Arachidonic, eicosapentaenoic, and biosynthetically related fatty acids in the seed lipids from a primitive gymnosperm, Agathis robusta.

The fatty acid composition of the seeds from Agathis robusta, an Australian gymnosperm (Araucariaceae), was determined by a combination of chromatographic and spectrometric techniques. These enabled the identification of small amounts of arachidonic (5,8,11,14-20:4) and eicosapentaenoic (5,8,11,14,17-20:5) acids for the first time in the seed oil of a higher plant. They were apparently derived from gamma-linolenic (6,9,12-18:3) and stearidonic (6,9,12,15-18:4) acids, which were also present, via chain elongation and desaturation, together with other expected biosynthetic intermediates [bis-homo-gamma-linolenic (8,11,14-20:3) and bishomo-stearidonic (8,11,14,17-20:4) acids]. Also present were a number of C20 fatty acids, known to occur in most gymnosperm families, i.e., 5,11-20:2, 11,14-20:2 (bishomo-linoleic), 5,11,14-20:3 (sciadonic), 11,14,17-20:3 (bishomo-alpha-linolenic), and 5,11,14,17-20:4 (juniperonic) acids. In contrast to most other gymnosperm seed lipids analyzed so far, A. robusta seed lipids did not contain C18 delta5-desaturated acids [i.e., 5,9-18:2 (taxoleic), 5,9,12-18:3 (pinolenic), or 5,9,12,15-18:4 (coniferonic)]. These structures support the simultaneous existence of delta6- and delta5-desaturase activities in A. robusta seeds. The delta6-ethylenic bond is apparently introduced into C18 polyunsaturated acids, whereas the delta5-ethylenic bond is introduced into C20 polyunsaturated acids. A general metabolic pathway for the biosynthesis of unsaturated fatty acids in gymnosperm seeds is proposed. When compared to Bryophytes, Pteridophytes (known to contain arachidonic and eicosapentaenoic acids), and species from other gymnosperm families (without such acids), A. robusta appears as an "intermediate," with the C18 delta6-desaturase/C18-->C20 elongase/C20 delta5-desaturase system in common with the former subphyla, and the unsaturated C18-->C20 elongase/C20 delta5-desaturase system specific to gymnosperms. The following hypothetical evolutionary sequence for the C18 delta6/delta5-desaturase class in gymnosperm seeds is suggested: delta6 (initial)-->delta6/delta5 (intermediate)-->delta5 (final).