“These data indicate that de novo synthesis of Pip in distal tissues is dependent on both SA and G3P and that distal levels of SA and G3P play an important role in SAR”
Although plants do not have the complex adaptive immune system present in mammals, they do have limited ability to defend themselves against disease-causing organisms. The basal immune program in plants means that they can recognize conserved molecular patterns of potentially pathogenic micro-organism and limit the invasive spread of virulent pathogens.
Furthermore, the recognition of pathogens in infected leaves initiates an additional resistance mechanism known as systemic acquired resistance (SAR) that can activate a state of enhanced immunity throughout the plant’s entire foliage. Thus, a localized leaf infection can protect uninfected leaves against future invasion by similar pathogens.
The SAR response is mediated through the production of chemical signals. Consequently, a range of plant metabolites are important for conferring immunity against invading pathogens. Compounds known to be key regulatory components of the SAR response include salicylic acid, azelaic acid, nitric oxide, reactive oxygen species, glycerol-3-phosphate (G3P) and galactolipids3.
In addition, there are indications that pipecolic acid is a regulator of plant SAR, but this has not been confirmed with direct evidence. Accumulation of pipecolic acid has been observed in uninfected (distal) leaves, but the putative involvement of pipecolic acid in SAR has not been studied.
Pipecolic acid is a non-protein amino acid. Its synthesis is catalysed by the enzyme aminotransferase (LAT). It can also be obtained from a metabolite of lysine catabolism by the enzyme SARD4.
In plants with defective LAT, levels of pipecolic acid are reduced in both local and distal leaves. In contrast, plants with defective SARD4 show normal levels of pipecolic acid in local leaves but lack pipecolic acid accumulation in uninfected distal leaves.
The involvement of pipecolic acid in SAR has recently been investigated. Pipecolic acid was applied locally to the leaves of a plant infected with either a weak strain of Pseudomonas syringae (Pst) or magnesium chloride. Methanol was used as a negative control. The untreated leaves of all plants were then exposed to a virulent strain of Pst, and the extent of the infection assessed up to 3 days later. Homegenized leaf samples were also analysed by electron paramagnetic resonance (EPR) using a Bruker ESP 300 X-band spectrometer.
Infection with the virulent Pst strain was 10‑15 times lower among the plants previously infected with a weak Pst strain than in the plants infiltrated with magnesium chloride. Similarly, plants treated with pipecolic showed significantly less growth of the virulent Pst strain compared with those treated with methanol.
ESR spectra revealed higher levels of the free radical, nitric oxide (NO), and other reactive oxygen species (ROS) in the plants pre-treated with pipecolic acid. These are compounds known to play a part in the SAR signalling pathway, which indicates that pipecolic acid is indeed a chemical inducer of SAR.
Accumulate of pipecolic acid in distal uninfected tissues was also reduced in plants with defective in NO, ROS, G3P, or salicylic acid biosynthesis. This reflects the effects of plants with defective SARD4.
These latest findings demonstrate the importance of pipecolic acid for the induction of SAR. They also show that salicylic acid and NO are required for distal biosynthesis of pipecolic acid, which in turn initiates the de novo synthesis of G3P.
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Caixia Wang C, et al. Pipecolic acid confers systemic immunity by regulating free radicals. Science Advances 2018;4(5):eaar4509 https://advances.sciencemag.org/content/4/5/eaar4509