Effects of arbuscular mycorrhizal fungi and exogenous polyamines on the growth and root system architecture of tree peony seedlings from embryo culture

Tree peony (Paeonia sect. Mouton) is a world-fa-mous ornamental and economically important spe-cies, but its conventional propagation methods se-verely limit industrial development. When tissue culture is an attractive option, the acclimatization becomes a bottleneck due to the poor root system architecture (RSA) with limited lateral roots and ar-rested growth of seedlings during ex vitro establish-ment. In this paper, we studied the seedling growth of P rockii 'Ding Hong' during acclimatization under various treatments with Arbuscular mycorrhizal fungi (AMF) inoculation or/and putrescine (Put) ap-plication. We found that AMF or Put alone optimized seedling growth and RSA in a species-dependent and concentration-dependent manner, respectively, and the combined uses of Claroideoglomus etunicatum (C.E.) with exogenous Put promoted additionally survival, growth and RSA traits of the seedlings ver-sus individual treatment, and the effects were de-pendent on concentration, with 0.05 mM Put in the C.E.-inoculated treatments being the most effective. Our studies also suggested that AMF inoculation and exogenous Put application improved morphological parameters via alterations of endogenous polyam-ines (PAs). The most obvious change was the dra-matic increase in spermidine (Spd), thus leading to a significant increase in both total PAs and the ratio of [spermine (Spm)+Spd]/Put and a decrease in Put/ PAs. Correlation analysis indicated that the overall balance of endogenous PM might be more import-ant than a specific one in root development, with the number of correlated parameters ranked as Put/ PAs > Spm+Spd/Put> Spd, total PM, Put> Spm. These results provide additional information for an im-proved ex vitro establishment of peony seedlings and enrich PA regulation theory.