Xylem‐transported glucose as an additional carbon source for leaf isoprene formation in Quercus robur

Abstract

Summary In order to test whether xylem‐transported carbohydrates are a potential source for isoprene biosynthesis, [U‐ 13 C]‐labelled α‐ d ‐glucose was fed via cut ends of stems into the xylem of Quercus robur seedlings and the incorporation of 13 C into isoprene emitted was studied. Emission of 13 C‐labelled isoprene was monitored in real time by proton‐transfer‐reaction mass spectrometry (PTR‐MS). A rapid incorporation of 13 C from xylem‐fed glucose into single (mass 70) and double (mass 71) 13 C‐labelled isoprene molecules was observed after a lag phase of approx. 5–10 min. This incorporation was temperature dependent and was highest (up to 13% 13 C of total carbon emitted as isoprene) at the temperature optimum of isoprene emission (40–42°C), when net assimilation was strongly reduced. Fast dark‐to‐light transitions led to a strong single or double 13 C‐labelling of isoprene from xylem‐fed [U‐13C]glucose. During a period of 10–15 min up to 86% of all isoprene molecules became single or double 13 C‐labelled, resulting in a 13 C‐portion of up to 27% of total carbon emitted as isoprene. The results provide evidence that xylem‐transported glucose or its degradation products can potentially be used as additional precursors for isoprene biosynthesis and that this carbon source becomes more important under conditions of limited photosynthesis.

<sup>13</sup>C labelling study of constitutive and stress-induced terpenoide missions from Norway spruce and Scots pine

“…This finding supports results from previous studies on isoprene and monoterpenes from deciduous species (Loreto et al, 1996;Kreuzwieser et al, 2002;Schnitzler et al, 2004; Biogeosciences Discuss., https://doi.org /10.5194/bg-2017-260 Manuscript under review for journal Biogeosciences Discussion started: 4 July 2017 c Author(s) 2017. CC BY 4.0 License.…”

“…The contributions of multiple carbon sources to isoprene biosynthesis are well discussed in previous studies (Kreuzwieser et al, 2002;Schnitzler et al, 2004;Ghirardo et al, 2011): carbon taken from atmospheric CO 2 , xylem transported glucose and other leaf internal carbon pools, e.g. starch, can contribute together to isoprene biosynthesis and cause the uncomplete labelling of the emitted molecules.…”

“…On 25 the day before the 13 C-glucose experiment, small branches of the individual plants were carefully cut and immediately transferred into a small glass chamber of 3 L volume. They were re-cut under water and initially fed with 10mM glucose solution (Kreuzwieser et al, 2002;Ghirardo et al, 2011) containing the natural abundance of 13 C. The small chamber was placed in the same walk-in climate chamber as the tested plants and equipped with an extra discharge lamp causing a light intensity of approximately 600 μmol m -2 s -1 , similar as for the living plants. Between 1.5-2 L min -1 of the air containing 350 30 ppm CO 2 with the natural 13 C abundance was pumped through the small chamber.…”

“…Alternative carbon sources are suggested to be refixation of unlabelled respiratory CO 2 (Loreto et al, 2004), xylemtransported glucose (Kreuzwieser et al, 2002) or starch-breakdown (Schnitzler et al, 2004). The labelling during feeding 13 C-glucose showed that this well-known alternative carbon source provided 13 C-atoms for all tested terpenoids.…”

“…Experiments with feeding 13 C-enriched carbon sources such as 13 CO 2 and 13 C-glucose allow studying the contributions of different carbon sources to the terpenoid synthesis (Karl et al, 2002;Kreuzwieser et al, 2002;Loreto et al, 2004;Schnitzler et al, 2004;Ghirardo et al, 2011). Besides atmospheric CO 2 , alternative carbon sources contribute simultaneously to isoprene and monoterpene biosynthesis and their contribution become especially important when photosynthesis is limited 25 (Brilli et al, 2007).…”