Abstract: [Objective] To elucidate the molecular mechanism for enhancing photosynthetic capacity of the exocarp of Carya carthayensis fruits pollinated by Carya illinoensis in order to provide a theoretic basis for further research on metaxenia.[Method] Transcriptome sequencing was performed for exocarp of C. cathayensis respectively pollinated by C. cathayensis (marked as hp) and C. illinoensis (marked as pp) with samples collected 65 days after pollination (65DAP) when the fruit rapidly expanding in size. Gene enrichment analysis was conducted for pathways of chlorophyll synthesis, photo-response and carbon assimilation in combination with variation in chlorophyll content, photosynthetic rate and activity of key enzymes involved in the photosynthesis.[Result] Compared to the fruit pollinated with hp, the chlorophyll content, photosynthetic rate, Rubisco activity, and phosphoenolpyruvate carboxylase (PEPC) activity of exocarp in the fruit pollinated with pp were 1.31 (P=0.047), 1.12 (P=0.000 43), 1.65 (P=0.036), and 1.23 (P=0.001 3) folds, respectively. A total of 32 908 scaffolds were generated by transcriptome sequencing, and 66 photosynthesis-related genes were screened from 1 894 differentially expressed genes (DEGs) (P <0.05,fold change>1.5), which were mainly involved in pathways of chlorophyll synthesis and photosynthetic carbon fixation. The expression of magnesium-chelatase subunit (CHLH), agnesium-protoporphyrin IX monomethyl ester cyclase (CRD), and chlorophyll(ide) b reductase(NYC) of the fruit pollinated with pp was significantly up-regulated while that of pheophorbide a oxygenase (PAO) was significantly down-regulated in the chlorophyll biosynthesis pathway, and the expression of 16 light harvesting pigment protein complexes (LHCⅡ)-coding genes, ribulose bisphosphate carboxylase/oxygenase activase (RCA), alpha carbonic anhydrase (CA) in the photosynthetic carbon fixation pathway were significantly up-regulated. 42 genes involved in photoprotection of exocarp are also showed significantly high expression in the fruit pollinated with pp than those in the fruit pollinated with hp.[Conclusion] In the rapid growth period of Carya carthayensis fruits, the expression of magnesium-chelatase subunit (CHLH), agnesium-protoporphyrin IX monomethyl ester cyclase (CRD), chlorophyll(ide) b reductase (NYC), light harvesting pigment protein complexes-coding genes (LHCⅡ), light repair protein-coding genes (PSAN, PSAB27, STN7), 38 heat shock proteins (HSP), ribulose bisphosphate carboxylase/oxygenase activase (RCA) and alpha carbonic anhydrase (CA) was significantly up regulated for the pp cross combination. It indicates that the significantly up-regulated expression of genes involved in chlorophyll synthesis, light capture and carbon assimilation might be related to the significantly higher photosynthetic rate in the exocarp of pp cross combination.

Key words: Carya cathayensis, Carya illinoensis, metaxenia, photosynthetic capacity, differential expression of gene