Osmotic stress induces the activation of a mitogen-activated protein kinase (MAPK) and a calcium-independent protein kinase from beet root ( Beta vulgaris L.)

Abstract

Beet root ( Beta vulgaris L.) is a halophytic plant adapted to soil with a high salt content. Since beet root is a stress-tolerant crop plant, we investigated the participation of protein kinases as possible mediators of osmotic stress tolerance. Activation of two protein kinases was detected by osmotic stress in beet roots. A 51 kDa kinase was observed when tissue was treated in hypoosmotic and moderate or high hyperosmotic media, whereas activation of a 46 kDa kinase was detected only after hyperosmotic stress at high osmolyte concentrations. Both enzymes preferentially phosphorylated MBP and their activity was partially inhibited in vitro by staurosporine and 6-dimethylaminopurine (6-DMAP). In contrast to the 51 kDa kinase, the activity of the 46 kDa kinase was not sensitive to the MAP kinase inhibitor apigenine. Mammalian anti-ERK1/2 and anti-phospho-ERK1/2 antibodies recognized only the 51 kDa protein. These biochemical and immunological data indicate that the 51 kDa kinase belongs to the MAP kinase family, whereas the 46 kDa kinase is activated in a calcium-independent manner. To our knowledge, this is the first report that identifies a MAP kinase and a calcium-independent protein kinase activated by osmotic stress in beet roots.