Phytohormone homeostasis in in vitro grown kohlrabi: the effect of cytokinins and sucrose

Abstract

This study fathoms a comprehensive hormone profiling of four growth stages of kohlrabi (Brassica oleracea var. gongylodes) seedlings in vitro (T1-T4), including the effects of cytokinins, CKs (trans-zeatin, transZ; thidiazuron, TDZ), and high sucrose concentration (6% and 9%). Depending on the type of investigated phytohormone and its metabolite, individual changes were observed on a time scale, showing specific hormone profiles. A significant increase in the content of endogenous zeatin nucleobases and gibberellin GA20 was detected at T2 stage (plantlets with two emerged leaves). However, levels of jasmonic acid, JA-isoleucine, indole-3-acetic acid (IAA) and indole-3-acetamide significantly increased as development progressed (T3), whereas content of most of analysed IAA metabolites decreased. Supplementing growth media with CKs induced de novo shoot formation during seedling development, and both CK and sucrose treatments caused significant changes in levels of most of the phytohormone groups at each developmental stage compared to control. Principal component analysis indicated that sucrose treatments, particularly 9%, had a stronger effect on the content of endogenous hormones than the CK treatments. The difference between two CK treatments proved to be remarkable at T1, T2, and T3 stages, while at the last stage difference was much smaller. The joint application of CKs and high sucrose concentration showed diverse relations depending on the metabolite, growth stage and amount of sucrose used. The most striking increase was induced in T4 for zeatin nucleobases when transZ and 9% sucrose were applied. Our results indicate specific patterns of metabolic balance during kohlrabi in vitro development and regeneration, revealing a complex interplay of different phytohormonal groups with CKs and sucrose.