High and low acid lemons: origin and transcriptome comparisons.

Citrus fruits are characterized by the accumulation of high levels of citric acid, which account for 90% of the total organic acids. Citric acid is an important determinant of fruit taste because it affects the sourness of the fruit as well as the perceived sweetness, by masking the taste of sugars. To study the role of gene expression in citric acid accumulation we compared the transcriptomes of juice sac cells in acid and acidless lemon genotypes using an Affymetrix microarray. Three genotypes were used for the transcriptome analysis: ‘Frost Lisbon’ (L), a standard sour lemon, and two ‘Faris’ genotypes obtained from a chimera that produces on the same tree both sweet (‘Faris’ non-acid, FNA) and sour fruit (‘Faris’ acid, FA). After the transcriptome comparison was completed, we discovered that ‘Faris’ is a graft chimera. Here we present SSR marker data showing that layer 1 of ‘Faris’ is derived from ‘Millsweet’ lime or a very similar cultivar, whereas layer 2 is derived from an acid lemon similar to ‘Eureka’ or ‘Lisbon’. Transcriptome comparisons among the three genotypes during two ripening stages showed high similarity between the two acid fruits and considerable divergence of FNA. Among the more interesting differences are genes involved in the GABA shunt pathway and a plasma-membrane type H+-ATPase.