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. 2023 May 8;14:1146577. doi: 10.3389/fpls.2023.1146577

Figure 5.

Figure 5

Priming by hexanoic acid. Hx is a monocarboxylic acid that functions as a natural priming agent in different plant species. The mechanisms enabling priming by Hx are best described in tomato plants. Exogenously applied Hx does not accumulate in tomato plants but still positively influences the plants’ resistance to pathogen infections. In tomato, Hx induces the expression of genes that are typically regulated in response to Botrytis cinerea infection (in addition genes are induced that respond specifically to Hx). Hx counteracts the pathogen-induced imbalance of the plant redox state, thereby enhancing the plants’ resistance. Especially JA signaling is potentiated by Hx treatment upon pathogen exposure. In response to B. cinerea infection (dark green color), Hx-primed tomato plants accumulate JA-isoleucine (JA-Ile) and 12-oxo-phytodienoic acid (OPDA) and increase the expression of the JA marker gene LoxD, while the expression of marker genes for SA and ET signaling is not enhanced. In addition, priming by Hx against B. cinerea requires the COI1-homolog JAI1 to induce downstream responses. Furthermore, in Hx-primed plants ABA signaling enhances callose deposition. This is not observed in Hx-primed tomatoes infected with Pst (grey color), indicating that priming by Hx activates different mechanisms depending on the pathogen. This is further supported by the observation that in response to Pseudomonas infection, priming by Hx requires functional SA signaling. Hx-priming increases the expression of SA signaling genes, such as PR1 and PR5, upon pathogen infection. Hx-priming inhibits stomatal reopening, which is mediated by the bacterial-derived coronatine (COR) mimicking plant JA-Ile. Thereby, Hx likely prevents the bacteria from entering the plant mesophyll. In addition, Hx counteracts negative effects on SA signaling, which are mediated by COR and JA-Ile, and thus improves the plants’ resistance. Similar as Botrytis-infected tomato plants, Hx-primed tomato plants infected with Pst accumulate OPDA (but not JA or JA-Ile) and induce the expression of OPDA- and JA biosynthesis and signaling genes, such as LoxD, OPR3, and JAZ1.