Abstract
N-acyl homoserine lactones (AHLs) act as quorum sensing signals that regulate cell-density dependent behaviors in many gram-negative bacteria, in particular those important for plant-microbe interactions. AHLs can also be recognized by plants, and this may influence their interactions with bacteria. Here we tested whether the exposure to AHLs affects the nodule-forming symbiosis between legume hosts and rhizobia. We treated roots of the model legume, Medicago truncatula, with a range of AHLs either from its specific symbiont, Sinorhizobium meliloti, or from the potential pathogens, Pseudomonas aeruginosa and Agrobacterium vitis. We found increased numbers of nodules formed on root systems treated with the S. meliloti-specific AHL, 3-oxo-C14-homoserine lactone, at a concentration of 1 μM, while the other AHLs did not result in significant changes to nodule numbers. We did not find any evidence for altered nodule invasion by the rhizobia. Quantification of flavonoids that could act as nod gene inducers in S. meliloti did not show any correlation with increased nodule numbers. The effects of AHLs were specific for an increase in nodule numbers, but not lateral root numbers or root length. Increased nodule numbers following 3-oxo-C14-homoserine lactone treatment were under control of autoregulation of nodulation and were still observed in the autoregulation mutant, sunn4 (super numeric nodules4). However, increases in nodule numbers by 3-oxo-C14-homoserine lactone were not found in the ethylene-insensitive sickle mutant. A comparison between M. truncatula with M. sativa (alfalfa) and Trifolium repens (white clover) showed that the observed effects of AHLs on nodule numbers were specific to M. truncatula, despite M. sativa nodulating with the same symbiont. We conclude that plant perception of the S. meliloti-specific 3-oxo-C14-homoserine lactone influences nodule numbers in M. truncatula via an ethylene-dependent, but autoregulation-independent mechanism.
Highlights
Many species of the legume family interact with nitrogen-fixing bacteria collectively called rhizobia, leading to the formation of root nodules, in which the bacteria are housed
EFFECTS OF acyl-homoserine lactone (AHL) ON NODULATION AND ROOT ARCHITECTURE OF WILD TYPE MEDICAGO TRUNCATULA To determine whether AHLs modulate the interaction of M. truncatula roots with its symbiont, Sinorhizobium meliloti, we exposed surface-sterilized, germinated seedlings to 15 different AHLs (Table 1 and Supplementary Figure 2), which are either known to be produced by S. meliloti or by other bacteria
We chose some well-studied AHLs synthesized by Pseudomonas aeruginosa and Agrobacterium vitis, some of these AHLs may be produced by other bacteria
Summary
Many species of the legume family interact with nitrogen-fixing bacteria collectively called rhizobia, leading to the formation of root nodules, in which the bacteria are housed. This provides a source of nitrogen to the plant, while the bacteria benefit from a carbon source from the plant host. Among the traits regulated by AHLs in bacteria, bacterial movement, biofilm formation, production of virulence factors and degradative enzymes have been shown to be important for bacteria-plant interactions (e.g., Parsek and Greenberg, 2000; von Bodman et al, 2003; De Angelis et al, 2008)
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Free) 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
