Evidence of the predominance of passive symplastic phloem loading and sugar transport with leaf ageing in Camellia oleifera

Phloem loading and transport of sugar from leaves to sink tissues such as fruits are crucial for yield formation.Camellia oleifera is an evergreen horticultural crop with high value;however,its low production limits the development of the C.oleifera industry.In this study,using a combination of ultrastructural observation,fluorescence loss in photobleaching(FLIP)and inhibitor treatment,we revealed that C.oleifera leaves mainly adopt a symplastic loading route from mesophyll cells to the surrounding vascular bundle cells in minor veins.HPLC assays showed that sucrose is the main sugar transported and only a small amount of raffinose or stachyose was detected in petioles,supporting a passive symplastic loading route in C.oleifera leaves.Compared to leaves grown this year(LT),the carbohydrate synthesis capacity in leaves grown last year(LL)was decreased while LL retained more soluble sugar,suggesting a decrease in transport capacity with leaf ageing.TEM and tissue staining showed that a reduction in plasmodesmata density leads to a decline in the degree of cellular coupling and is responsible for the weakening transport capacity in older leaves.RNA-seq revealed several differentially expressed genes(DEGs)including CoPDCB1-1,CoSUT1 and CoSWEET12,which are likely involved in the regulation of phloem loading and sugar transport.An expression correlation network is constructed between PD-callose binding protein genes,sugar transporter genes and senescence-associated genes.Collectively,this study provides the evidence of the passive symplastic phloem loading pathway in C.oleifera leaves and constructs the correlation between sugar transport and leaf ageing.

Comparative transcriptome profiling reveals the defense pathways and mechanisms in the leaves and roots of blueberry to drought stress

Blueberry is an important horticultural plant that is very susceptible to drought.However,the molecular regulation mechanisms of blueberry tolerance to drought remain elusive.In this study,the transcriptome profile of blueberry by RNA-seq under different degrees of drought treatment were conducted and drought-related genes and pathways were screened using weighted gene co-expression network analysis(WGCNA)and Venn analysis.Interestingly,the leaves and roots of blueberry exhibited different expression patterns under drought treatment and differentially expressed genes(DEGs),GO terms and KEGG pathways were more enriched in leaves than in roots.The majority of DEGs were associated with signal transduction,transcriptional regulation,and metabolism.Eight key pathways and eight DEGs were shared both in leaves and roots.Notably,the pathway with the most core genes in leaves is the plant hormone signal transduction pathway,and in roots are the MAPK signaling pathway,reactive oxygen species metabolism and the key genes such as VcXTH3,VcPP2C51,and VcPNC1 were identified.For transcription factors,VcABR1,VcABF2,VcMYB108 and VcMYB93 are likely involved in drought response.In the metabolism category,VcCYP75A1 was likely involved in anthocyanin biosynthesis,and VcPNC1 in the monoterpenoid biosynthesis pathway.The eight DEGs markedly induced under drought exhibited differential expression in leaves and roots.Correspondingly,physiological assays showed that POD activity,ABA content,and anthocyanin biosynthesis in leaves and roots were significantly increased.Collectively,our study revealed the synergistic and distinct defense pathways and mechanisms in leaves and roots of blueberry and explored the potential regulatory network in blueberry response to drought stress.