The starch-sugar interconversion mechanism during bulb development of Cardiocrinum giganteum (Wall.) Makino revealed by transcriptome and metabolite analysis

Cardiocrinum giganteum bulbs are rich in starch and natural compounds that benefit health. To better utilize the bulb starch of C. giganteum as an edible resource, understanding of the kinetics of starch and sucrose accumulation as well as the involved metabolic pathways in different developmental stages of the bulbs are required. In this study, we compared the dynamic changes in the transcriptomes and metabolomes of the bulbs over different developmental stages. A total of 43,692 differentially expressed genes and 508 metabolites were identified, which were clustered into nine co-expression clusters based on biological functions. Starch and sucrose metabolic pathways were identified at both the transcriptome and metabolome levels, revealing that activating the pathways of starch synthesis as well as sucrose synthesis and hydrolysis could be main conversion modes in starch-sugar interconversion. The data showed that the starch synthesis genes GBSS, SS3, GBE and ISA were expressed to highest levels at the stage of three-four years of C. giganteum development (c3y-c4y), concomitantly with the highest starch contents in this period. This revealed that the starch from the developing bulbs is optimally harvested at this stage. Meanwhile, sucrose synthesis genes were active and sucrose metabolites were maintained in balance during this stage, providing carbohydrate resources for starch biosynthesis. In the four-five year stage of development (c4y-c5y), the contents of sucrose metabolites increased, starch synthesis slowed down and sucrose hydrolysis genes were activated. Moreover, qRT-PCR analysis showed that the temporal expression patterns of starch and sucrose biosynthesis genes were largely consistent with the RNA-seq results. Taken together, these findings provide important information on the regulatory mechanisms of starch-sugar interconversion during C. giganteum bulb development.