Abstract
Burkholderia kururiensis is a diazotrophic bacterium originally isolated from a polluted aquifer environment and presents a high level of similarity with the rice endophyte "B. brasilensis" species. This work assessed the ability of B. kururiensis to endophytically colonize rice plantlets by monitoring different tissues of root-inoculated plants for the presence of bacterial growth in different media, electron microscopy and by 16S rDNA analysis. Observations of roots, stems and leaves of inoculated rice plantlets by electron microscopy revealed B. kururiensis colonization predominantly on root hair zones, demonstrating endophytic colonization primarily through the endodermis, followed by spreading into xylem vessels, a possible pathway leading to aerial parts. Although indifferent for the bacterial growth itself, addition of a nitrogen source was a limiting factor for endophytic colonization. As endophytic colonization was directly associated to an enhanced plant development, production of phytohormone auxin/indole-3-acetic acid by B. kururiensis was assayed with transgenic rice plantlets containing an auxin-responsive reporter (DR5-GUS). Our findings suggest the ability of auxin production by plant-associated B. kururiensis which may have a stimulatory effect on plant development, as evidenced by activation of DR5-GUS. We hereby demonstrate, for the first time, the ability of B. kururiensis to endophytically colonize rice, promoting both plant growth and rice grain yield.
Highlights
Burkholderia a phylogenetically well-defined genus which is remarkably diverse in view of its wide environmental distribution and its capabilities for biodegra-ciations (Barraquio et al 1997, Sevilla et al 2001, Caballero-Mellado et al 2004, Chen et al 2006, Perin et al 2006)
Efficient establishment of endophytic colonization of rice plants by B. kururiensis was demonstrated with a gnotobiotic system, using axenic rice plantlets
The main driving force to determine this putative plant-bacteria association was the phylogenetic closeness between B. kururiensis and “B. brasilensis” (Baldani and Baldani 2005, Estradade los Santos et al 2001, Marin et al 2003), an endophytic diazotrophic bacterium isolated from rice in Brazil (Baldani et al 1997) which have a 100% similarity with the nifH gene sequence with B. kururiensis (Marin et al 2003)
Summary
Burkholderia a phylogenetically well-defined genus which is remarkably diverse in view of its wide environmental distribution and its capabilities for biodegra-ciations (Barraquio et al 1997, Sevilla et al 2001, Caballero-Mellado et al 2004, Chen et al 2006, Perin et al 2006). Unlike legume-nodulating symbionts, endophytically associated species have developed intimate and mutually beneficial interactions with different parts of the plants, colonizing roots, stems and leaves. Those include B. vietnamiensis (Gillis et al 1995, Govindarajan et al 2008), B. unamae (Caballero-Mellado et al 2004), B. tropica (Reis et al 2004), “B. brasilensis” (Baldani et al 1997), and B. phytofirmans (Sessitsch et al 2005). A detailed investigation of a putative B. kururiensisplant association is of great interest, specially with rice, which is one the most important staple crop in the developing world (Barraquio et al 1997, Kuklinsky-Sobral et al 2004), since many studies have been focused on the search for natural diazotrophs that can possibly act as plant-growth promoting associative bacteria (Baldani et al 2000, Biswas et al 2000, Trân Van et al 2000, Ciccillo et al 2002, Compant et al 2005, Muthukumarasamy et al 2006, Rosenblueth and Martinez-Romero 2006, Govindarajan et al 2008)
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