Piperonylic acid (PA) treatment induces systemic plant resistance against plant-parasitic nematodes, without negative effects on plant growth and development
Foliar PA pre-treatment enhanced resistance to the root-knot nematodeMeloidogyne incognita, as evidenced by a reduced number of galls 28 dai (-56%, P < 0.001), an effect that showed no dose-dependence over the tested concentration range (100 - 1000 µM;Figure 1a ). PA also induced resistance against the rice root-knot nematode M. graminicola at 14 dai (-40%, P < 0.001; Figure 1b ). As PA is not acutely toxic to M. incognita or M. graminicola (Table 1 ), reduced susceptibility is likely due to induced resistance (IR) rather than direct toxicity. Our data suggest a conservation of PA-IR between dicots and monocots.
To verify that PA-IR is not triggered by an unknown off-target effect of PA, we tested whether two other previously described C4H inhibitors, 4-Propynyloxybenzoic acid and 3-(4-Pyridyl)-acrylic acid (Schochet al. 2002), also induced resistance against M. graminicola in rice (see Supplementary Figure S1 ). Since these two inhibitors induced the same resistant phenotype as PA, despite their very different chemical structures and inhibition mechanisms (Schochet al. 2002), it is highly likely that C4H inhibition is indeed the root cause of PA-IR.
Since IR may have negative effects on plant growth and development, the effect of weekly foliar PA treatment at concentrations ranging from 100 - 1000 µM on tomato growth was investigated. No effect on leaf area, shoot dry mass or root length was seen (P = 0.97, P = 0.71 and P = 0.45, respectively; Figures 1c-e ). 300 µM was chosen as the standard dose for all following experiments.
The efficacy of PA-IR against root-knot nematodes was validated in a greenhouse naturally infested with M. incognita and M. javanica. Soil samples taken before the trial contained 14 000 J2s per liter of soil, indicating a severe infestation.
The effect of PA on plant health and yield was evaluated, and its effect on root-knot nematode disease severity was compared to both an untreated control and the commercial nematicide Cedroz. Both PA and Cedroz reduced disease severity, as indicated by plant gall index (P = 0.042 and P = 0.025 respectively, Figure 1f ). By the end of the trial, untreated plants showed a mean gall index (± SEM) of 8.1 ± 0.4, versus 6.3 ± 0.3 in PA-treated plants and 4.6 ± 1.1 in Cedroz-treated plants. Neither treatment significantly affected cumulative yield (PA: +13%, P = 0.77; Cedroz: +34%, P = 0.21 – Figure 1g ). No phytotoxicity or growth reduction was observed with any treatment.
Piperonylic acid treatment enhances tomato resistance to the foliar pathogens Botrytis cinerea and Pseudomonas syringae.
Foliar PA treatment enhances resistance against two foliar pathogens,B. cinerea R16 (Figure 2a-c ) and P. syringaeDC3000 (Figure 2d ). PA treatment increased the percentage of resistant interactions with B. cinerea from 34% in the control to 64% (P = 0.014), and inoculation sites also exhibited higher Fv/Fm (and thus higher photosynthetic efficiency) in PA-treated plants (+11%, P = 0.018). In an experiment with higher disease pressure, PA pre-treatment still reduced B. cinerea disease pressure (3% resistant interactions in mock-treated plants versus 34% in PA-treated plants, P < 0.001;Supplementary Figure S2a ) and again increased Fv/Fm (+20%, P = 0.003;Supplementary Figure S2b). Reduced B. cinerea disease severity was also observed in a greenhouse experiment with strawberry (Fragaria x ananassa Duchesne ‘Elsanta’), shown in