Mechanisms of nitric oxide and salicylic acid signaling in the plant immune response

The participation of nitric oxide (NO) and salicylic acid (SA) as disease resistance signals in plants has been studied by means of two different approaches. NO's ability to inhibit tobacco catalase (CAT) and ascorbate peroxidase (APX), two enzymes that play a key role in H₂O₂ metabolism, was assessed in vitro using a variety of NO donors. H₂O₂ accumulation is an early step in the establishment of disease resistance, and inhibition of H₂O₂-scavenging enzymes has been proposed to contribute to this phenomenon. NO reversibly inhibited both CAT and APX from tobacco, suggesting that NO generated in plants as part of the defense response might regulate H₂O₂ accumulation in vivo through reversible inhibition of these enzymes.; To unravel the mechanisms of action of SA, a search for proteins that physically interact with it and serve as effectors in disease resistance signaling was undertaken. A tobacco chloroplast protein, the salicylic acid-binding protein 3 (SABP3), was purified on the basis of its ability to bind SA, and identified by partial sequencing as the chloroplast carbonic anhydrase (CA). The protein specifically binds SA with moderate affinity (Kd = 3.7 μM). Restoration of yeast strain ΔNCE103 growth under aerobic conditions by expression of CA/SABP3 initially suggested that the tobacco chloroplast CA had antioxidant activity. However, this activity could not be demonstrated in either ΔNCE103 or tsa1Δtsa2Δ (an oxidant sensitive yeast strain) cells expressing the tobacco chloroplast CA and treated with oxidants. Moreover, by carrying out an in-depth complementation analysis it was demonstrated that complementation of ΔNCE103 by CA was due to its CO₂ hydration/dehydration (CA enzymatic) activity. This finding was supported by the fact that the yeast protein NCE103 also exhibited CA enzymatic activity. Since CA/SABP3 appears to only have CA enzymatic activity, which is neither activated nor inhibited by SA, the functional relevance of SA binding to CA/SABP3 in disease resistance remains unclear. Experiments performed with tobacco and Arabidopsis transgenic lines that over- or under-express CA challenged with several pathogens showed that altering CA expression suppressed resistance or enhanced susceptibility in some plant-pathogen interactions. However, since these pathogen studies did not reveal a clear pattern of effects of CA expression on disease resistance, they have not provided a better understanding of the significance of SA-CA/SABP3 interaction.