Isolation and Characterization of Nodule-AssociatedExiguobacterium sp. from the Root Nodules ofFenugreek(Trigonella foenum-graecum) and Their Possible Role in Plant Growth Promotion

Abstract

<jats:p>One of the ways to increase the competitive survivability of rhizobial biofertilizers and thus achieve better plant growth under such conditions is by modifying the rhizospheric environment or community by addition of nonrhizobial nodule-associated bacteria (NAB) that cause better nodulation and plant growth when coinoculated with rhizobia. A study was performed to investigate the most commonly associated nodule-associated bacteria and the rhizospheric microorganisms associated with the<jats:italic>Fenugreek</jats:italic>(<jats:italic>Trigonella foenum-graecum</jats:italic>) plant. Isolation of nonrhizobial isolates from root nodules of<jats:italic>Fenugreek</jats:italic>was carried out along with the rhizospheric isolates. About 64.7% isolates obtained from<jats:italic>Fenugreek</jats:italic>nodules were gram-negative coccobacilli, 29.41% were gram-positive bacilli, and all rhizospheric isolates except one were gram-positive bacilli. All the isolates were characterized for their plant growth promoting (PGP) activities. Two of the NAB isolates M2N2c and B1N2b (<jats:italic>Exiguobacterium sp.</jats:italic>) showed maximum positive PGP features. Those NAB isolates when coinoculated with rhizobial strain—<jats:italic>S. meliloti</jats:italic>, showed plant growth promotion with respect to increase in plant’s root and shoot length, chlorophyll content, nodulation efficiency, and nodule dry weight.</jats:p>

The Nodule Microbiome: N2-Fixing Rhizobia Do Not Live Alone

“…They are now known as nonrhizobia endophytes (NRE) (De Meyer et al 2015), nodule endophytes , or nodule-associated bacteria (NAB) (Rajendran et al 2012). In this review, we will use the abbreviation NAB, which is broader in scope than the other two designations.…”

“…Also, nodules of Hedysarum, a forage legume, were found to harbor Enterobacter cloacae, Enterobacter kobei, Escherichia vulneris, Pantoea agglomerans, and Leclercia adecarboxylata (Muresu et al 2008). Because it has long been known that coinoculation of rhizobia and other bacteria, particularly Bacillus species, promotes not only nodulation (Schwartz et al 2013 and papers cited therein), but also N availability in sustainable agricultural systems (Rajendran et al 2012), it is assumed that many of the nonpathogenic bacteria found within nodules or in plant tissues as endophytes could be safe and effective partners for enhancing nitrogen fixation in legumes (Sturz et al 2000).…”

“…trifolii with the clover nodule isolates Bacillus insolutis, Bacillus brevis, and Agrobacterium rhizogenes A promoted plant growth and nodulation compared with inoculating with Rhizobium alone (Sturz et al 1997). In a more recent study, 17 different NAB were isolated from Trigonella foenum-graecum (fenugreek) nodules, and two isolates, both species of Exiguobacterium, enhanced plant growth when coinoculated with Sinorhizobium meliloti (Rajendran et al 2012).…”

Endophytic occupation of legume root nodules by nifH-positive non-rhizobial bacteria, and their efficacy in the groundnut (Arachis hypogaea)

“…and Paenibacillus sp. have been found to be present inside the root nodules (Pandya et al 2013;Zhang et al 1996;Ibanez et al 2009;Tariq et al 2012;Rajendran et al 2011;Egamberdieva et al 2010). Enterobacter and Klebsiella are members of the family Enterobacteriaceae, and many species of these two genera are known to occur endophytically in the root nodules of a number of crops (Pandya et al 2013;De Meyer et al 2015;Selvakumar et al 2008).…”

“…In the early days, it was believed that the root nodules of leguminous plants accommodated only endophytic bacteria of the genera Rhizobium, Mesorhizobium, Bradyrhizobium, Ensifer and Azorhizobium-collectively known as rhizobial bacteria-with fixation of atmospheric nitrogen occurring symbiotically. However, root nodules also accommodate various non-rhizobial bacteria, which have a definite influence on the survival, nodulation and grain yield of the crop (Lin et al 2008;Pandya et al 2013;Zhang et al 1996;Ibanez et al 2009;Tariq et al 2012;Rajendran et al 2011 andEgamberdieva et al 2010).…”