Transition from somatic embryo to friable embryogenic callus in cassava: Dynamic changes in cellular structure, physiological status, and gene expression profiles

Friable embryogenic callus (FEC) has been considered as the most suitable materials for efficient genetic transformation of cassava. Due to heavy genotype dependence on FEC induction and being amenable to somaclonal variation, the production and maintaining of reliable FEC is a limitation factor for cassava genetic transformation. Identification of key elements involving in biological processes from somatic embryos (SEs) to the FEC at different stages will provide critical insights for FEC improvement. Cytological observation showed the dramatic change of subcellular structures among SEs, fresh FEC (FFEC) and old FEC (OFEC). Decrease of sucrose and increase of fructose and glucose were detected in OFEC. A total of 6871 differentially expressed genes (DEGs) were identified from SEs, FFEC and OFEC by RNA-seq. Analysis of the DEGs showed that FEC induction was accompanied by the process of dedifferentiation, whereas the epigenetics modification occurred during the continuous subculturing process. The cell structure was reconstructed, mainly including the GO terms of "cell periphery" and "external encapsulating structure"; in parallel, the internal mechanisms were changed correspondingly, including the metabolisms of process (glycolysis) and compounds(alanine, aspartate, and glutamate). The significant reduction of genomic DNA methylation in OFEC indicates altered gene expression via chromatin modification. These results indicate that the induction and long-term subculture of FEC is a complicated biological process involving changes of genome modification, gene expression and subcellular reconstruction. The findings will be useful for improving FEC induction and maintenance from farmer-preferred cassava cultivars that are recalcitrant to genetic transformation, hence improving cassava through genetic engineering. Overall design: Two-week-old SEs cultured on GD, FFEC emerging from SEs, and 9-month-old OFEC were used for analysis. Each sample was mixtured from six indepedent replicates and were collected for RNA extraction respectively.

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Ma Q, Zhou W, Zhang P