Differential Gene Expression Analysis of Yunnan Red Pear, Pyrus Pyrifolia, During Fruit Skin Coloration

The color of fruit skin is an important quality parameter, and in many plants, it is the result of coordinative regulation of the anthocyanin pathway. To characterize the mechanism involved in fruit peel coloration of Yunnan red pear (Pyrus pyrifolia), we constructed a subtractive cDNA library using the suppression subtractive hybridization (SSH) technology. cDNA of red peel exposed to sunlight (for 2, 4, 6, and 8 days) was subtracted from that of white skin unexposed to sunlight. Over 100 differentially expressed ESTs were obtained, putatively involved in primary and secondary metabolism, stress, and defense response. Expression analysis using semiquantitative reverse transcription polymerase chain reaction (RT-PCR) for 13 genes was performed with two pear cultivars, light-skinned 'Zaobaimi' and red-skinned 'Yunhong-1', which had been bagged and then exposed to sunlight for 0, 1, 2, 3, 5, and 7 days before harvest. This analysis showed that genes encoding for a metallothionein-like protein and a NADP-malic acid enzyme were constitutively expressed, whereas other selected genes were either down- or up-regulated. Semiquantitative RT-PCR analysis for 7 anthocyanin biosynthetic pathway genes and 3 putative regulatory genes was also performed. Results showed that an R2R3 MYB transcription factor PyMYB10 was up-regulated in both the less-colored pear 'Zaobaimi' and well-colored red pear Yunhong-1 after the bag was removed, but that kinetics differed between cultivars. Other anthocyanin-related genes appeared to be coordinately regulated by the MYB–bHLH–WD40 complex. DFR and ANS genes seemed to be limiting factors for the peel coloration of less-colored pear 'Zaobaimi', while all biosynthetic steps are up-regulated by 7 days after bag removal in red fruit. This study suggests the regulation of red pear coloring is via differential effects of the MYB–bHLH–WD40 complex on the pear anthocyanin pathway genes.