Conversion of protocorm-like bodies of Dendrobium huoshanense to shoots: the role of polyamines in relation to the ratio of total cytokinins and indole-3-acetic acid.

In the present paper, a correlation between enhanced conversion of protocorm-like bodies (PLBs) of Dendrobium huoshanense to shoots by free polyamines (PAs) and changes in the levels of endogenous hormones is described. The endogenous levels of free spermidine (Spd) and putrescine (Put) increased during the conversion of PLBs to shoots. The exogenous addition of PAs, mainly Spd or Put at 2.0mM, not only elevated the endogenous levels of PAs but also promoted the frequency of conversion of PLBs to shoots. As compared with control, the enhanced conversion of PLBs to shoots by exogenous PAs was accompanied by an increase in ratio of total cytokinins (CTKs) to indole-3-acetic acid (IAA), which was due to decrease in the endogenous level of IAA and increase in the endogenous level of total CTKs, including the levels of isopentenyladenine+isopentenyladenine 9-riboside and zeatin+zeatin riboside. Analysis of enzyme activities showed that the increased endogenous level of total CTKs by PAs was related to the inhibition of CTK decomposition by CTK oxidase, while the decreased endogenous level of IAA was related to the promotion of IAA decomposition by IAA oxidase. Addition of PA biosynthetic inhibitors, involving alpha-difluoromethylornithine for Put and methylglyoxal(bis)-guanylhydrazone for Spd and Spm, decreased the conversion of PLBs to shoots, the ratio of total CTKs to IAA, and the levels of endogenous Put and Spd. This inhibition could be partly reversed by the application of exogenous Put or Spd.