Abstract
Root endophytic nitrogen-fixing bacteria (reNFB) have been proposed as important contributors to the invasiveness of exotic legumes; however, the reNFB of invasive nonlegumes has received less attention. In particular, the growth-promoting effect of reNFB on invasive plants remains unknown. In this study, 131 strains of potential nitrogen-fixing bacteria were isolated and purified from the roots of the invasive plant, Ageratina adenophora, in Southwest China. Phylogenetically, these reNFB were categorized into three phyla at 97% sequence identity that included Proteobacteria (92.4%), Actinobacteria (4.6%), and Firmicutes (3.1%). The dominant isolates ranked by number were Pseudomonas (80 isolates, 61.1%), Rhizobium (12 isolates, 9.2%), and Duganella (11 isolates, 8.4%). The community composition and diversity of A. adenophora reNFB were markedly different across study regions. The capacity of these reNFB to accumulate indolyl-3-acetic acid (IAA), solubilize phosphate, and produce siderophores was determined. All 131 isolates of reNFB accumulated IAA, 67 isolates solubilized phosphate, and 108 isolates produced siderophores. Among the three dominant genera of reNFB, Pseudomonas had the highest phosphorus solubilization and siderophore production, while the accumulation of IAA in the genus Duganella was the lowest. Interestingly, the calculated reNFB Shannon diversity index of each A. adenophora individual was negatively correlated with the capacity of reNFB to produce growth-promoting products. Six randomly selected isolates from three dominant genera were further used to conduct inoculation experiments, and all isolates showed significant positive growth-promoting effects on A. adenophora seedlings. The contribution of reNFB to the root biomass was higher than that to the shoot biomass. Our results suggest that reNFB, similar to soil or nodular nitrogen-fixing bacteria, can potentially promote plant growth and may play an important role in the invasion of nonleguminous plants. More detailed studies on the correlation between reNFB and invasive plants are necessary.
Highlights
Ongoing globalization and climate warming have enhanced the migration of species (Lehan et al, 2013; Walther et al, 2002) and have exacerbated the colonization of invasive plants in new habitats (Moles, Gruber & Bonser, 2008)
Species composition of Root endophytic nitrogen-fixing bacteria (reNFB) In total, 150 isolates were obtained from the roots of 15 A. adenophora individuals across the five regions mentioned above by using yeast-mannitol agar (YMA) selective medium, but only 131 isolates were able to grow continuously for seven culture generations in nitrogen-free medium, including 29 isolates from XM, 24 from CY, 23 from XS, from WS, and from YL
There were a variety of endophytic nitrogen-fixing bacteria (NFB) inhabiting the roots of A. adenophora, and the dominant genera ranked by number were Pseudomonas, Rhizobium, and Duganella
Summary
Ongoing globalization and climate warming have enhanced the migration of species (Lehan et al, 2013; Walther et al, 2002) and have exacerbated the colonization of invasive plants in new habitats (Moles, Gruber & Bonser, 2008). The invaded ecosystems tend to have higher net primary production compared to native ecosystems (Wilsey & Wayne Polley, 2006), indicating that the growth requirements of invasive plants have a high environmental capacity. Invasive plants promote the development of their own communities through highly efficient nutrient cycling (Ehrenfeld, 2003). Increased nitrogen input was well documented to facilitate the invasion of exotic species in new habitats (Siemann & Rogers, 2003) and enhance the dominance of invasive plants (Brooks, 2003)
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