Assessing root nodule microsymbionts in healthy and declined rooibos (Aspalathus linearis burm f.) at a plantation in South Africa

Ahmed Idris Hassen,Johannes H Habig,Sandra C Lamprecht

doi:10.1080/17429145.2018.1473514

Abstract

ABSTRACTAspalathus linearis (burm f.), commonly known as rooibos, grows in nutrient and organic matter poor sandy soils that limit its growth. In this study, samples of nodules from both declined and healthy rooibos plants were collected to determine the frequency of nodule nitrogen-fixing and endophytic bacteria. Standard microbiological procedures as well as sequence analysis of the 16S rRNA revealed that more than 75% of the bacterial isolates from the healthy plants contained microsymbionts belonging to the Rhizobium group and the remaining 25% were characterized as Pseudomonas and Burkholderia spp. The nodule from the declined plants lacks a sufficient number of rhizobia and was mostly white in color, small and contains the free-living endospore-forming Bacillus and other endophytic Burkholderia and Pseudomonas spp. The results provide a baseline data on the microsymbionts of rooibos nodules in Citrusdal and highlighted the need for further investigation using additional techniques.

Highlights

Aspalathus linearis, commonly known by the name rooibos, is an economically important beverage legume endemic to the Cape floristic region in South Africa (Hassen et al 2012)
The nodules were crushed using a sterile glass rod and a loopful of the suspension was streak plated on Yeast Mannitol (YM) agar that supports the growth of the root nodule rhizobia as well as on Luria Bertani (LB) agar for the cultivation of a wide range of non-fastidious bacteria (Bertani, 1951) The plates were incubated at 28°C for 24–48 h for the fast growers and for 3–5 days for the slow-growing Bradyrhizobium group
Sequence analysis of the 16S ribosomal RNA of the isolates revealed that more than 75% of the isolates from the nodules of healthy soils belonged into the Rhizobium–Bradyrhizobium group and the remaining 25% were identified as Bacillus, Burkholderia and Pseudomonas spp. (Figure 2)

Summary

Introduction

Aspalathus linearis (burm f.), commonly known by the name rooibos, is an economically important beverage legume endemic to the Cape floristic region in South Africa (Hassen et al 2012). Among the various factors that contributed to crop decline in many agricultural fields is a change or decrease in beneficial soil microbial communities. A good example of the role of soil microbiota in controlling crop decline is the shift in soil microbial communities that favors the growth of antagonists of the pathogen Gaeumannomyces graminis (Weller et al 2002). Agricultural practices that result in change or reduction in the beneficial soil microbial communities may favor the proliferation of pathogens resulting in crop decline. Soil microorganisms such as Bacillus, Trichoderma and Pseudomonas spp. increase the level of soil suppressiveness in many agro ecosystems and contribute to prevent crop declines caused by Fusarium wilt in many crops, potato scab and take-all of wheat. The major modes of action in the suppressive effects of these rhizobacteria include antibiosis and siderophore-mediated iron competition with the pathogens (Kloepper et al 1980)

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