Transcriptomic Analyses of Taproot Growth and Sucrose Accumulation at Different Developmental Stages in Sugar Beet (Beta vulgaris L.)

In sugar beet (Beta vulgaris L.), taproot size and sucrose content are important determinants of yield and quality. However, high yield and low sucrose content are two tightly bound agronomical traits. The advances in next-generation sequencing technology and publication of sugar beet genome have provided a method for the study of molecular mechanism underlying the regulation of taproot growth and sucrose accumulation. In this work, we performed comparative transcriptome analyses of the high yield cultivar SD13829 (SD) and the high sucrose content cultivar 04BS02 (BS) at five developmental stages. More than 50,000,000 pair-end clean reads for each library were generated. When taproot turned into the rapid growth stage of 82 day after emergence (DAE), three enriched GO terms, “cell wall”, “cytoskeleton” and “enzyme linked receptor protein signaling pathway”, occurred in both SD and BS. Differentially expressed genes (DEGs) of paired comparison for BS and SD were enriched in “cell wall”. For pathway enrichment analyses of DEGs that were respectively generated at 82 DAE compared to 59 DAE (before taproot turning into the rapid growth stage) in both SD and BS, “plant hormone signal transduction pathway” was enriched. Several transcription factor family members were up-regulated in rapidly increased taproot. An antagonistic expression of brassinosteroid- and auxin-related genes in taproots was necessary for the rapid enlargement of taproots at stage 82 DAE. In SD, the growth strategy was relatively focused on cell enlargement promoted by brassinosteroid signaling, whereas in BS was relatively focused on secondarily cambial cell division regulated by cytokinin, auxin and brassinosteroid signaling. Taken together, our data demonstrate that the size and sucrose content of taproots rely on the taproot growth strategy, which is controlled by cytokinin, auxin, GA and brassinosteroid signaling.