A mutant tobacco plant cell having one or more mutations in at least one isopropylmalate synthase gene, wherein the isopropyl malate synthase comprises:(i) a polynucleotide comprising a sequence having at least 80% sequence identity to SEQ ID NO:1 or SEQ ID NO:10 or SEQ ID NO: 12 or SEQ ID NO:14; or(ii) a polypeptide encoded by the polynucleotide(s) set forth in (i); or(iii) a polypeptide having at least 80% sequence identity to SEQ ID NO:2 or SEQ ID NO:11 or SEQ ID NO:13 or SEQ ID NO:15;wherein the mutant tobacco plant cell is not exclusively obtained by an essentially biological process.