Gene Expression Analysis In Mesophyll Cell And Bundle Sheath Cell Of Rice And Maize

C₄ photosynthesis performs approximately 50%higher efficiency than that of the C₃ pathway.To successfully install the C₄ photosynthetic pathways into a C₃ plant,it is essential to clarify the molecular mechanisms regulating photosynthetic genes that are differentially expressed in leaf mesophyll cells（Mc）and bundle sheath cell（BSc）.Therefore,the aims of this study are:（1）to obtained pure Mc and BSc population from rice and maize leaf for cell specific RNA-seq by employing LCM technique;（2）to analyse differential expression of BSc-,and Mc-specific genes in rice–a C₃ plant;and（3）to analyse differential expression of BSc-,and Mc-specific genes in maize–a C₄ plant.In rice,a total of 35,956 genes of either BSc or MSc were identified,with 2,068 BSc-specific genes and 5,842 Mc-specific genes.Photosynthetic genes,including OsSET13 and OsSET26（Rubisco LS methyltransferase substrate-binding domain）,CAB2R/OsLhcb1.2 and RCABP89/Os Lhcb2（Chlorophyll A-B binding protein）,and PsaH and Psb X（components of photosystems）,and hypothetical genes CHP01589,K09122,and OsPM8ATP were preferably expressed in Mc;however,minor expression of these gene in BSc indicated photosynthesis activities also took place in this cell.In the other hand,most rice BSc-specific genes,including GST and OsATL5（substrate transporters）,AGPS and UBC2（biosynthesis activities）,and an uncharaterised gene,are involved in substrate transportation,and metabolic pathways.Gene Ontology（GO）classification showed that differentially expressed genes in both two cells were mostly enriched in cellular component and biological processes.Transcriptome analysis of maize BSc and Mc showed that there are 4,344 genes differentially expressed between BSc and Mc,among which 1,038 genes were up-regulated and 3,302 genes were down-regulated in BSc.Transcripts of core photosynthetic genes accumulated at least 2 folds higher in one cell type versus the other.Genes involved in CO₂ concentrating mechanism（CA6,PEP4,and MDH6）,triose phosphate reduction（PGK,and TIMs）,and photosystems（ZmLhcb3 and FDX6）were preferentially expressed in Mc,whereas genes encoding Calvin cycle enzymes（RBCS1 and RLSB）,gene Tie-dyed2 responsible for vein development,and genes involved in transcription factors,transporters,abiotic stress response and protein degradation（YAB11,ZmNAC89,UMC1236,and ZmUBC-02）highly expressed in BSc.These results showed the tight regulation and specific expression of core C₄ genes in maize BSc and Mc,ensuring effective cooperated functioning of the two cells in photosynthesis.In addition to well-identified genes,some uncharacterised genes were also highly enriched in one cell type,this suggested that cooperating activities of C₄ genes might be regulated by some other pathways.Moreover,the preferential cell-specific expression of these genes might imply an overall differentiation in the regulatory networks and mentioned pathways underlying the functional differentiation of the two photosynthetic cell types in C₄ leaves.