Genetic control of fruit size in apple
Fruit size is a commercially valuable trait in apple as well as in other fruit crops. These studies investigated the role of the cell division cycle in regulating fruit size of apple. Final cell number within the fruit cortical region was positively related to the fruit diameter (r2 = 0.90). The large fruit size cultivar 'Golden Delicious' differed from the smaller fruit size cultivars both in early cell production rate (CPR) and duration of the cell production suggesting that both factors influence final cell number and fruit size in apple. Four core cell cycle genes, Md;CDKA, Md;CDKB1, Md;CycB2 and Md;CycD3, were cloned from 'Gala' using reverse transcription mediated polymerase chain reaction (RT-PCR) and rapid amplification of cDNA ends (RACE). Md;CDKB1, Md;CycB2 and Md;CycD3 showed cell division phase specific transcript expression. The transcript accumulation pattern of the above genes during early fruit development was closely related to cell production dynamics. Md;CycD3 transcripts accumulated to greater than 2 fold higher levels 'Golden Delicious' compared with the crabapple, 'Hopa,' suggesting a role for this gene in controlling cell production and thereby, fruit size. Analysis of the basis of alteration in fruit size of a large fruited mutant of 'Gala,' 'Grand Gala' (GG), indicated that larger cell size was responsible for a 40 % increase in fruit size of GG. Fruit cortical cells of GG contained larger nuclei with ploidy levels of 2C, 4C, 8C and 16C. This alteration is hypothesized to be due to fruit specific endoreduplication, normally absent in commercial apple cultivars. Transcript abundance of Md;CDKB1 was increased while that of Md;CycB2 and Md;CycD3 was reduced by 30 % during early fruit development in GG compared with 'Gala.' Early season warm temperature increased fruit size through an increase in cell size and not cell number. The above studies suggest that fruit size in apple is regulated through control of cell production and that Md;CycD3 may play an important role in this control through an effect on CPR and cell production duration.
Hirst, Purdue University.
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