Morphocytological Observation Of Rhizome Formation And Transcriptome Analysis In Gynostemma Pentaphyllum

Gynostemma pentaphyllum(Thunb.)Makino,belonging to the genus Gynostemma in the Cucurbitaceae family,is a perennial herb.It has important medicinal value because its capability of synthesizing and accumulating ginsenosides.Before entering the winter,the subapical regions of some aerial stems swell and drill into the soil to form rhizomes that produce new plants in the coming year.The transition of aerial stem-to-rhizome is an adaptive reproductive strategy of G.pentaphyllum.However,the molecular regulation of aerial stem-to-rhizome transition is poorly understood.In this study,transcriptome sequencing is performed for representative developmental stages of aerial stem-to-rhizome transition,namely unswelled aerial stem(stage 1),aboveground moderately swelling stem(stage 2),and underground newly formed rhizome(stage 3).And this study also combined it with morphocytological observation to explore the molecular mechanism of the aerial stem to rhizome transition in G.pentaphyllum.The main results of this study are as follows:(1)In the process of aerial stem-to-rhizome transition of G.pentaphyllum,there was no obvious change in tissue structure,and stems at transition stages 1,2,and 3 were all composed of epidermis,cortex,vascular bundles arranged along the stem circumference,and pith from outside to inside.But the cells became much bigger as the transition of aerial stem-to-rhizome.Compared with the stage 1 and stage 2,the number of starch grains increased dramatically and their volumes also became larger in the cell of stage 3.The starch grains mainly accumulated in the starch sheath of cortex and pith.(2)Transcriptome sequencing was performed on the nine RNA libraries derived from three developmental stages of aerial stem-to-rhizome transition in G.pentaphyllum,and total of 207,635 transcripts and 100,1 19 unigenes were generated.After aligning the sequences of all-unigenes with NR,GO,KEGG,KOG,COG,Pfam,Swiss-Prot and EggNOG databases,55,333 unigenes genes were annotated.(3)Based on the cut-off values of FDR<0.01 and log2 fold change?1 or ?-1,pairwise comparisons were conducted among three developmental stages of aerial stem-to-rhizome transition,and a total of 5428 differentially expressed genes(DEGs)were identified.KEGG pathway enrichment analysis identified that these DEGs were significantly enriched in the pathways related to plant hormone signal transduction pathways,phenylpropanoid biosynthesis,photosynthesis,as well as starch and sucrose metabolism.(4)Among the 5,428 identified DEGs,the gravitropism-related DEGs cooperatively controlled the gravitropic response during aerial stem-to-rhizome transition,thereby enabling the rhizome to acquire a positive gravitropism phenotype similar to that of roots and then growing into soil.Meanwhile,DEGs related to phenylpropanoid biosynthesis,photoperiod,hormone signal and transduction,carbohydrate metabolism,and other DEGs associated with rhizome formation were also found to be involved in regulation of the aerial stem-to-rhizome transition.In this study,the regulatory molecular networks and mechanisms of the aerial stem to rhizome transition in G.pentaphyllum were investigated through the morphocytological observation and DEGs mining,which broaden our understanding of mechanism of developmental transition in plants.