GERMIN, A PROTEIN MARKER OF EARLY PLANT DEVELOPMENT, IS AN OXALATE OXIDASE

Germin is a homopentameric glycoprotein, the synthesis of which coinci des with the onset of growth in germinating wheat embryos. There have been detailed studies of germin structure, biosynthesis, homology with other proteins, and of its value as a marker of wheat development. Ge rmin isoforms associated with the apoplast have been speculated to hav e a role in embryo hydration during maturation and germination. Antige nically related isoforms of germin are present during germination in a ll of the economically important cereals studied, and the amounts of g ermin-like proteins and coding elements have been found to undergo con spicuous change when salt-tolerant higher plants are subjected to salt stress. In this report, we describe how circumstantial evidence arisi ng from unrelated studies of barley oxalate oxidase and its coding ele ments have led to definitive evidence that the germin isoform made dur ing wheat germination is an oxalate oxidase. Establishment of links be tween oxalate degradation, cereal germination, and salt tolerance has significant implications for a broad range of studies related to devel opment and adaptation in higher plants. Roles for germin in cell wall biochemistry and tissue remodeling are discussed, with special emphasi s on the generation of hydrogen peroxide during germin-induced oxidati on of oxalate.