Catalyzes the conversion of threonine to alpha-keto butyrate in isoleucine (Ile) biosynthesis (PubMed:17085687). Required for JA-Ile biosynthesis, a signaling molecule involved in defense and resistance to the herbivore Manduca sexta caterpillars (PubMed:17085687). L-threonine = 2-oxobutanoate + NH4(+) Amino-acid biosynthesis; L-isoleucine biosynthesis; 2-oxobutanoate from L-threonine: step 1/1. Induced by methyl jasmonate and feeding with the herbivore Manduca sexta caterpillars. Plants silencing TD exhibit a stunt growth phenotype. Belongs to the serine/threonine dehydratase family.