Accumulation of and Response to Auxins in Roots and Nodules of the Actinorhizal Plant Datisca glomerata Compared to the Model Legume Medicago truncatula.

Irina V Demina,Ulrike Mathesius,Thomas Roitsch,Anurupa Nagchowdhury,Eric Van Der Graaff,Katharina Pawlowski,Pooja Jha Maity,Jason L P Ng,Kirill N Demchenko

doi:10.3389/fpls.2019.01085

Abstract

Actinorhizal nodules are structurally different from legume nodules and show a greater similarity to lateral roots. Because of the important role of auxins in lateral root and nodule formation, auxin profiles were examined in roots and nodules of the actinorhizal species Datisca glomerata and the model legume Medicago truncatula. The auxin response in roots and nodules of both species was analyzed in transgenic root systems expressing a beta-glucuronidase gene under control of the synthetic auxin-responsive promoter DR5. The effects of two different auxin on root development were compared for both species. The auxin present in nodules at the highest levels was phenylacetic acid (PAA). No differences were found between the concentrations of active auxins of roots vs. nodules, while levels of the auxin conjugate indole-3-acetic acid-alanine were increased in nodules compared to roots of both species. Because auxins typically act in concert with cytokinins, cytokinins were also quantified. Concentrations of cis-zeatin and some glycosylated cytokinins were dramatically increased in nodules compared to roots of D. glomerata, but not of M. truncatula. The ratio of active auxins to cytokinins remained similar in nodules compared to roots in both species. The auxin response, as shown by the activation of the DR5 promoter, seemed significantly reduced in nodules compared to roots of both species, suggesting the accumulation of auxins in cell types that do not express the signal transduction pathway leading to DR5 activation. Effects on root development were analyzed for the synthetic auxin naphthaleneacetic acid (NAA) and PAA, the dominant auxin in nodules. Both auxins had similar effects, except that the sensitivity of roots to PAA was lower than to NAA. However, while the effects of both auxins on primary root growth were similar for both species, effects on root branching were different: both auxins had the classical positive effect on root branching in M. truncatula, but a negative effect in D. glomerata. Such a negative effect of exogenous auxin on root branching has previously been found for a cucurbit that forms lateral root primordia in the meristem of the parental root; however, root branching in D. glomerata does not follow that pattern.

Highlights

Plant productivity depends on the successful acquisition of water and nutrients from the soil
A detailed analysis of auxin composition and concentrations was performed for roots and nodules of D. glomerata and M. truncatula, using the extraction method described by Müller and Munné-Bosch (2011) followed by liquid chromatography– mass spectrometry (LC-Murashige and Skoog (MS))/MS
indole-3-acetic acid (IAA), indole-3-butyric acid (IBA), 4-Cl-IAA, and phenylacetic acid (PAA) are considered as active auxins, while conjugated auxins are considered as inactive storage compounds (Korasick et al, 2013); it should be pointed out that conjugated forms of PAA were not analyzed as no standards were available

Summary

Introduction

Plant productivity depends on the successful acquisition of water and nutrients from the soil. (Cannabaceae, Rosales), and soil actinobacteria of the genus Frankia enter so-called actinorhizal symbioses with a diverse group of dicotyledonous plants from eight families from three orders (Datiscaceae and Coriariaceae from the Cucurbitales; Betulaceae, Casuarinaceae, and Myricaceae from the Fagales; and Elaeagnaceae, Rhamnaceae, and Rosaceae from the Rosales). Both nodules and lateral roots are formed postembryonically from preexisting roots. Actinorhizal nodules and rhizobia-induced nodules of the non-legume Parasponia are coralloid organs composed of multiple lobes, each of which represents a modified lateral root with a central vascular system and Frankia-infected cells in the nodule cortex, surrounded by a periderm. E.g., from Medicago truncatula, Cicer arietinum, and Vicia faba, and all actinorhizal nodule lobes are of the indeterminate type, i.e., they have an apical meristem whose function leads to the formation of a developmental gradient of infected cells in the nodule inner tissue (legume nodules) or nodule cortex (actinorhizal nodules; Pawlowski and Bisseling, 1996)

Objectives

Methods

Results

Discussion

Conclusion