Induced epidermal permeability modulates resistance and susceptibility of wheat seedlings to herbivory by Hessian fly larvae

Abstract

Salivary secretions of neonate Hessian fly larvae initiate a two-way exchange of molecules with their wheat host. Changes in properties of the leaf surface allow larval effectors to enter the plant where they trigger plant processes leading to resistance and delivery of defence molecules, or susceptibility and delivery of nutrients. To increase understanding of the host plant's response, the timing and characteristics of the induced epidermal permeability were investigated. Resistant plant permeability was transient and limited in area, persisting just long enough to deliver defence molecules before gene expression and permeability reverted to pre-infestation levels. The abundance of transcripts for GDSL-motif lipase/hydrolase, thought to contribute to cuticle reorganization and increased permeability, followed the same temporal profile as permeability in resistant plants. In contrast, susceptible plants continued to increase in permeability over time until the entire crown of the plant became a nutrient sink. Permeability increased with higher infestation levels in susceptible but not in resistant plants. The ramifications of induced plant permeability on Hessian fly populations are discussed.

Fig. 1.

Controls for induced permeability. H9-Iris (resistant) and Newton (susceptible) seedlings received the following treatments before staining with neutral red: infestation with Biotype L Hessian fly larvae that hatched 3 d before staining (A–D; same plants before and after staining), no infestation (E and F), puncturing with a minuten pin (diameter 200 μm) immediately before staining (G and H), puncturing and infestation (I and J), infestation with just one larva per plant (K and L: the length of dark streaks in L is 13.4 mm and the length of one first-instar larva is ∼770 μm), and infestation by 10 larvae (M). Seedling diameter is ∼2 mm. Black ink from a Sharpie pen was used to mark the locations of larvae (which often dislodged during staining) and punctures, but did not induce permeability. Compatible interactions between H9-Iris and vH9 larvae (not shown) were indistinguishable from compatible interactions between Newton and Biotype L larvae.

Fig. 2.

Effect of the infestation level on rating of six characters associated with permeability. Leaf 3 was stained 3 DAH. The same data set for 212 plants from 16 infestation level groups were analysed in Fig. 2, Fig. 3, and Table 1. Univariant comparisons utilized unpaired Student's t-test analysis. Characteristics of neutral red stain absorption were either rated (red staining intensity rating scale from 0 to 7 and background blush rating scale from 0 to 5) or counted (number of dark streaks, spots, broken lines, and solid lines) and then averaged for plants undergoing varying levels of infestation (larvae per plant). Examples of staining intensities in the rating scale are shown in Fig. 1, with ratings of ‘0’ representing plants with no absorbed stain and ratings of ‘7’ (red staining-intensity rating) and ‘5’ (background blush rating) representing plants stained the darkest. White bars correspond to characteristic averages for resistant H9-Iris infested with Biotype L larvae (incompatible). Black bars correspond to susceptible H9-Iris infested with vH9 larvae (compatible). Striped bars correspond to susceptible Newton infested with Biotype L larvae (compatible). Standard error of the mean analysis is shown.

Fig. 3.

Multivariate canonical discriminant plot analysis of six characters associated with permeability at different infestation levels. Centroids of the 16 infestation level groups are shown inside boxes marked with the corresponding group numbers from Table 1. Gradient vectors show the relationship of the six scored characteristics [red staining intensity rating (RR), background blush score (BS), spot count (SC), broken line count (BL), solid line count (SL), and dark streak count (DS)] with regards to canonical axes 1 and 2. Data for uninfested control plants are indicated with filled triangles, while data for resistant plants are shown by open circles and for susceptible plants by filled circles. See the Materials and methods for a detailed explanation of the plot.

Fig. 4.

Changes in plant cell permeability over time. H9-Iris seedlings were infested with Biotype L (resistant plant/avirulent larvae/incompatible interaction) or vH9 Hessian fly (susceptible plant/virulent larvae/compatible interaction). Plant tissues were harvested after 12 h and at 1, 2, 3, 4, 5, 6, 8, 10, and 12 DAH. Examples from 12 h and days 5, 6, 10, and 12 are not shown. After removal of the outer leaf, the tissues were stained with neutral red. Seedling diameter is ∼1.5 mm at 1 DAH and 2.5 mm at 8 DAH.

Fig. 5.

Influence of Hessian fly larvae on wheat GDSL-lipase (EST CA709446) mRNA abundance. Transcript levels were quantified by qRT-PCR in samples from developing leaf tissue at the crown of seedlings involved in incompatible (resistant H9-Iris wheat infested with Biotype L larvae; black bars) and compatible (susceptible Newton wheat infested with Biotype L larvae; white bars) interactions or from uninfested control plants (baseline of 0). Bars represent log fold change ±SE of infested samples with respect to uninfested controls. Numbers above or below bars indicate non-log fold change in samples showing significant differences from control levels (P <0.05). The experiment was conducted with three biological replicates each subjected to qRT-PCR three times (three technical replicates).

Fig. 6.

Summary of trends over time for the wheat leaf hosting Hessian fly larvae in resistant and susceptible plants. Information about incompatible interactions (resistant wheat and avirulent larvae) is shown by dashed lines and italic text, and information about compatible interactions (susceptible plants and virulent larvae) is show by solid lines and Roman text. Data are summarized from the following: changes in permeability, this study; changes in leaf growth rate, Anderson and Harris (2008); abundance of lectin mRNAs, Subramanyam et al. (2006) and Giovanini et al. (2007); abundance of waxes, cutins, and mRNAs encoding proteins involved in maintenance of cuticle integrity, Kosma et al. (2010) and Saltzmann et al. (2010); abundance of peroxidase mRNAs encoding proteins that produce reactive oxygen species in plant defence, Liu et al. (2010); timing of appearance of features including epidermal ruptures, abundance of organelles (endoplasmic reticulum, small vesicles), increase in epidermal cell wall thickness, development of nutritive tissue, and diversion of photo-assimilates to the nutritive tissue, Harris et al. (2006, .