A possible role for phenyl acetic acid (PAA) on Alnus glutinosa nodulation by Frankia

Phenylacetic hopanetetrol is a Frankia specific lipid present in vesicles. Phenylacetic acid (PAA) is known as an auxinomimetic, exhibiting the same effect on plant growth as indole acetic acid (IAA). We hypothesize that PAA, only bound by an ester link to the hopanetetrol basic unit, would be easily released and could thus play a role in nodule formation. HPLC and mass spectrometry analysis allowed us to show that 2 Alnus- (ACoN24d and ACN14a) and 2 Elaeagnus-infective strains (EaI1 and EaI3) released PAA into the culture medium, at concentrations of about 10^–5 to 10^–6 M, whereas IAA was not detected. Furthermore, exogenous PAA added to axenically-grown Alnus glutinosa roots at a concentration of 5×10–5 M, resulted in the formation of thick, short lateral roots which resembled actinorhizal nodules. phenylalanine ammonia lyase (PAL) and chalcone syntase (CHS) induction by incompatible and compatible Frankia strains in A. glutinosa roots and the different contents in salicylic acid precursors (cinnamic acid and benzoic acid) observed between nodules and roots support the idea that PAA would be produced in nodules to the detriment of salicylic acid production. These results provide evidence that in actinorhizal root nodules, phenylpropanoid metabolism may play a multiple role in symbiotic interactions including the limitation of the induction of the systematic acquired resistance (SAR) by the plant.