Nitric oxide (NO) is a diatomic gas that performs crucial functions in a wide array of physiological processes in animals.The past several years have revealed much about its roles in plants.It is well established that NO is synthesized from nitrite by nitrate reductase (NR) and via chemical pathways.There is increasing evidence for the occurrence of an alteative pathway in which NO production is catalysed from L-arginine by a so far non-identified enzyme.Contradictory results have been reported regarding the respective involvement of these enzymes in specific physiological conditions.Although much remains to be proved,we assume that these inconsistencies can be accounted for by the limited specificity of the pharmacological agents used to suppress NO synthesis but also by the reduced content of L-arginine as well as the inactivity of nitrate-permeable anion channels in nitrate reductase- and/or nitrate/nitrite-deficient plants.Another unresolved issue conces the molecular mechanisms underlying NO effects in plants.Here,we provide evidence that the second messenger Ca2+,as well as protein kinases including MAPK and SnRK2,are very plausible mediators of the NO signals.These findings open new perspectives about NO-based signaling in plants.