Aluminum induces cross-resistance of potato to Phytophthora infestans

Abstract

The phenomenon of cross-resistance allows plants to acquire resistance to a broad range of stresses after previous exposure to one specific factor. Although this stress-response relationship has been known for decades, the sequence of events that underpin cross-resistance remains unknown. Our experiments revealed that susceptible potato (Solanum tuberosum L. cv. Bintje) undergoing aluminum (Al) stress at the root level showed enhanced defense responses correlated with reduced disease symptoms after leaf inoculation with Phytophthora infestans. The protection capacity of Al to subsequent stress was associated with the local accumulation of H2O2 in roots and systemic activation of salicylic acid (SA) and nitric oxide (NO) dependent pathways. The most crucial Al-mediated changes involved coding of NO message in an enhanced S-nitrosothiol formation in leaves tuned with an abundant SNOs accumulation in the main vein of leaves. Al-induced distal NO generation was correlated with the overexpression of PR-2 and PR-3 at both mRNA and protein activity levels. In turn, after contact with a pathogen we observed early up-regulation of SA-mediated defense genes, e.g. PR1, PR-2, PR-3 and PAL, and subsequent disease limitation. Taken together Al exposure induced distal changes in the biochemical stress imprint, facilitating more effective responses to a subsequent pathogen attack.

Fig. 1

The qRT-PCR analysis of PR-1, PR-2, PR-3 and PAL gene expression in roots (a) and leaves (b) of potato exposed to 250 μM AlCl3 at 48 h. β-1,3-Glucanase (c) and chitinase (d) activities in roots and leaves of Al-treated potato. Asterisks indicate values that differ significantly from the non-treated (control) potato plants at P < 0.05, n = 3

Fig. 2

The level of free SA and SA conjugated with Glc in potato plants exposed to 250 μM AlCl3 at 48 h. SA and SAG content was measured in roots (a), shoots (b) and leaves (c). Asterisks indicate values that differ significantly from the non-treated control plants at P < 0.05

Fig. 3

Bio-imaging of NO with a Cu-FL fluorescent probe in potato roots (a–c) and leaves (d, e, g–i) at 48 h after root exposure to 250 μM AlCl₃. Images show general phenomena representative of three individual experiments; control of background where the fluorescent probe was omitted (f). Bars indicate 200 μm (d–f), 100 μm (a, c, g, h) and 20 μm (b, i). Measurement of FL-NO fluorescence in extracts of potato roots (j), stems (k) and leaves (l) exposed to aluminum. NO production was assayed spectrofluorimetrically using a selective NO sensor (Cu-FL). FL-NO fluorescence intensity represents mean values for the average of data ± SD of three independent experiments

Fig. 4

The effect of aluminum stress at 48 h, supplied as 250 μM AlCl₃, on hydrogen peroxide accumulation in roots (a), stems (b) and leaves (c) of potato cv. ‘Bintje’. Asterisks indicate values that differ significantly from non-treated control potato plants at P < 0.05

Fig. 5

Total contents of S-nitrosothiols (SNOs) in roots (a), stems (b) and leaves (c) of potato cv. ‘Bintje’ treated with 250 μM AlCl₃ or without AlCl₃ (acidic control) at 48 h. Nitrosothiol content was determined by chemiluminescence using a Sievers^® Nitric Oxide Analyzer NOA 280i. Detection of SNOs in potato leaves by immunofluorescence histochemistry using Alexa Fluor 405 Hg-Link reagent phenylmercury. Blue fluorescence attributable to SNOs in roots (d, e) and leaves (f–j) of control and Al-treated potato. Bars indicate 250 μm (d, e), 200 μm (h, i), 100 μm (f, g, j). Asterisks indicate values that differ significantly from non-treated control potato plants at P < 0.05

Fig. 6

The effect of aluminum stress at 48 h, supplied as 250 μM AlCl₃, on GSNO reductase activity in roots (a), stems (b) and leaves (c) of potato cv. ‘Bintje’. Asterisks indicate values that differ significantly from non-treated control potato plants at P < 0.05

Fig. 7

Systemic protection of potato cv. ‘Bintje’ against P. infestans. Potato plants were treated at root level with water (acid control) or exposed to aluminum stress for 48 h and then inoculated at the leaf level. The index of disease development in potato leaves at 24, 48, 72 h after P. infestans challenge inoculation (hpi) represents the percentage of leaf area covered by late blight symptoms. Values are means of the disease index of 20 leaves from three independent experiments

Fig. 8

The effect of pretreatment with aluminum followed by challenge inoculation with P. infestans on PR-1, PR-2, PR-3 and PAL gene expression in potato leaves. The qRT-PCR analyses of PRs and PAL were performed at 24, 48 and 72 h after challenge inoculation. Asterisks indicate values that differ significantly from non-treated, P. infestans inoculated leaves at P < 0.05, n = 3

Fig. 9

The proposed mechanism of Al induced cross-resistance of potato to P. infestans, in which moderate Al stress triggered changes in short-term biochemical imprint in distal potato leaves, facilitating effective defense responses against a subsequent pathogen attack. First, plant pretreatment with Al at the root level provoked redox imbalance manifested in H₂O₂ overproduction and diminished NO synthesis. These local changes might create a redox background for distal NO and SA-dependent signal generation. The complex regulatory networks facilitated systemic activation of Al stress responses engaged in the biochemical imprint linked to the coding of NO message in reversible SNO storage. Finally, signal amplification leading to potato resistance manifested in distal leaves of Al-treated plants was related to expression of SA-mediated defense genes (PRs and PAL) early after contact with a challenging pathogen and to subsequent disease limitation