Insulin‐stimulated ribosomal protein synthesis in maize embryonic axes during germination

Abstract

Addition of insulin to maize seed (Zea mays L. cv. Chalqueño) was found to accelerate germination and seedling growth. Insulin‐stimulated maize axes showed enhancement of S‐methionine incorporation into ribosomal proteins (rp) and mobilization of S6 rp mRNA into polysomes. Increase in S6 rp phosphorylation of the small ribosomal subunit (40S) was observed in P‐orthophosphate pulse‐labeled experiments when maize axes were stimulated by insulin. Application of either wortmannin or rapamycin, inhibitors of protein kinases of the insulin transduction pathway, abolished the insulin stimulatory effect on S6 rp phosphorylation and on ribosomal protein synthesis. The above data are interpreted as an indication of the existence of an insulin‐stimulated signal transduction pathway in maize tissues that is involved in the regulation of translation.

Functional Characterization of a Maize Ribosomal S6 Protein Kinase (ZmS6K), a Plant Ortholog of Metazoan p70S6K

“…Insulin and insulin-like growth factors (IGFs), widely distributed among eukaryotic organisms (26), stimulate a signal transduction pathway that targets p70 S6K activation and the subsequent S6rp phosphorylation on the 40S ribosomal subunit (5). Because it has been shown that insulin induces maize S6rp phosphorylation in growing tissues (12), the insulin effect on ZmS6K activity was measured in maize embryonic axes germinated for 24 h. For these experiments, Crude extracts from each axes set were prepared and the same amount of proteins as in A) were resolved in the 2D electrophoresis and analyzed by Western blot (upper panels) and autoradiography (lower panels). The asterisk marks a protein that was nonspecifically recognized by the antibody.…”

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

“…The effect of the insulin-like protein isolated from B. variegata leaves on glucose levels in the serum of diabetic mice was demonstrated by intravenous injection of the protein fraction extracted from electrophoresis gels. 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).…”

The leaves of green plants as well as a cyanobacterium, a red alga, and fungi contain insulin-like antigens

“…In plants, apart from the many reports on the lowering of blood sugar levels in diabetic animals by extracts of plant parts (4,16,17), no place for insulin as a member of signaling pathways has been suggested by researchers in the fields of glucose mobilization or transport (18)(19)(20)(21). Nevertheless, it has been shown that added insulin increases germination of some seeds (22; Oliveira AEA and Xavier-Filho J, unpublished results), accelerates synthesis of ribosomal proteins in germinating maize embryos (23,24), and increases the activity of glyoxysomal enzymes involved in the conversion of fat to carbohydrate in maize (22).…”