Effect of fruit maturity on UV-B-induced post-harvest anthocyanin accumulation in red Chinese sand pear

The effect of fruit maturity on UV-B-induced post-harvest anthocyanin accumulation in red Chinese sand pear (Pyrus pyrifolia Nakai) cultivar ‘Mantianhong’ was evaluated. During the irradiation, compared with the fruit harvested at 20 days before harvest (DBH) and 10 DBH, the mature fruit (harvested at commercial harvest date) had higher soluble solids content, soluble sugars concentration but lower firmness and starch content. In addition, higher content of anthocyanin has been detected in mature fruits than in immature fruits due to the significant increase in the expression of genes related to anthocyanin biosynthesis, especially PpCHS, PpF3H, PpANS, PpUFGT, PyMYB10 and PpbHLH in red Chinese sand pears. Hierarchical clustering analysis suggested that most genes related to anthocyanin biosynthesis showed a coordinate expression pattern. These findings are helpful in understanding the molecular mechanism of anthocyanin biosynthesis and regulation, which could lead to the development of new technologies for improving fruit color in Chinese sand pears and other fruits.     

BBX16, a B‐box protein,positively regulates light‐induced anthocyanin accumulation by activating MYB10 in red pear

The red coloration of pear (Pyrus pyrifolia) results from anthocyanin accumulation in the fruit peel. Light is required for anthocyanin biosynthesis in pear. A pear homolog of Arabidopsis thaliana BBX22, PpBBX16, was differentially expressed after fruits were removed from bags and may be involved in anthocyanin biosynthesis. Here, the expression and function of PpBBX16 were analysed. PpBBX16's expression was highly induced by white‐light irradiation, as was anthocyanin accumulation. PpBBX16's ectopic expression in Arabidopsis increased anthocyanin biosynthesis in the hypocotyls and tops of flower stalks. PpBBX16 was localized in the nucleus and showed trans‐activity in yeast cells. Although PpBBX16 could not directly bind to the promoter of PpMYB10 or PpCHS in yeast one‐hybrid assays, the complex of PpBBX16/PpHY5 strongly trans‐activated anthocyanin pathway genes in tobacco. PpBBX16's overexpression in pear calli enhanced the red coloration during light treatments. Additionally, PpBBX16's transient overexpression in pear peel increased anthocyanin accumulation, while virus‐induced gene silencing of PpBBX16 decreased anthocyanin accumulation. The expression patterns of pear BBX family members were analysed, and six additional BBX genes, which were differentially expressed during light‐induced anthocyanin biosynthesis, were identified. Thus, PpBBX16 is a positive regulator of light‐induced anthocyanin accumulation, but it could not directly induce the expression of the anthocyanin biosynthesis‐related genes by itself but needed PpHY5 to gain full function. Our work uncovered regulatory modes for PpBBX16 and suggested the potential functions of other pear BBX genes in the regulation of anthocyanin accumulation, thereby providing target genes for further studies on anthocyanin biosynthesis.     

Evidence for a photoprotective function of low-temperature-induced anthocyanin accumulation in apple and pear peel

The light requirement and low-temperature stimulation of anthocyanin synthesis in peel of apple ( Malus domestica ) and pears ( Pyrus communis ) and the presence of anthocyanins in immature fruits are not congruent with a visual function in dispersal. We hypothesized that anthocyanins afford photoprotection to peel during low-temperature-induced light stress and that the protection is not a fortuitous side-effect of light absorption by anthocyanin. The extent of photoinhibition at harvest and after light stress treatment in pear cultivars differing in redness decreased with increasing red color on the sun-exposed sides of fruits. Green-shaded sides of the pears showed comparable levels of photoinhibition indicating that pears did not differ in their inherent photosensitivity. Apple and pear peel show considerable short-term fluctuation in redness in response to temperature, with red color increasing rapidly in response to low temperature and just as quickly fading in response to high temperature. Briefly, shading pears and apples during cold conditions for 2 days reduced the accumulation of anthocyanin and increased the photosensitivity of peel. Subsequent shading during warm conditions did not affect the accumulation of anthocyanin or the photosensitivity of peel indicating that the response at low temperature was not due to shade adaptation. The assessment of photosystem II (PSII) efficiency and quenching of chlorophyll fluorescence between 16 and 40°C indicated that 'Forelle' pear peel was particularly sensitive to photostress at low temperature. The photosynthetic system in mature 'Forelle' leaves was comparatively much less sensitive to light stress at low temperature. Results support the view that anthocyanins are adaptable light screens deployed to modulate light absorption in sensitive tissues such as fruit peel in response to environmental triggers such as cold front snaps.