research with arabidopsis has allowed dissection of the events taking place following treatment with baba . in summary , the stress encountered determines the events following it , and the treatment with baba can potentiate ( prime ) the most appropriate to counteract it . for example , bacterial infection by pseudomonas syringae pv tomato ( pst ) dc3000 and attack by the fungal pathogen botrytis cinerea are both combatted through potentiation of salicylic acid ( sa)-dependent defenses , while defense against plectosphaerella cucumerina is obtained through an enhanced callose deposition depending on a functional abscisic acid ( aba ) signaling pathway . baba does not usually induce directly defense responses , but primes the plant to react faster and/or stronger to a given stress . interestingly , the priming state induced by baba can also be transmitted to the descendants of a plant via its seeds . baba perception involves an aspartyl - trna synthetase , and this could be interpreted as a first clue to the possibility that baba might play a natural role in a plant 's physiology . the recent discovery of baba as natural molecule synthesized by plants and induced by stress sheds new light on the research presented until now , and many phenomena known up to date have to be seen and interpreted in the light of this new finding . in particular , baba seems now to possess the features of a novel plant stress hormone . the finding of endogenous baba in plants raises 2 questions : a ) is the amount of baba produced by plants comparable to main plant stress hormones ? and b ) how do the temporal dynamics of its induction relate to those of hormones ? in this addendum , we will try to answer these questions . in our last paper we measured the levels of baba in a. thaliana plants before and after exposure to several biotic and abiotic stressors . unstressed columbia-0 plants at the age of 5  weeks showed a basal level of baba in leaves usually lower than 10  ng / g of fresh weight ( fw ) . if we compare with sa , which can be found , as the free form , in amounts between 30 and 500  ng / g fw in unstressed a. thaliana leaves , baba can be considered less present than sa . however , the molecular weight of baba is the lowest when compared , for example , to sa , aba , jasmonic acid ( ja ) or indole-3-acetic acid ( iaa ) . therefore , in comparison to aba and ja , which are similarly found in amounts lower than 10  ng / g fw , or iaa , which is present in amounts around 20  ng / g fw , the basal amount of baba in a. thaliana leaves can be considered in line with , or in some cases higher than these hormones . how and how much do plant hormones vary in similar circumstances ? by making comparisons with the literature it is possible to observe , for example , that aba has been reported to accumulate during the first 48  hours after p. cucumerina infection with a similar pattern to baba . in that study , report that aba levels increased about 2.5fold after 48  hours of infection , and the aba peak was reached at that time point . in our study , baba similarly accumulated 2.5-fold after 48  hours , although it kept increasing up to 5-fold after 72  hours . in a different study with p. cucumerina , sa and ja have also been reported to increase , but neither hormone was found to increase over 2-fold after 48  hours . during pst dc3000 infection , baba showed a continuous increase over time until reaching 15-fold the control value after 48  hours , whereas in a study by gao et  al . both aba and in addition to p. cucumerina and pst dc3000 infection , we also tested the levels of baba after infection with a virulent strain of hyaloperonospora arabidopsidis . interestingly , 3  d of infection with this biotrophic pathogen were not enough for baba to significantly accumulate , as similarly reported by other authors for free sa . finally , salt stress led baba to increase already after 24  hours of treatment , whereas ellouzi et  al . report on aba accumulation in leaves of a. thaliana after 3  d of nacl treatment to the roots . in conclusion , although the biologic role of the endogenous production of baba during stress has still to be clarified , its levels in planta and its temporal dynamics suggest , along with its outstanding efficacy in inducing resistance , that baba may be a novel plant ( priming ) hormone . the authors are grateful to all the colleagues which contributed to establishing the methodology to measure baba .
abstractthe capacity of -aminobutyric acid ( baba ) to induce resistance in plants against biotic and abiotic stresses has been known for more than 50  y. in the beginning reports were mainly descriptive of the phenomenon , but it became clear with the discovery of baba insensitive mutants in arabidopsis that there was definitely a genetic basis underlying baba - induced resistance . 
 the study of these mutants , along with the use of regular hormone mutants , allowed establishing the defense pathways activated upon defense induction . 
 to date it is clear that baba potentiates the defense pathway more appropriate to counteract the upcoming stress situation , through a phenomenon termed priming . 
 interestingly , plants possess a receptor for baba , but up to recently there was a general consensus on the fact that baba was a xenobiotic molecule . 
 the development of an accurate non - destructive assay for measuring aminobutyric acid isomers in planta and the finding of plant - produced baba , thus seems to represent the missing link for the discovery of a novel plant hormone . 
 differences and similarities with some of the classical plant hormones are presented here .