Downy mildew resistant lettuce mutant

A cultivated plant of the species Lactuca sativa which is homozygous for a mutant allele of the wild type homoserine kinase gene encoding SEQ ID NO:

• 1, wherein the mutant allele encodes a mutant homoserine kinase protein comprising one or more amino acids being substituted by a different amino acid compared to the wild type protein of SEQ ID NO:
• 1 resulting in the leaf tissue of said lettuce plant accumulating L-homoserine, while a plant heterozygous for the mutant allele or lacking the mutant allele does not comprise L-homoserine in the leaf tissue;
• wherein said L-homoserine in the leaf tissue confers resistance against Bremia lactucae, and wherein the one or more amino acids are selected from the group consisting of N64 (Asn at position 64 of SEQ ID NO:
• 1), D70 (Asp at position 70 of SEQ ID NO:
• 1), N118 (Asn at position 118 of SEQ ID NO:
• 1), C119 (Cys at position 119 of SEQ ID NO:
• 1), K144 (Lys at position 144 of SEQ ID NO:
• 1), G149 (Gly at position 149 of SEQ ID NO:
• 1), G151 (Gly at position 151 of SEQ ID NO:
• 1), L152 (Leu at position 152 of SEQ ID NO:
• 1), G153 (Gly at position 153 of SEQ ID NO:
• 1), S154 (Ser at position 154 of SEQ ID NO:
• 1), S155 (Ser at position 155 of SEQ ID NO:
• 1), S158 (Ser at position 158 of SEQ ID NO:
• 1), D196 (Asp at position 196 of SEQ ID NO:
• 1), N197 (Asn at position 197 of SEQ ID NO:
• 1), T24 (Thr at position 241 of SEQ ID NO:
• 1), R245 (Arg at position 245 of SEQ ID NO:
• 1), R293 (Arg at position 293 of SEQ ID NO:
• 1), A320 (Ala at position 320 of SEQ ID NO:
• 1), and M244 (Met at position 244 of SEQ ID NO:
• 1).