Evolution and Distribution next up previous contents
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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.

Table 1.2: Distribution of SFTI-1 within the Helianthinae$ ^a$
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
$ ^a$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$ _r$ of 11,350 that shared limited homology with the BBI's. Likewise a number of trypsin inhibitory molecules with M$ _r$'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 up previous contents
Next: Summary Up: SFTI Previous: Biosynthesis
Jason Mulvenna