Water Stress and Aphid Feeding Differentially Influence Metabolite Composition in <em>Arabidopsis thaliana</em> (L.)

<div><p>Little is known about how drought stress influences plant secondary metabolite accumulation and how this affects plant defense against different aphids. We therefore cultivated <em>Arabidopsis thaliana</em> (L.) plants under well-watered, drought, and water-logged conditions. Two aphid species were selected for this study: the generalist <em>Myzus persicae</em> (Sulzer) and the crucifer specialist <em>Brevicoryne brassicae</em> (L.). Metabolite concentrations in the phloem sap, which influence aphid growth, changed particularly under drought stress. Levels of sucrose and several amino acids, such as glutamic acid, proline, isoleucine, and lysine increased, while concentrations of 4-methoxyindol-3-ylmethyl glucosinolate decreased. <em>M. persicae</em> population growth was highest on plants under drought stress conditions. However, <em>B. brassicae</em> did not profit from improved phloem sap quality under drought stress and performed equally in all water treatments. Water stress and aphids generally had an opposite effect on the accumulation of secondary metabolites in the plant rosettes. Drought stress and water-logging led to increased aliphatic glucosinolate and flavonoid levels. Conversely, aphid feeding, especially of <em>M. persicae</em>, reduced levels of flavonoids and glucosinolates in the plants. Correspondingly, transcript levels of aliphatic biosynthetic genes decreased after feeding of both aphid species. Contrary to <em>M. persicae</em>, drought stress did not promote population growth of <em>B. brassicae</em> on these plants. The specialist aphid induced expression of <em>CYP79B2</em>, <em>CYP79B3</em>, and <em>PAD3</em> with corresponding accumulation of indolyl glucosinolates and camalexin. This was distinct from <em>M. persicae</em>, which did not elicit similarly strong camalexin accumulation, which led to the hypothesis of a specific defense adaptations against the specialist aphid.</p> </div

Overall effects of aphid species and water stress on secondary metabolite accumulation in <i>A. thaliana</i>.

<p>Scheme depicting the effect of aphid infestation (biotic stress) or water stress (abiotic stress) on inducing (+), reduction (−), or not changing (+/−) plant metabolites.</p

Concentrations of amino acid in the phloem sap of <i>A. thaliana</i> plants cultivated under different water stress treatments.

<p>n = 4, Mean values followed by different letters within a row and for one sampling day indicate significant differences for treatments, Tukey’s HSD test <i>P</i><0.05;</p>*<p>Essential amino acids as defined by Morris (1991).</p

Drought stress reduced indolyl glucosinolate concentrations in the phloem sap of plants.

<p>Glucosinolate (GS) levels in the phloem sap of <i>A. thaliana</i> plants cultivated for one week under different water conditions (n = 3, different letters indicate significant differences in total glucosinolate levels between water treatments, and different Greek letters indicate significant differences in single compounds between the water treatments, Tukey’s HSD test <i>P</i><0.05).</p

Sucrose levels in phloem sap increased under drought stress.

<p>Sugar levels in the phloem sap of <i>A. thaliana</i> plants cultivated under different water conditions (n = 3, different letters indicate significant differences between treatments, * indicates significant differences among treatments of sucrose only, Tukey’s HSD test <i>P</i><0.05).</p

The aphid species elicited different camelexin accumulation in plants under different water treatments.

<p>Camalexin levels in control and aphid infested <i>A. thaliana</i> plants after two weeks cultivation under different water conditions (n = 5, different letters indicate significant differences between water treatments and bars with an asterisk indicate significant differences in <i>B. brassicae</i> infestation compared to <i>M. persicae</i> and controls, Tukey’s HSD test, <i>P</i><0.05; WC: well-watered control, DC: drought control, LC: water-logged control, WM: well-watered + <i>M. persicae</i>, DM: drought + <i>M. persicae</i>, LM: water-logged + <i>M. persicae</i>, WB: well-watered + <i>B. brassicae</i>, DB: drought + <i>B. brassicae</i>, LB: water-logged + <i>B. brassicae</i>).</p

Transcript levels of glucosinolate biosynthetic, regulatory and camalexin biosynthesis genes in <i>A. thaliana</i> after two weeks’ water stress and one week of aphid feeding (plants39 days old, n = 3 biological replicates).

<p>Positive value: fold increase, negative value fold decrease, *and **indicate significant differences in transcript levels at <i>P</i><0.1 and <i>P</i><0.05, Tukey’s HSD test.</p

Water stress reduced plant water content and dry weight.

<p>Mean water content (A) and final dry weight (B) of <i>A. thaliana</i> plants after cultivation under different water conditions (n = 10, error bars: SD, different letters indicate significant differences in values obtained between treatments, Tukey’s HSD test <i>P</i><0.05).</p

Especially <i>M. persicae</i> reduced aliphatic glucosinolate levels under all water conditions.

<p>Glucosinolate (GS) levels in <i>A. thaliana</i> plants cultivated under different water conditions and after one week feeding of <i>M. persicae</i> (Mp) and <i>B. brassicae</i> (Bb) (n = 5, different letters indicate significant differences in total glucosinolate levels of control and aphid-treated plants within a water treatment, different Greek letters indicate significant differences in single compounds within each water treatments for the main compounds, Tukey’s HSD test <i>P</i><0.05; minor compounds not significantly different).</p

Drought stress favors the population growth of <i>Myzus persicae</i> on <i>A. thaliana</i>.

<p>Average population size of <i>B. brassicae</i> and <i>M. persicae</i> after feeding for 7 days on <i>A. thaliana</i> plants cultivated under different water conditions (n = 10, error bars: SE, different letters indicate significant differences in aphid species numbers between treatments, Tukey’s HSD test <i>P</i><0.05).</p