Insect oral secretions suppress wound-induced responses in Arabidopsis

Abstract

The induction of plant defences and their subsequent suppression by insects is thought to be an important factor in the evolutionary arms race between plants and herbivores. Although insect oral secretions (OS) contain elicitors that trigger plant immunity, little is known about the suppressors of plant defences. The Arabidopsis thaliana transcriptome was analysed in response to wounding and OS treatment. The expression of several wound-inducible genes was suppressed after the application of OS from two lepidopteran herbivores, Pieris brassicae and Spodoptera littoralis. This inhibition was correlated with enhanced S. littoralis larval growth, pointing to an effective role of insect OS in suppressing plant defences. Two genes, an ERF/AP2 transcription factor and a proteinase inhibitor, were then studied in more detail. OS-induced suppression lasted for at least 48 h, was independent of the jasmonate or salicylate pathways, and was not due to known elicitors. Interestingly, insect OS attenuated leaf water loss, suggesting that insects have evolved mechanisms to interfere with the induction of water-stress-related defences.

Fig. 1.

Insect OS suppress Arabidopsis defences. (A) Weight gain of S. littoralis larvae feeding on control plants (white bar), wounded plants (black bar), or on plants treated with S. littoralis OS (dark grey bar) was measured after 3 d. Every day, 12 larvae were transferred to freshly treated plants. Values (±SE) are the mean of eight independent measurements. Bars with different letters differ at P value <0.05 (Student–Newman–Keuls Method). (B) Weight gain of S. littoralis larvae feeding on wounded plants (black bar) or on plants treated with P. brassicae OS (light grey bar) was measured after 7 d. Values (±SE) are the mean of three independent experiments. *P value <0.05 (Student’s t test).

Fig. 2.

Kinetics of suppression of wound-responsive genes by insect oral secretions (OS). Expression of ERF/AP2 TF (A) and Protease Inhibitor (B) was monitored for several hours after wounding (black bars), and after treatment with S. littoralis OS (dark grey bars), or P. brassicae OS (light grey bars) by qPCR. Untreated plants were used as controls (white bars). Values (±SE) are normalized to the reference gene and each time point is the mean of at least three biological replicates. Statistical differences between wounding and OS treatment are indicated (Student’s t test, *P value <0.05, **P value <0.01).

Fig. 3.

Suppression of wound-responsive genes is independent of JA and SA pathways. Expression of ERF/AP2 TF and Protease Inhibitor was monitored by qPCR after wounding (black bars), and after treatment with S. littoralis OS (dark grey bars), or P. brassicae OS (light-grey bars) on the jasmonate-insensitive mutant coi1-1 (A) and the SA-deficient mutant sid2-1 (B). Untreated plants were used as controls (white bars). Values (±SE) are normalized to the reference gene and are the mean of two biological replicates. Statistical differences between wounding and OS treatment are indicated (Student’s t test, *P value <0.05, **P value <0.01).

Fig. 4.

Role of FACs in the suppression of wound-responsive genes. Expression of ERF/AP2 TF was monitored by qPCR after wounding (black bars), and after treatment with S. littoralis OS (dark grey bars), P. brassicae OS (light grey bars), or FACs (hatched bars) for 10 hr. Untreated plants were used as controls (white bars). (A) Larvae were fed with Col-0 or fad378 mutant plants lacking 18:3. (B) 1, 18:3-GLN; 2, 18:3-GLU; 3, 18:2-GLN; 4, 18:2-GLU. Values (±SE) are normalized to the reference gene and are the mean of two biological replicates. Statistical differences between wounding and OS treatment are indicated (Student’s t test, *P value <0.05, **P value <0.01).

Fig. 5.

Insect OS reduce wound-induced water loss. Leaves were wounded (open squares) or treated with P. brassicae OS (open circles). Untreated plants were used as controls (filled circles). Fresh weight of detached leaves was measured for 6 h. Values (±SE) are the mean of 30 leaves and are expressed relative to the initial fresh weight.