Amino acid and nucleotide sequences relating to the glutamate dehydrogenase (GDH) enzyme are described. The GDH enzymes described herein were discovered in the alga Chlorella sorokiniana in the form of seven different inducible isoenzymes. These isoenzymes are found in the algae as chloroplast-localized hexamers composed of alpha- and beta-subunits. Plants transformed with nucleotide sequences encoding the alpha- or beta-subunits of the enzyme show improved properties, for example, increased growth and improved stress tolerance. A heterohexamer having both .alpha.- and .beta.-subunits can have higher aminating:deaminating activity ratio than .alpha.-homohexamers or .beta.-homohexamers.