Proteomic analysis of the key mechanism of exocarp russet pigmentation of semi-russet pear under rainwater condition

Abstract

This study aimed to explore the mechanism of exocarp russet pigmentation of semi-russet ‘Cuiguan’ sand pears at the proteome level following rainwater. Exocarp russeting was characterized in semi-russet sand pears cultivated in the rain shelter and control (rainwater) groups via microscopy and ultra-microscope observations, followed by determination of lignin content and enzyme activities. The differentially expressed proteins between different cultivation conditions were analyzed using the isobaric tags for relative or absolute quantitation (iTRAQ) technology, followed by confirmation by real-time quantitative polymerase chain reaction (qPCR). The rust color of the outer pericarp of ‘cuiguan’ was rapidly accumulated on 60–80 days after full bloom (DAFB) in both two groups. Under the shelter condition, most stratum corneum cracks were shallow and had less suberization. The lignin content as well as the enzyme activities of phenylalanine ammonia lyase (PAL), cinnamyl alcohol dehydrogenase (CAD), catechol-O-methyltransferase (COMT), cinnamoyl-CoA reductase (CCR) and peroxidase (POD) in exocarp from the rain shelter group were all lower than that in control group. Additionally, 309 differentially expressed proteins were identified in the rain shelter group, and eight down-regulated proteins associated with lignin monomer synthesis were identified, such as PAL (E9JWF1), CCR1 (F4Y9E7), CAD (U3PGC0), two peroxidases (K9URQ0 and M5W3Y0, respectively), two COMT (S5TPC3 and Q09K02, respectively) and omega-hydroxypalmitate O-feruloyl transferase (HHT1) (M5VMR7), and their expression was confirmed by qPCR. Moreover, many defense proteins were significantly down-regulated. These findings suggest that the rain shelter treatment may affect phenylalanine lignin monomer synthesis and subsequent defense metabolism by regulating corresponding proteins, thus decreasing exocarp russet accumulation in semi-russet pear. This study provides a further understanding of pear russeting and plant defense mechanisms.