Arbuscular mycorrhizas alter root system architecture of Citrus tangerine through regulating metabolism of endogenous polyamines

Arbusculuar mycorrhizal fungi (AMF) optimize root system architecture (RSA) of their host plants, whilst polyamines (PAs), including putrescine (Put), spermidine (Spd) and spermine (Spm), play an important role in primary, lateral and adventitious root development. However, the interaction of AMF and PAs on RSA is less studied. In a pot study, 0, 5, 10, 20 and 40 g fresh inocula of Glomus mosseae (Nicol. & Gerd.) Gerd. & Trappe were inoculated into rhizosphere of 9-day-old red tangerine (Citrus tangerine Hort. ex. Tanaka) seedlings. After 164 days of inoculation, AM colonization ranged from 26.6 to 54.2% and increased with the increase of G. mosseae inocula amount. Mycorrhization generally significantly increased RSA traits such as total length, projected area, surface area, volume, 0-1 cm classified root length, and the ratio of the 0-1 cm classified root length to the total length, but decreased root average diameter and the proportion of 1-2 cm classified root length. Mycorrhization also generally increased both leaf and root arginine and ornithine decarboxylase (ADC and ODC) activity, thus stimulating the synthesis of Put. In leaves, more Put was converted to Spd, but not to Spm, resulting in a Spd increment in leaf. By contrast, root Put was induced by AMF, which indirectly improved the root average diameter and proportion of fine roots, since the RSA traits were significantly correlated with root Put synthetases through ADC and ODC. Our results indicate that AMF might regulate endogenous PAs metabolism, thus resulting in a synergetic impact on RSA.