Plant insulin or glucokinin: a conflicting issue

Abstract

The presence of insulin in plants is not accepted by the scientific community in general. In this review we discuss this paradigm and retrieve information that strongly suggests that insulin is indeed found in plants. We present results, which indicate that a protein molecule with the same amino acid sequence as bovine insulin is expressed in leguminous plants. Additionally, we provide evidence that proteins associated with insulin signalling pathways in vertebrates are also found in association with insulin-like molecules in plants.

Isolation and intracellular localization of insulin-like proteins from leaves of Bauhinia variegata

“…Several lines of evidence suggest that hexoses (glucose) generated by starch breakdown in the dark in chloroplasts are transported into the cytosol via a glucose translocator (26,27). In view of the results reported here and of others previously published (23), it is tempting to speculate that insulin in chloroplasts may be involved in carbohydrate biosynthesis and transport. Experiments using pedigree strains of maize, additions of insulin and glucokinin (insulinlike antigens isolated from maize) showed accelerated development of young seedlings (28,29) and increased development of chloroplast pigments (29).…”

“…While it has been previously shown that insulin is present in the testa of C. ensiformis (11), in developing fruits of Vigna unguiculata (12) and in M. charantia seeds (13), its functions in plants are not clear at present (23). Nevertheless, its localization in these tissues and the timing of its appearance suggest that insulin in plants has metabolic functions similar to those it exerts on animals, at least in connection with carbohydrate metabolism (23).…”

“…Nevertheless, its localization in these tissues and the timing of its appearance suggest that insulin in plants has metabolic functions similar to those it exerts on animals, at least in connection with carbohydrate metabolism (23). Several lines of evidence suggest that hexoses (glucose) generated by starch breakdown in the dark in chloroplasts are transported into the cytosol via a glucose translocator (26,27).…”

“…This fraction caused a significant decrease in blood glucose levels in diabetic mice similar to that isocitrate lyase, and malate synthase) involved in the conversion of stored fat to carbohydrate in the fat-storing seed species, sunflower (Helianthus annuus), watermelon (Citrullus vulgaris) and cucumber (Cucumis sativus) (31) and to induce ribosomal protein synthesis and seedling development from maize seeds (32). Yet, despite the lack of direct evidence of insulin effects on carbohydrate metabolism in plants, many literature reports suggest the existence of proteins with functions, localization and sequences of the corresponding gene or protein similar to proteins that are members of the insulin pathways characteristic of vertebrates (23). Some of these proteins are insulin receptor (tyrosine kinase) (33), insulin receptor substrate proteins 1 and 2 (34), glucose transporter (35), phosphatidylinositol 3-kinase (36), hexokinase (37), MAPK pathway (38), and ribosomal S6 kinase (39).…”

“…Very few peptides have been recognized as signal molecules functioning in plants (40) but the presence of insulin-like molecules, as described here, suggests that they may participate in plant signaling pathways akin to those in which it is involved in animals. Also, the presence of insulin in plants, besides suggesting that traditional remedies obtained from them and utilized worldwide may have a scientific basis, strongly suggests that the signaling pathways in which this hormone is involved may have been conserved throughout evolution (23).…”