## 4 Discussion
Therefore, a subpopulation of cells that are exposed to these harsh environmental conditions can enter into the persister state when the energetic cost to successfully infect the host would be too high. Furthermore, if cells enter into the persistence state, they become more tolerant to antimicrobial compounds applied to citrus orchards, raising the question of whether these applications should be shifted to colder periods of the day. We also found that the small antioxidant molecule NAC can prevent metal-induced persistence, shedding light on how we can more readily control persister cells in the field. NAC is a sustainable alternative to control _Xanthomonas_ spp. Diseases that affect both citrus (citrus canker) and tomato (bacterial spot of tomato) [54; 55]. Since metallic compounds can create an imbalanced internal oxidative environment [51], we presume that NAC could be acting directly as an antioxidant molecule, lowering oxidative stress [53] and blocking persistent induction, which can be triggered by oxidative stress [30; 40; 64]. This \"anti-persistence\" effect has been previously suggested for _Mycobacterium_ spp. [65; 66] and our data support this previous finding. Of note, the combination of NAC and copper has better results in the control of both citrus canker and bacterial spot disease of tomato [54; 55]. Finally, besides chemical inducers and killers of persistent cells, we also looked for molecules that could specifically wake _X. citri_ persisters. Based on the finding that alanine is a signal recognized by _E. coli_ persisters to restart growth [18], we identified proline as a signal to wake _X. citri_ persisters. Curiously, proline is also the best amino acid to wake _Pseudomonas aeruginosa_ from persistence [17]. Moreover, we observed that isoleucine has the opposite effect, blocking persister resuscitation, even when proline is also present, which is, to the best of our knowledge, a first report that an amino acid can prevent persister waking. It is interesting to note that valine inhibits _E. coli_ growth due to the internal accumulation of an isoleucine precursor [67], which may involve ReIA/ppGpp pathways [68]. Nevertheless, the reasons why these amino acids present these opposing roles are still unknown and require further investigation. Overall, this work presents a basis for future studies on _X. citri_ persistence to identify chemical and additional physical elements that can induce, kill, prevent and modulate persister waking. We recently found that the alarmone ppGpp directly causes the formation of _E. coli_ persisters by dimerizing ribosomes [15] and that persister cells resuscitate by sensing nutrients through chemotaxis/sugar transporters, reducing cAMP, and stimulating ribosome activity [16; 18], so perhaps these pathways hold as well for _X. citri_. This study opens new insights to better understand basic aspects of the biology of a previously ignored stage of this phytopathogen's life cycle. Indeed, in-planta experimentation is still needed to confirm the extent to which persistence impacts disease control in the field, but this is beyond the scope of our study. Nevertheless, we expect that the knowledge of persister cell behavior under chemical and physical stresses can be used for further improvement of plant disease management in the field in order to obtain a more efficient and sustainable disease control. P.M.M.M. experiments execution, original manuscript writing, experiment design, and result analysis. A.A.d.S. and T.K.W. original idea, experimental design, results analysis/discussion and original manuscript correction. All authors have read and agreed to the published version of the manuscript. [PERSON] is a recipient of a research fellowship from the Research Internships Abroad from Sao Paulo Research Foundation-FAPESP 2018/18550, supported by FAPESP 2013/10957-0 and also by INCT 14/50880-0. TKW was supported by funds derived from the Biotechnology Endowed Professorship at the Pennsylvania State University.