Transcriptome Profiling Of Secondary Cell Wall Growth In Zenia Insignis

Zenia insignis is an ecologically and economically important tree species that grows in karst habitats of southern China and southern Asia. The trees are well adapted to the rocky desert conditions of karst because of their ability to produce a large root system and woody biomass. Despite of its importance, genomic information for this valuable tree species is very limited. Here, we characterize the transcriptome of Z. insignis stem tissues undergoing different phases of secondary cell wall formation and lignification, using the Illumina Hi Seq2500. The following summarizes our major research results:(1) More than 26.22 Gb clean data were generated and assembled into 94,580 unigenes with a mean length of 726 nt. 31,802 unigenes were annotated with different databases. We gained a large volume of data on genetic information of Z. insignis from this RNA-seq.(2) We used biochemical methods to analyze the patterns of lignin, cellulose and hemicellulose deposition in cell walls of different samples. The basal stem segments show significant amounts of lignification and secondary cell wall deposition in the xylem. The lignocellulose content in bottom stem segments was higher than in other tissues.(3) A comparative anatomy of three different stem segments of Z. insignis(apical, middle and basal) was analyzed. We found the xylem cells in basal stem segments to be more developed than the apical stem segments. The cell wall thickness reached a maximum of 2.066 ?m in basal stem, which far exceeded the cell wall thickness of other tissues. Using the xylan specific antibody LM10, we detected strong signals in the middle and basal stem segments. The FT-IR results correspond with the biochemical analysis.(4) RNA-Seq analysis of Z. insignis tissues revealed the expression characteristics of genes related to secondary cell wall synthesis. The quantitative PCR results of 19 genes, related to cell wall synthesis in Z. insignis, were consistent with the FPKM values of the transcriptome.(5) We dyed the sclerenchyma ring that is composed of two or three layers of cells in the cortex via phloroglucinol. This resulted in red staining and revealed a high degree of lignification. The RNA-seq of sclerenchyma generated more than 19.06 Gb clean data and assembled into 75,172 unigenes with a mean length of 853 bp. 39,159 unigenes were annotated with different databases.(6) Our analysis of the cell wall polysaccharide composition showed the sclerenchyma polysaccharide composition to be similar to that of the xylem. Using the xylan specific antibody LM10 resulted in strong signals in both sclerenchyma and xylem. The unigene expression related to cell wall synthesis of the sclerenchyma was similar to that of the xylem. We conclude from these results, that the molecular mechanisms of synthesis are similar for sclerenchyma and xylem.