## 6 Rhizobial AHL in Interkingdom Communication  
Several studies indicate that AHLs not only mediate cell-to-cell communication between bacteria, but also can be recognized by eukaryotic hosts and induce diverse reactions in them (reviewed in [104]). The reaction of plants to AHLs is specific and depends on the type of AHL and on the plant species. In contrast to prokaryotic signal detection, the lactone ring does not need to be intact for plants to exhibit some or all of their responses to AHLs, a feature that could be exploited to distinguish between plant and bacterial responses to the signal molecules [105]. The first study analyzing the effects that AHLs cause in plants was based on a proteomic analysis performed on the model legume _Medicago truncatula_ treated with nanomolar concentrations of 3-oxo-C12-HL produced by _Pseudomonas aeruginosa_ or 3-oxo-C16:1 HL produced by _S. melioti_[106]. Accumulation of a large number of root proteins related to plant defense, stress responses, flavonoid synthesis, hormones, and regulatory functions was detected. Some specific proteins were differentially produced depending on AHL structure, indicating that the plant could distinguish between different AHLs. Later studies performed using different plant species found that treatment with different pure AHLs could influence plant growth and/or plant-microbe interactions (reviewed in [104]). The concentration and length of the acyl chain are important attributes for the plant response. In general, AHLs with a short acyl chain promote plant growth, whereas long-chain AHLs (\(\geq\) C12) affect interaction of plants with microbes but not plant development [107]. _Arabidopsis thaliana_ plants inoculated with _S. melioti expR\({}^{+}\)_ showed increased resistance against _Pseudomonas syringae_ pv. tomato (Pst), whereas reduced or no effect on resistance could be observed when plants were treated with ExpR-deficient or AttM lactonase-expressing strains, which show lower or lactone hydrolyzed signal production, respectively [108]. Likewise, inoculation with a _R. etli_ strain, which produces oxo-C8-HSL, had no impact on plant resistance. _S. melioti expR\({}^{+}\)_ also induced resistance in barley, wheat and tomato plants against several fungal pathogens [109]. The effects caused by _S. melioti expR\({}^{+}\)_ on plant defense responses were associated with _N_-3-oxo-tetradcanoyl-homoserine lactone (3-oxo-C14-HL) since chemical application of this AHL increased plant resistance towards several pathogens [107; 110]. In _Arabidopsis_, 3-oxo-C14-HL-induced resistance is caused by priming, a process in which plants are able to respond faster and/or more strongly to a pathogen challenge [110; 111; 112]. Interestingly, whereas inoculation of _Arabidopsis_ plants with 3-oxo-C14-HL induces resistance, the same treatment in _M. truncatula_ was found to increase total nodule number upon rhizobial inoculation [113]. The positive influence of 3-oxo-C14-HL on nodulation is host-specific since the effect was not observed in _M. sativa_ (alfalfa) despite being nodulated by the same rhizobial species. The effect on nodulation, which was not associated with changes in flavonoid production, lateral root formation or root growth, relies on an ethylene-dependent, but autoregulation-independent mechanism since the effects were observed in the autoregulation mutant _sunn4_ (super numeric nodules4) but not in the ethylene-insensitive _sickle_ mutant [113]. A more recent study found that total nodule number of _M. truncatula_ increases upon treatment with different long-chain AHLs (\(\geq\) 12 carbons). Most of these AHLs also increased the rate of nodulation. However, whereas the latter response required the integrity of the lactone ring, the effect of AHLs on total nodule number was also observed with the ring-opened AHL, indicating that this response is driven by plant responses to these signals rather than a QS-mediated effect by the bacteria [105].