Investigating Hormone Regulation and Sugar Storage during Tuber Development in Turnip Plants (Brassica rapa)

Brassica rapa belongs to Brassicaceae family with diploid (2n=20) genotype and this species consists of morphological diverse crops. It includes leafy vegetables, turnip vegetables, and vegetable oil. This study focused on investigation of hormones and sugar storage during turnip tuber formation in turnip plants. First, sugar and starch content in mature turnip tubers of 19 turnip accessions from different geographical origin and growth conditions (Greenhouse and field) were measured by enzymatic analysis. Glucose and fructose content were higher than sucrose content in most of the accessions and the total sugar and starch content in tubers grown in greenhouse were higher than the ones grown in the field. VT-053 was unique from all accessions, as it had high sucrose content in both field and greenhouse grown conditions. For sugar translocation study, four (SM-17, FT-004, FT-086 & DH-VT-117) accessions were used which collected at different growth stage. Average total sugar content was lower in leaf tissues than in turnip tissues in all tested growth stages. In addition, the average total glucose content was also lower at later growth stages (4 & 5) in shoot and tuber tissues. This might show the flow of sugar from the leaves (source) to storage organ. Based on these data, VT-115, VT-117, VT-012, VT-053, VT-052, FT-002, YS-143 and red round turnip accessions were selected and grown in vitro condition to assess the effect of sucrose (5, 6, 7, & 8%), medium (Standard MS-medium and Tuber Inducing Medium), photoperiod (long day (LD16) & the shortest day (SD8)) and hormones (BAP and GA3) in vitro condition. The result of this part showed that six percent sucrose with the standard MS medium in the long day condition induced turnip tuber formation. GA3 hormone did not show a significant difference in turnip formation for all (0, 3.5, 7.5 and 11.5mg/l) tested concentrations. Application of exogenous BAP (5, 10, & 15mg/l) hormone inhibited turnip tuber formation and turnip growth. Third, gene expression patterns of 21 selected genes in two tissue types (shoot and tuber) from six different growth stages of VT-117 were quantified using qRT-PCR. The result of this part indicated that Out of 21 genes VTuni_02, VTuni_05, VTuni_09, VTuni_14, VTuni_30, BraDDF1, BraARF1 and BraARF3_2 might be involved in turnip tuber formation.