Abstract
BackgroundPlant-endophyte symbioses often revolve around nitrogen metabolism, and involve varying degrees of intimacy. Although evidence for vertical inheritance of nitrogen-fixing endophytic bacteria is increasing, it is confined mostly to crop plants, and to date no such system has been reported for geophytes.MethodsBacterial endophytes associated with Oxalis, the most species-rich geophytic genus form the Cape Flora in southern Africa was studied. Culturable endophytes were isolated from surface-sterilized vegetative and reproductive plant organs for six host species at three locations. Colonies of microbes on various artificial media were morphotyped, enumerated and identified using sequence data. Filter exclusion experiments were conducted to determine if endophytes were vertically transmitted to seeds, determine if mucilage plays a role to actively attract microbes from the soil and to assess microbial richness isolated from the mucilage of Oxalis seedlings. Fluorescent microscopy was implemented in order to visualize endophytic bacteria in cryo-sectioned seeds.ResultsEvidence for a novel, vertically transmitted symbiosis was reported. Communities of nitrogen-fixing and plant growth-promoting Bacillus endophytes were found to associate with selected Oxalis hosts from nitrogen-deficient environments of the Cape. Bacillus endophytes were ubiquitous and diverse across species and plant bodies, and were prominent in seeds. Three common nitrogen-fixing Bacillus have known oxalotrophic properties and appear to be housed inside specialised cavities (containing oxalates) within the plant body and seeds.ConclusionsThe discovery of vertical transmission and potential benefits to both host and endophyte suggest a particularly tight mutualism in the Oxalis-endophyte system. This discovery suggests unexpected ways in which geophytes might avoid nitrogen deficiency, and suggest that such symbioses are more common than previously expected.
Highlights
Plant-endophyte symbioses often revolve around nitrogen metabolism, and involve varying degrees of intimacy
Bacterial species found in vegetative and reproductive organs The presence and diversity of endophytic bacteria (EB) were assessed amongst vegetative and reproductive organs of six Oxalis host species sampled from three different locations (Fig. 1a)
Using the sequenced and identified EB from macerated plant material as reference material, this study has shown that the same suite of Bacillus species and no other cultivable bacteria were present in the mucilage of developing seedlings when germinated on sterile media (Fig. 2a–b)
Summary
Plant-endophyte symbioses often revolve around nitrogen metabolism, and involve varying degrees of intimacy. The sandstone derivation and low pH of most soils, together with predictable winter rainfall and relatively frequent wildfires all contribute to dystrophic conditions with severe nitrogen deficiency [4]. These include some of the lowest nitrogen and phosphorus levels measured globally [5]. There is growing evidence that diverse Cape plant lineages have adapted by forming associations with growth promoting and nitrogen-fixing micro-organisms [9]. Relatively little research attention has been given to these associations; it is evident that limited information is available addressing the mechanisms, diversity and role of microbial associations of Cape plants [18]
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