with its substrates. Cd2+ had no effect on the constituent enzymes that catalyse the synthesis of free protoehlorophyllide from 5‐aminolaevulinic acid. The results obtained are consistent with Cd2+ inhibiting the formation of chlorophyll by reacting with essential thiol groups in both the protochlorophyllide reductase protein and the enzyme(s) involved in the light dependent synthesis of 5‐aminolaevulinic acid.

Isochrysis galbana, a marine prymnesiophyte micro-