The folate precursor para-aminobenzoic acid elicits induced resistance against Cucumber mosaic virus and Xanthomonas axonopodis

Abstract

Background and aims: The use of vitamins including vitamin B1, B2 and K3 for the induction of systemic acquired resistance (SAR) to protect crops against plant pathogens has been evaluated previously. The use of vitamins is beneficial because it is cost effective and safe for the environment. The use of folate precursors, including ortho-aminobenzoic acid, to induce SAR against a soft-rot pathogen in tobacco has been reported previously.

Methods: In the present study, para-aminobenzoic acid (PABA, also referred to as vitamin Bx) was selected owing to its effect on the induction of SAR against Xanthomonas axonopodis pv. vesicatoria in pepper plants through greenhouse screening.

Key results: Dipping of pepper seedlings in a 1 mm PABA solution in field trials induced SAR against artificially infiltrated X. axonopodis pv. vesicatoria and naturally occurring cucumber mosaic virus. Expression of the Capsicum annuum pathogenesis-related 4 gene was primed in response to pathogen infection as assessed by quantitative real-time PCR. The accumulation of cucumber mosaic virus RNA was reduced in PABA-treated pepper plants at 40 and 105 d post-treatment. Unexpectedly, fruit yield was increased in PABA-treated plants, indicating that PABA-mediated SAR successfully protected pepper plants from infection by bacterial and viral pathogens without significant fitness allocation costs.

Conclusions: The present study is the first to demonstrate the effective elicitation of SAR by a folate precursor under field conditions.

Fig.1.

Screening of folate precursors for the induction of systemic resistance in pepper plants under greenhouse conditions. (A) The chemical structures of OABA, MABA and PABA are shown. (B) The folate precursors were inoculated into the potting media containing 3-week-old pepper seedlings at concentrations of 10 mm and 100 µm. (B and C) Disease severity (0–5) of pepper seedlings treated with folate precursors at 10 mm and 100 µm PABA was assessed 7 d after infection with X. axonopodis pv. vesicatoria at 10⁴ and 10⁶ cfu mL⁻¹. Water and 0.5 mm BTH were used as negative and positive controls, respectively. Asterisks in (B) and (C) indicate statistically significant differences compared with water-treated control plants (P = 0.05). Error bars indicate the s.e.

Fig. 2.

Induction of systemic resistance by 1 mm PABA under field conditions. (A) Disease severity was measured at 20, 30 and 40 d after X. axonopodis pv. vesicatoria challenge at 10⁶ cfu mL⁻¹ in plants pretreated with 1 mm PABA and 0.5 mm BTH. Bars represent the mean ± s.e. (sample size, n = 12 replications per treatment). Different letters indicate significant differences between treatments in each time point (P = 0.05 according to least significant difference). The experiment was repeated four times with similar results. (B) The expression of coat protein genes was measured 20, 30 and 40 d after treatment with PABA, BTH and the control in pepper plants. Bars represent the mean value ± s.e. (n = 3). The housekeeping gene CaActin was used as a control. The experiment was repeated twice with similar results.

Fig. 3.

Transcriptional expression of defence-related genes. The expression levels of the pepper resistance genes CaPR4, CaPR9, CaTin1 and CaPIN2 were assessed by qRT-PCR. Bars represent the mean value ± s.e. (n = 3). The housekeeping gene CaActin was used as a control. The experiment was repeated twice with similar results.

Fig. 4.

PABA elicited SAR against naturally occurring bacterial pathogens. (A, B) Disease severity (0–5) and a population of the naturally occurring pathogen X. axonopodis pv. vesicatoria at 77 dpt. (C, D) Disease severity (0–5) and expression of the CMV coat protein gene at 77 dpt. Water and 1 mm BTH were used as negative and positive controls, respectively. Four independent experiments were performed with ten pepper plants per treatment. Different letters indicate statistically significant differences compared with water-treated control plants (P = 0.05). Error bars indicate the s.e.

Fig. 5.

Increase in pepper yield induced by PABA. Fruit yield of PABA- or control-treated pepper plants was assessed at 64 and 77 dpt. Water and 1 mm BTH were used as the negative and positive controls, respectively. Different letters indicate statistically significant differences compared with water-treated control plants (P = 0.05). Error bars indicate the s.e.