Secrets of the Forest: Volatiles First Discovered in Pine Trees Propagate Defense Signals within and between Plants.

Secrets of the Forest: Volatiles First Discovered in Pine Trees Propagate Defense 2 Signals Within and Between Plants 3 4 Systemic acquired resistance (SAR)—a plant-wide heightened state of defense following 5 localized exposure to a pathogen—is characterized by increased salicylic acid (SA) and ROS 6 levels and elevated expression of pathogenesis-related genes. SAR depends on ENHANCED 7 DISEASE SUSCEPTIBILITY1 (EDS1), which both generates the SAR signal at the primary infection 8 site and perceives it in distal, uninfected tissues (Breitenbach et al., 2014). Various vascular 9 mobile signals function within SAR, including methyl salicylate (Park et al., 2007), the non10 proteogenic amino acid pipecolic acid (Návarová et al., 2012), and the dicarboxylic acid azelaic 11 acid (Jung et al., 2009). 12 13 In a new study, Riedlmeier et al. (2017) examined the role of volatile organic compounds (VOCs) 14 in SAR. Using gas chromatography/mass spectrometry, they compared the emissions released 15 by wild-type Arabidopsis plants and SAR-deficient eds1-2 mutants several hours after induction 16 of SAR. Whereas the wild-type plants released substantial amounts of the bicyclic 17 monoterpenes α-pinene and β-pinene—volatiles best known for giving pine forests their 18 enchanting scent—the mutants with compromised SAR released almost undetectable levels of 19 these VOCs. 20 21 Based on these findings, the authors hypothesized that α-pinene and β-pinene induce SAR. To 22 test this possibility, they exposed wild-type plants to a mixture of α-pinene and β-pinene or to 23 the VOC solvent control for 3 days and then challenged the plants with the pathogenic 24 bacterium Pseudomonas syringae pv tomato strain DC3000 (Pst). They found that bacterial 25 growth was ~10-fold less in the leaves of plants that had been exposed to the monoterpenes, 26 and that the effect depended on pinene concentration. Bacterial growth was not limited in the 27 non-expressor of PR genes1-1 (npr1-1) mutant, which has defects in signaling downstream of 28 SA, or in the sa induction deficient2-1 (sid2-1) mutant, which has defects in SA biosynthesis, 29 after 3 days of monoterpene exposure. Furthermore, ROS O2 ̄ accumulated in the leaves of 30 plants treated with pinenes, but not in those of control plants. Microarray analysis showed that 31 pinene treatment directly influenced key regulatory genes in SA biosynthesis and SAR (e.g., the 32 gene encoding the lipid transfer protein AZELAIC ACID INDUCED1). Therefore, pinenes appear 33 to induce SAR, in a manner that depends on SA signaling. 34 35 As monoterpenes are highly volatile, the authors directly tested whether these VOCs could 36 function as mobile plant-to-plant defense signals. Wild-type receiver plants enclosed in a 37 vacuum dessicator for three days with SAR-induced (i.e., monoterpene-emitting) sender plants 38 were indeed more resistant to Pst infection than were receiver plants enclosed with control 39 plants. 40 41 Plant Cell Advance Publication. Published on June 13, 2017, doi:10.1105/tpc.17.00462