The production of reactive oxygen species (ROS) by NADPH oxidase, which is also referred to as respiratory burst oxidase homolog (RBOH), affects several processes in plants. However, the role of RBOHs in cell wall lignification is not well understood. In this study, we show thatPuRBOHF, an RBOH isoform, plays an important role in secondary wall formation in pear stone cells. ROS were closely associated with lignin deposition and stone cell formation according to microscopy data. In addition, according to the results of an in situ hybridization analysis, the stage-specific expression ofPuRBOHFwas higher in stone cells than in cells of other flesh tissues. Inhibitors of RBOH activity suppressed ROS accumulation and stone cell lignification in pear fruit. Moreover, transient overexpression ofPuRBOHFcaused significant changes in the amount of ROS and lignin that accumulated in pear fruit and flesh calli. We further showed that PuMYB169 regulatesPuRBOHFexpression, whilePuRBOHF-derived ROS induces the transcription ofPuPOD2andPuLAC2. The findings of this study indicate thatPuRBOHF-mediated ROS production, which is regulated by a lignin-related transcriptional network, is essential for monolignol polymerization and stone cell formation in pear fruit.

The improvement of fruit quality, in particular sugar content, has been a major goal of plant breeding programs for many years. Here, two varieties of the Ussurian pear (Pyrus ussuriensis), Nanguo, and its high-sucrose accumulation bud sport, Nanhong, were used to study the molecular mechanisms regulating sucrose transport in fruits. Comparative transcriptome analysis showed that in Nanhong fruit, a MYB transcription factor, PuMYB12 and a sucrose transporter protein, PuSUT4-like were expressed at higher levels, while a PRE transcription factor, PuPRE6 and a histone deacetylase, PuHDAC9-like were expressed at lower levels than in Nanguo fruit. PuSUT4-like silencing and overexpression experiments in Nanguo pear showed that PuSUT4-like is essential for sucrose transportation. PuPRE6 and PuMYB12 act as antagonistic complexes to regulate PuSUT4-like transcription and sucrose accumulation. The histone deacetylation levels of the PuMYB12 and PuSUT4-like promoters were higher in Nanguo fruit than in Nanhong fruit, and Y1H assays showed that histone deacetylase PuHDAC9-like bound directly to the promoters of PuMYB12 and PuSUT4-like. Our results uncovered transcription regulation and epigenetic mechanisms underlying sucrose accumulation in pears.