Abstract
Quorum sensing auto-inducers of the N-acyl homoserine lactone (AHL) type produced by Gram-negative bacteria have different effects on plants including stimulation on root growth and/or priming or acquirement of systemic resistance in plants. In this communication the influence of AHL production of the plant growth promoting endophytic rhizosphere bacterium Acidovorax radicis N35 on barley seedlings was investigated. A. radicis N35 produces 3-hydroxy-C10-homoserine lactone (3-OH-C10-HSL) as the major AHL compound. To study the influence of this QS autoinducer on the interaction with barley, the araI-biosynthesis gene was deleted. The comparison of inoculation effects of the A. radicis N35 wild type and the araI mutant resulted in remarkable differences. While the N35 wild type colonized plant roots effectively in microcolonies, the araI mutant occurred at the root surface as single cells. Furthermore, in a mixed inoculum the wild type was much more prevalent in colonization than the araI mutant documenting that the araI mutation affected root colonization. Nevertheless, a significant plant growth promoting effect could be shown after inoculation of barley with the wild type and the araI mutant in soil after 2 months cultivation. While A. radicis N35 wild type showed only a very weak induction of early defense responses in plant RNA expression analysis, the araI mutant caused increased expression of flavonoid biosynthesis genes. This was corroborated by the accumulation of several flavonoid compounds such as saponarin and lutonarin in leaves of root inoculated barley seedlings. Thus, although the exact role of the flavonoids in this plant response is not clear yet, it can be concluded, that the synthesis of AHLs by A. radicis has implications on the perception by the host plant barley and thereby contributes to the establishment and function of the bacteria-plant interaction.
Highlights
In the rhizosphere, microbes are selectively enriched as compared to the surrounding bulk soil due to the availability of plant root exudates
The results indicated that the acyl homoserine lactone (AHL) produced by A. radicis N35 reduced systemic defense responses like flavonoid accumulation in response to the colonization by this endophytic bacterium
AHL production by A. radicis N35 has a positive effect on the colonization of roots, since an AHL defective mutant was less successful in root colonization (Figure 3)
Summary
Microbes are selectively enriched as compared to the surrounding bulk soil due to the availability of plant root exudates. Induced systemic resistance (ISR) caused by root associated bacteria enhances the defense even in foliar tissues for later pathogen attack (Lugtenberg and Kamilova, 2009). For instance 2R, 3R-butanediol, were shown to induce plant resistance (Cortes-Barco et al, 2010). PGPB cause ISR because they initiate a priming of specific initial plant responses, upon surface or endophytic colonization. Quorum sensing (QS) compounds of Gram-negative bacteria, like N-acyl homoserine lactones (AHL), were found to cause systemic ISR-like responses in different plants (De Vleesschauwer et al, 2008; De Vleesschauwer and Höfte, 2009; Cortes-Barco et al, 2010). Compared to the already advanced knowledge on the responses of plants to a large number of systemic defense elicitors, details about the perception of plants regarding these QS signal molecules are still scarce, despite the importance these bacterial messenger molecules must have considering their presence throughout the entire plant evolution
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