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Evolution and Distribution
SFTI-1 appears to be restricted to several species within the subtribe Helianthinae of the Compositae (Asteraceae). Table 1.2 sets out the species in which it has been described and it can be seen that it is present in the seeds of Helianthus annus (the common sunflower), several other diploid annual species and in all tetraploid and hexaploid helianthus species, including H. tuberosus (topinambour or Jerusalem artichoke) and Tithonia diversifolia (Mexican sunflower). As SFTI-1 appears to be derived from the BBIs it is interesting to note that these proteins have only been located in the seeds of species from the Fabaceae and Poaceae plant families. As monocots and dicots these two families are somewhat distinct from each other and the Asteraceae (asterids), in turn, is removed from the Fabaceae (rosids). The isolated instance of a single, highly unusual, BBI inhibitor in a phylogenetically isolated species is somewhat reminiscent of the ``patchy'' distribution of the cyclotides within the plant kingdom.
| Species | Common Name |
| Genus Helianthus | |
| Diploid Species | |
| Helianthus annuus | Common sunflower |
| Helianthus debilis | Cucumberleaf sunflower, beach sunflower |
| Helianthus nuttallii | Nuttall's sunflower |
| Helianthus petiolaris | Prairie sunflower |
| Helianthus praecox | Texas sunflower |
| Tetraploid species | |
| Helianthus decapetalus | Thin-leaved sunflower, forest sunflower |
| Helianthus hirsutus | Hairy sunflower |
| Helianthus laetiflorus | Cheerful sunflower |
| Helianthus strumosus | Pale-leaved woodland sunflower, red-leaved sunflower |
| Hexaploid species | |
| Helianthus californicus | California sunflower |
| Helianthus multiflorus | Manyflower sunflower |
| Helianthus resinosus | Resindot sunflower |
| Helianthus rigidus | Stiff sunflower |
| Helianthus tuberosus | Topinambour, Jerusalem artichoke |
| Genus Tithonia | |
| Tithonia diversifolia | Mexican sunflower |
 Data derived from [112] |
|
If SFTI-1 is derived from a larger BBI-like protein then it would be
reasonable to expect that other members of the Compositae would
contain SFTI-1 or BBI-like proteins. However a study of the
proteinase content of seeds from the Compositae found no BBI like
proteases in this family [112]. The unusual
distribution of the BBI inhibitors in the plant kingdom was noted in a
study that analysed large sequence databases for the occurrence of
BBIs [113]. The authors speculated that
the BBIs may have been lost in the majority of plant species due to
their functional role being fulfilled by a variety of other proteins.
If this is the case it is possible that SFTI-1 was derived from a BBI
that existed in the ancestors of the Compositae but was consequently
lost. Depending on the timing of this event a number of plants
related to the sunflower would then have had to lose SFTI-1 as other
proteins were evolved to fulfil similar functional roles, a process
that did not occur in the sunflower. Intriguingly the study of
trypsin inhibitors in the Compositae did partially characterise an
unidentified trypsin inhibitor in Zinnia elegans with a M
of
11,350 that shared limited homology with the BBI's. Likewise a number
of trypsin inhibitory molecules with M's of approximately 7600
were also identified in sunflower seeds but not further characterised.
In principle these masses could represent BBI-like proteins, or
descendants of BBIs, within sunflower seeds. One possible hypothesis
could be the separate evolution of a single headed BBI within the
Compositae from an ancestral single headed monocot BBI. The single
headed variant is thought to be ancestral to modern BBIs
[113] and derivation of SFTI-1 from a
single headed descendant in the Compositae is certainly possible.
Along with the partially characterised inhibitor from Z.
elegans these masses may therefore represent a faint evolutionary
echo of ancestral BBIs in the Compositae.
Next: Summary Up: SFTI Previous: Biosynthesis Jason Mulvenna
2005-04-24
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