Research progress in the regulation mechanism of lignin synthesis in pear stone cells

Abstract: Stone cells are sclerenchyma cells formed by deposition of lignin, which is the most significant factor limiting the quality of pears. Therefore, it is of great significance to explore the mechanism of lignin synthesis regulation in pear fruit development for the genetic regulatory network of stone cell traits. In this paper, the mechanisms of transcriptional factors, hormones, sugars, calcium, reactive oxygen species, light quality and pollen sensitivity in lignin synthesis in pear fruits are reviewed, aiming to provide reference for the regulatory network of lignin in pear fruits and genetic improvement of fruit quality. Several transcription factors involved in lignin biosynthesis have been identified in pears, such as MYB, NAC, bZIP, KNOX and zinc finger protein, among which MYB and NAC transcription factors play key regulatory roles in lignin synthesis. Study has showed that PbrMYB169, an R2R3MYB transcription factor of Pyrus bretschneideri, positively regulates lignification of stone cells in pear fruits. On the one hand, PbrMYB24 activates the transcription of lignin and cellulose biosynthesis genes by binding to different cis- element. On the other hand, PbrMYB24 binds directly to the promoters of PbrMYB169 and PbrNSC, activating the gene expression. Moreover, both PbrMYB169 and PbrNSC activate the promoter of PbrMYB24, enhancing gene expression. Research has identified PbMYB61 and PbMYB308 as candidate transcriptional regulators of stone cell formation, revealing that PbMYB61 regulates stone cell lignin formation by binding to the AC element in the PbLAC1 promoter to upregulate expression. Exogenous application of 200 μmol·L-1 NAA can reduce stone cell content and also significantly decrease the expression level of PbrNSC encoding a transcriptional regulator. In addition, PbrARF13-PbrNSC-PbrMYB132 regulatory cascade mediates the biosynthesis of lignin and cellulose in stone cells of pear fruit in response to auxin signals. Research has found several hormone-responsive el-ements in the upstream regulatory sequences of PbPALs family members. ABA, SA and MeJA could regulate the expression of PbPAL1 and PbPAL3 genes, and affect the formation of fruit stone cells. A series of experiments have proved that PbUGT72AJ2 mediates glycosylation by catalyzing the glucose conjugation of monolignols and may affect the expression of downstream genes as well as the content of monolignols to affect the lignin deposition and stone cell development in pear fruit. The treatment of exogenous glucose has significantly enhanced the accumulation of lignin in pear calli. Expression of structural genes (PbPAL, PbHCT, PbCOMT and PbPRX) in lignin biosynthesis is up-regulated after glucose treatment. Transien expression of PbPFP has resulted in a significant increase of lignin content in Dangshansuli fruits on 35th day after full bloom (DAB) and in tobacco leaves, indicating that PbPFP might be associated with the enhancement of lignin biosynthesis in response to glucose treatment. Ca2+ is known to inhibit stone cells in pear fruits. In order to further explore how calcium-nitrate treatment affects lignin synthesis, the PbCML3 has been identified in pears and relevant experiments have been conducted to find that the overexpression of PbCML3 would increase the content of pear stone cell. Further analysis has identified a transcription factor, PuDof2.5, and its targets gene PuPRX42- like (lignin polymerase gene) expression has decreased in CaCl2-treated samples, which are involved in suppressing lignin biosynthesis in pear fruit. ROS is closely associated with lignin deposition and stone cell formation. Research has showed that PuRBOHF, an RBOH isoform, plays an important role in secondary wall formation in pear stone cells. Inhibitors of RBOH activity suppress ROS accumulation and stone cell lignification in pear fruit. Moreover, it has been showed that PuMYB169 regulates PuRBOHF expression, while PuRBOHF- derived ROS induces the transcription of PuPOD2 and PuLAC2. Research has showed that secondary cell wall thickening and lignin accumulation in pears may regulate by different wavelengths of light. It is reported that CRY-mediated blue-light signal plays an important role in cell wall lignification and promotes the formation of stone cells in pears by regulating downstream genes. Results have showed that blue light induces the expression of lignin structure genes and promotes lignin accumulation. Furthermore, four blue light receptors cryptochromes have been identified in white pear, named PbCRY1a, PbCRY1b, PbCRY2a and PbCRY2b. Previous studies have reported that pollination affects the expression of laccase gene microRNA in pear fruits, and the expression of peroxidase 47 (PER47), β-glucosidase (BGLU15) and laccase-4 (LAC4), thus affecting lignin synthesis. This finding demonstrates that pollination with different sources of pollens affects the synthesis of lignin in pear fruit on the levels of gene and protein expression.