Arbuscular mycorrhiza enhances auxin levels and alters auxin biosynthesis in Tropaeolum majus during early stages of colonization

Abstract

Plant hormones, including auxins, might be signals during the establishment of an arbuscular mycorrhizal (AM) symbiosis. Here, we report on the concentrations of three auxins native to nasturtium (Tropaeolum majus L.) during early AM development. Indole‐3‐acetic acid (IAA), indole‐3‐butyric acid (IBA), and phenylacetic acid (PAA) were previously identified as endogenous compounds in this species by full‐scan gas chromatography–mass spectrometry. All auxinic compounds were influenced by AM colonization but showed completely different patterns. At very early stage, free IAA and IBA were lower in infected than in control roots, whereas PAA concentration was higher in infected roots than in controls. At later stages, PAA was reduced in colonized roots, whereas, especially, IBA was increased in colonized roots compared with controls. Measurement of total auxins confirmed a complex regulation pattern for the three compounds. In hyphae of Glomus intraradices, none of the auxins was detectable. Biosynthesis of the three auxins was measured using heavy labeled isotopes as precursors in control and AM‐inoculated roots. While not much difference was found in the IAA labeling pattern between controls and AM‐inoculated roots at both time points, IBA synthesis was slightly higher in AM‐inoculated roots. Double labeling experiments showed that two distinct pathways, a tryptophan‐dependent and a tryptophan‐independent biosynthetic pathway contribute to the synthesis of IAA in T. majus roots. Because T. majus is difficult to genetically manipulate, we have used tobacco plants transformed with the auxin‐inducible promoter GH3 fused to the β‐glucuronidase (GUS) reporter gene to investigate whether AM structures would co‐localize to cells harboring the auxin‐inducible promoter. Although the GUS activity increased significantly in AM‐inoculated roots, there was no obvious correlation between GH3::GUS expression and fungal structures.

Stars and Symbiosis: MicroRNA- and MicroRNA*-Mediated Transcript Cleavage Involved in Arbuscular Mycorrhizal Symbiosis

“…Targets for both miRNAs were confirmed independently by our degradome analysis and by Jagadeeswaran et al (2009). Auxin has been demonstrated to play a crucial role in AM symbiosis, probably in the presymbiotic phase of AM fungal development (Hanlon and Coenen, 2011), and the auxin content is elevated in mycorrhizal roots (Fitze et al, 2005;Jentschel et al, 2007). The fact that both miRNAs were significantly up-regulated in mycorrhizal roots and target transcripts of the same gene family implies that miR160c and miR167 might be involved in the fine regulation of auxin responses in AM symbiosis.…”

Regulação do desenvolvimento de micorrizas arbusculares

“…Análises de auxinas por cromatografia gasosaespectrometria de massas mostraram que as concentrações de AIA, AIB e ácido fenilacético (AFA) em raízes de Tropaeolum majus colonizadas por G. intraradices são diferencialmente alteradas em relação ao controle não-micorrizado (Jentschel et al, 2007). Nos estádios inicias (10 dias após a inoculação), as concentrações de AIA e AIB livres são menores nas raízes colonizadas, enquanto a concentração de AFA aumenta em relação aos controles não-micorrizados.…”

“…Fenômeno similar foi observado em raízes de M. truncatula colonizadas por G. intraradices (Campanella et al, 2008). Esses aumentos de concentração de AIB são decorrentes de maior síntese do FH, conforme demonstrado com o uso de precursores marcados com isótopos estáveis ( 13 C, 15 N e 2 H) em T. majus (Jentschel et al, 2007). Esses resultados sugerem que o acúmulo preferencial de AIB em MAs pode contribuir para a regulação do desenvolvimento da simbiose.…”

Auxins as Signals in Arbuscular Mycorrhiza Formation

“…Although the GUS activity increased significantly in AM inoculated roots, there was no obvious correlation between GUS expression patterns and fungal structures. 13 Nasturtium (Tropaeolum majus L.) is an interesting species in which to study the relationship between auxin and glucosinolate (GSL) metabolism as well as the role of these compounds during symbiosis, due to its relative simple GSL pattern compared to other plant species. 14 Despite the fact that high levels of benzylglucosinolate are present in roots of nasturtium, the colonization rate with different AM fungal species was comparable to non-GSL plant species.…”