Pummelo fruit transcript homologous to ripening-induced genes.

Organic acid accumulation in the vacuole of juice cells of citrus fruits is a developmentally regulated process, the degree and timing of which varies greatly among species and varieties and is highly susceptible to environmental conditions (Clements, 1963; Ting and Vines, 1966; Vandercook, 1977). Total titratable acidity of the fruit juice of the acidless pummelo 2240 (Citrus maxima [Burm.] Merrill) is only 0.10% on a weight basis; HPLC analysis indicates that the citric acid concentration of pummelo 2240 fruit juice is 10 times lower than that in normal fruits. The acidless phenotype of pummelo 2240 is caused by a mutation involving a single nuclear gene, acitric, that does not affect other traits (Cameron and Soost, 1977; M.L. Roose, unpublished results). To generate citrus varieties of moderate acidity, pummelo 2240 was used in crosses with varieties of i n te rmed ia te and high ac id levels (Soost and Cameron, 1961). F, and backcross populations obtained from these crosses include low-acid individuals that are homozygous for acitric and high-acid individuals that carry at least one copy of the wild-type allele. As part of a continuing effort to identify acitric, two-dimensional IEF/SDS-PAGE protein patterns of tonoplast-enriched fractions obtained from the juice of lowand high-acid fruits were compared. A moderately abundant polypeptide of 10 to 15 kD and approximately neutra1 pI was identified in the juice of immature low-acid fruits that was not detected in juice from highacid fruits at the same developmental stage. Mouse polyclonal antibodies raised against the electroeluted protein reacted with a single low-molecular-weight protein of juice tissues and seeds; the protein was not detected in roots, epicotyl, fruit rind, or unstressed leaves. Full-length cDNA clones encoding the purified polypeptide were isolated that showed strong nucleotide sequence similarity to the Lycopersicon esculentum genes asrl and asr2 (GenBank accession Nos. L08255 and L20756). The tomato genes encode 13-kD proteins, whose expression is induced in leaves by water stress and in fruit pericarp by ripening (Iusem et al., 1993). The citrus gene, designated asr l , encodes a 98-amino acid polypeptide of 10.87 kD. Like the tomato asrl product, the citrus protein is rich in Ala, Glu, His, and Lys (Table I).