beta-Lactam Antibiotics Modify Root Architecture and Indole Glucosinolate Metabolism in Arabidopsis thaliana

The presence of antibiotics in soils could be due to natural production by soil microorganisms or to the effect of anthropogenic activities. However, the impact of these compounds on plant physiology has not been thoroughly investigated. To evaluate the effect of beta-lactam antibiotics (carbenicillin and penicillin) on the growth and development of Arabidopsis thaliana roots, plants were grown in the presence of different amounts and we found a reduction in root size, an increase in the size of root hairs as well as an abnormal position closer to the tip of the roots. Those phenomena were dependent on the accumulation of both antibiotics inside root tissues and also correlated with a decrease in size of the root apical meristem not related to an alteration in cell division but to a decrease in cell expansion. Using an RNA sequencing analysis, we detected an increase in the expression of genes related to the response to oxidative stress, which would explain the increase in the levels of endogenous reactive oxygen species found in the presence of those antibiotics. Moreover, some auxin-responsive genes were misregulated, especially an induction of CYP79B3, possibly explaining the increase in auxin levels in the presence of carbenicillin and the decrease in the amount of indole glucosinolates, involved in the control of fungal infections. Accordingly, penicillin-treated plants were hypersensitive to the endophyte fungus Colletotrichum tofieldiae. These results underscore the risks for plant growth of beta-lactam antibiotics in agricultural soils, and suggest a possible function for these compounds as fungus-produced signaling molecules to modify plant behavior.