Short-chain fatty acids (SCFAs), produced by colonic bacteria and obtained from the diet, have been linked to beneficial effects on human health associated with their metabolic and signaling properties. Their physiological functions are related to their aliphatic tail length and dependent on the activation of specific membrane receptors. In this review, we focus on the mechanisms underlying SCFAs mediated protection against oxidative and mitochondrial stress and their role in regulating metabolic pathways in specific tissues. We critically evaluate the evidence for their cytoprotective roles in suppressing inflammation and carcinogenesis and the consequences of aging. The ability of these natural compounds to induce signaling pathways, involving nuclear erythroid 2-related factor 2 (Nrf2), contributes to the maintenance of redox homeostasis under physiological conditions. SCFAs may thus serve as nutritional and therapeutic agents in healthy aging and in vascular and other diseases such as diabetes, neuropathologies and cancer.

Some alternative control strategies of currently emerging plant diseases are based on the use of resistance inducers. This review highlights the recent advances made in the characterization of natural compounds that induce resistance by a priming mechanism. These include vitamins, chitosans, oligogalacturonides, volatile organic compounds, azelaic and pipecolic acid, among others. Overall, other than providing novel disease control strategies that meet environmental regulations, natural priming agents are valuable tools to help unravel the complex mechanisms underlying the induced resistance phenomenon. The data presented in this review reflect the novel contributions made from studying these natural plant inducers, with special emphasis placed on hexanoic acid (Hx), proposed herein as a model tool for this research field. Hx is a potent natural priming agent of proven efficiency in a wide range of host plants and pathogens. It can early activate broad-spectrum defenses by inducing callose deposition and the SA and JA pathways. Later it can prime pathogen-specific responses according to the pathogen’s lifestyle. Interestingly, Hx primes redox-related genes to produce an anti-oxidant protective effect, which might be critical for limiting the infection of necrotrophs. Our Hx-induced resistance (Hx-IR) findings also strongly suggest that it is an attractive tool for the molecular characterization of the plant alarmed state, with the added advantage of it being a natural compound.

Two structurally divergent endo-beta-1,4-glucanase (EGase) cDNAs were cloned from tomato. Although both cDNAs (Cel1 and Cel2) encode potentially glycosylated, basic proteins of 51 to 53 kD and possess multiple amino acid domains conserved in both plant and microbial EGases, Cel1 and Cel2 exhibit only 50% amino acid identity at the overall sequence level. Amino acid sequence comparisons to other plant EGases indicate that tomato Cel1 is most similar to bean abscission zone EGase (68%), whereas Cel2 exhibits greatest sequence identity to avocado fruit EGase (57%). Sequence comparisons suggest the presence of at least two structurally divergent EGase families in plants. Unlike ripening avocado fruit and bean abscission zones in which a single EGase mRNA predominates, EGase expression in tomato reflects the overlapping accumulation of both Cel1 and Cel2 transcripts in ripening fruit and in plant organs undergoing cell separation. Cel1 mRNA contributes significantly to total EGase mRNA accumulation within plant organs undergoing cell separation (abscission zones and mature anthers), whereas Cel2 mRNA is most abundant in ripening fruit. The overlapping expression of divergent EGase genes within a single species may suggest that multiple activities are required for the cooperative disassembly of cell wall components during fruit ripening, floral abscission, and anther dehiscence.

We have demonstrated that root treatment with hexanoic acid protects tomato plants against Botrytis cinerea. Hexanoic acid-induced resistance (Hx-IR) was blocked in the jasmonic acid (JA)-insensitive mutant jai1 (a coi1 homolog) and in the abscisic acid (ABA)-deficient mutant flacca (flc). Upon infection, the LoxD gene as well as the oxylipin 12-oxo-phytodienoic acid and the bioactive molecule JA-Ile were clearly induced in treated plants. However, the basal ABA levels were not altered. Hexanoic acid primed callose deposition against B. cinerea in a cultivar-dependent manner. Treated plants from Ailsa Craig, Moneymaker, and Rheinlands Ruhm showed increased callose deposition but not from Castlemart. Hexanoic acid did not prime callose accumulation in flc plants upon B. cinerea infection; therefore, ABA could act as a positive regulator of Hx-IR by enhancing callose deposition. Furthermore, although hexanoic acid protected the JA-deficient mutant defensless1 (def1), the priming for callose was higher than in the wild type. This suggests a link between JA and callose deposition in tomato. Hence, the obtained results support the idea that callose, oxylipins, and the JA-signaling pathway are involved in Hx-IR against B. cinerea. Moreover our data support the relevance of JA-signaling for basal defense against this necrotroph in tomato. Hexanoic acid also protected against Pseudomonas syringae, indicating a broad-spectrum effect for this new inducer.