Relationship between fruit cracking and expression of the expansin gene MdEXPA3 in ‘Fuji’ apples (Malus domestica Borkh.)

In ‘Fuji’ apples, fruit cracking causes great economic loss. To understand its mechanisms, we analyzed the relationship between fruit cracking and the expression of an apple expansin gene (MdEXPA3) in pericarp and mesocarp during fruit growth. Fruit cracking is divided into two types; internal ring cracking (IRC) and stem-end splitting (SES). The former is an early symptom sometimes followed by the lattar. In this study, IRC mostly was observed during the phase of rapid fruit growth. MdEXPA3 transcripts appeared in the mesocarp at 30 days after full bloom (DAFB), reached a maximum at 95 DAFB and then decreased, thus paralleling the fruit growth rate. In contrast, the transcript level in the pericarp was below the detection limit until 50 DAFB, then increased until 109 DAFB to remain high until the end of observation. As IRC began to occur just before the increase of MdEXPA3 transcript levels in the pericarp, the differential expression in pericarp and mesocarp may be related to the initiation of IRC. Bagging reduced the incidence rate of both IRC and SES to one eighth without affecting fruit enlargement, and induced MdEXPA3 expression at earlier stage in the pericarp but not in mesocarp. These results suggested that induced accumulation of MdEXPA3 mRNA in pericarp reduced the susceptibility of fruit cracking. Thus, early symptoms of fruit cracking coincide with situations in which MdEXPA3 expression in the mesocarp exceeds that in the pericarp. In such situations, pericarp cells may be unable to follow the expansion of mesocarp cells due to insufficient levels of growth promoting expansins. If so, IRC appears as a consequence of the imbalance of expansin-dependent tissue growth rates.