We have previously seen that insulin obtained from cows has the ability to induce cellular changes in Hydra. This led someone to ask me whether Hydra itself produces insulin. While it is assumed that Hydra must produce some insulin-like molecule to react with its insulin-like receptor, no evidence for such an intrinsic ligand has yet to be discovered.

That bovine insulin can activate a cnidarian RTK is intriguing enough, as it opens up some doors from the perspective of front-loading evolution, but what if we traveled further back in time? Could mammalian insulin have an analogous effect on something that is not an animal?

Here is a link to a review article from several Brazilian labs that argues for the presence in insulin in plants. Let me pick out some interesting excerpts.

First, there is evidence that extrinsically added bovine insulin does indeed influence plant development. The authors first cite some earlier work:

After a long period in which no report is found in the literature of any plant physiological work related to insulin, Goodman and Davis (1993) reported that added insulin, insulin like growth factors I and II (IGF-I and IGF-II) accelerate the post-germinative development of fat-storing seeds (sunflower, watermelon and cucumber). They also measured increased activities of enzymes necessary for the conversion of fat to carbohydrate like fatty acyl CoA dehydrogenase, citrate synthase, malate dehydrogenase, isocitrate lyase, and malate synthase. No mechanism is suggested by the authors to explain this increase in enzyme activities although they hint at the possible increase in protein synthesis. The authors call attention to “the possibility that there are hormones and/or growth factors that have a regulatory role in both plants and animals” and some of these could be insulin-, and IGF-like proteins (Goodman and Davis, 1993).

They then cite some of their own work:

We found by immunofluorescence microscopy analysis that insulin, insulin receptor and phosphoserine proteins are localized to an internal tissue layer of the seed coat but not in cotyledon tissues of C. ensiformis. This region is assumed to be important in sugar transport to the embryo. We then employed bovine insulin to test if it has any effect on germination of C. ensiformis seeds. The results showed that insulin, vanadyl sulfate (an insulin mimetic compound), pinitol (a chiro inositol analogue) and glucose were able to accelerate C. ensiformis seed radicle and epicotyl development and on the contrary, tyrphostin (an inhibitor of insulin receptor kinase activity) inhibited these processes (Oliveira AEA, Ribeiro ES, da Cunha M, Gomes VM, Fernandes KVS, Xavier-Filho J - Insulin accelerates germination and development of Canavalia ensiformis (Jack bean) seeds. Submitted for publication).

C. ensiformis is an annual, semi-domesticated legume with long germination and developmental times making it a less than ideal model for germination and developmental studies. Therefore we utilized common bean (Phaseolus vulgaris) as a more convenient model plant. We showed that increasing concentrations of added bovine insulin (for 72 h) promote an increase in the mass and size of radicles and epicotyls of P. vulgaris and also in the number of lateral roots. Additionally we extracted and purified a protein from embryonic axes (48 h), which cross-reacted with an anti-human insulin antibody (Santos, 2003).

Thus, not only does cow insulin influence development of Hydra, but it also influences the development of plants. And it gets even more interesting.

Our laboratory has mostly been directed to the elucidation of the biochemical basis of bruchid (insect) resistance shown by some legume seeds (Macedo et al., 1993; Fernandes et al., 1993; Xavier-Filho et al., 1996; Sales et al., 2000). As such, investigation of potentially toxic proteins from the seed coat of the legume Canavalia ensiformis to Callosobruchus maculatus (cowpea weevil) led Elenir Oliveira to isolate and purify a number of proteins from this material. One of these purified proteins was submitted to sequencing as an assignment for a training course. The resulting analysis showed unambiguously that the protein had the same amino acid sequence as bovine insulin (Table 1). To control for potential contamination, the analysis was repeated with different samples of the protein obtained from different batches of seeds and the amino acid sequencing analysis was also performed by two independent laboratories. After obtaining a total of seven analyses for the sequence we were convinced that the seed coat of C. ensiformis indeed contained a protein with a sequence equal to that of bovine insulin (Oliveira et al., 1999a). In this manuscript we suggested that molecules of insulin in seed coat tissues survive desiccation after maturation of the seed and, together with other proteins can be easily extracted. The high solubility of these proteins is certainly due to the lack of tannins and pigments in this tissue (Oliveira et al., 1999a; Oliveira et al., 1999b). These insulin molecules (and also a peptide fragment of a receptor-like-kinase accompanying the protein, see Table 2) in the seed coat seemed to be remains of constituents of signaling pathways probably involved in the transport of carbohydrate (Oliveira et al., 1999a). Contrary to our expectancies our results were received with disbelief.

and

We also choose a second fast growing plant, cowpea (Vigna unguiculata) to test for the presence of insulin during development. The protein was detected (by Western blotting) both in empty pods and seed coats but not in the embryo. Insulin was measured by an ELISA assay using an anti-human insulin antibody. The highest concentrations (about 0.5 ng.mg-1 of protein) of this protein were found in seed coats of 16 and 18 DAP (days after pollination) in which case the values were 1.6 to 4.0 times higher than the values found for isolated pods of any day. Insulin was isolated from 10 DAP empty pods by the method of Khanna et al. (Khanna et al., 1976), purified by C4-HPLC and submitted to N-terminal amino acid sequencing. The amino acid sequence was found to be equal to the sequence of bovine insulin and to the sequence of the insulin isolated from C. ensiformis seed coat (see above and Table 1) (Venâncio, 2001; Venâncio et al., 2003).

Here is Table 1:

Yet despite all these data, the authors acknowledge:

We know that up to now no gene sequence was found for insulin in the genome of Arabidopsis (Anon, 2000) or in any other plant genome already published. We do not have any explanation for the conflicting results and the others already referred to above.

This raises the interesting question of conflicting evidence. On one hand, we have a good bit of biochemical and cytological evidence that indicates insulin plays a role in plant development and that some plants not only possess insulin, but their insulin is the same as bovine insulin. On the other hand, sequence data to support this activity does not exist. When it comes to the sequence dilemma, one must ask whether this is this one widespread, yet subtle, contamination problem. Or is this an annotation problem? Or, if you really want to bake your noodle, might some plant genomes code for pieces of insulin sequence, such that insulin is created through elaborate RNA-processing events?

Whatever the answer, what does appear to be on solid ground is the theme whereby a mammalian hormone has the ability to influence not only the development of a simple animal like Hydria, but also of some plants. Yet does the story stop here?