Photoprotective isoprenoids as physiological markers for the adaptation of Douglas fir to drought stress and climate change
Final Report Abstract
Many plants emit volatile organic compounds (VOC) into the atmosphere. On a quantitative basis, the most important VOC are the isoprenoids isoprene, monoterpenes and sesquiterpenes. Once they are produced, they are released either from storage pools or directly from their compartment of biosynthesis. Although many studies have been performed, the function of volatile isoprenoids is still not completely understood. Isoprenoids seem to be involved in plant defense against biotic and abiotic stress. They might exert a role in protecting plants from thermal damage and/or serve as an antioxidant in leaves where they confer protection against singlet oxygen; they seem to exert this protective action at the membrane level. In good agreement with the assumed antioxidative role of volatile isoprenoids, maximal emission rates occur during periods of increased air temperature and under drought conditions, but not in all plant species. Douglas fir (Pseudotsuga menziesii) is a typical monoterpene emitter. This tree species has an extremely wide range of natural distribution in the west of North America where it spans from 23° to 55° in latitude, and from 100° to 128° in longitude. Consequently, there are numerous different ecotypes adapted to the specific environmental conditions. Douglas fir (Pseudotsuga menziesii) has been divided into the coastal variety (var. menziesii) occurring often at sites well supplied with water and the interior variety (var. glauca) originating at much drier sites and considered more drought tolerant. In the present project we aimed to clarify if the higher drought tolerance of interior provenances compared to the less tolerant coastal provenances is connected to the pattern of biosynthesis, abundance and/or emission of volatile isoprenoid compounds. It was planned to test the following working hypotheses: (i) Volatile isoprenoids are important drought defense compounds; they accumulate in needles during drought stress. (ii) Such accumulation occurs at the expense of non-volatile isoprenoids (e.g. carotenoids). A changed metabolic flux in the isoprenoid pathway therefore exists during stress periods. (iii) Trees of drought tolerant Douglas fir provenances produce constitutive defense features. Even under non stress conditions, therefore, stress tolerant (interior) Douglas fir provenances contain more isoprenoids and release them at higher rates into the atmosphere than less tolerant provenances. (iv) Volatile isoprenoids possess different efficiencies in alleviating drought stress. Different spectra of isoprenoids (pools and emitted) are therefore proposed for provenances with different stress tolerance (interior vs. coastal). In the present project, we performed both, field and laboratory based studies to test these hypotheses. In the field, we took advantage of a ‘common garden experiment’ which was established at different field sites in Baden-Württemberg in 1961. Trees of a variety of interior and coastal provenances originating from sites in North-West America were planted in defined plots at these sites. The field sites were managed and maintained since 1961 by the Forest Research Institute, Freiburg. For our studies, 4 provenances (1 interior, 3 coastal) were selected of which gas exchange, isoprenoid emission and needle stored isoprenoid content was determined. We observed that needles of coastal provenances stored similar amounts and composition of isoprenoids, interior provenances, however, contained considerably higher amounts of isoprenoids with clearly different composition. The emission of isoprenoids seemed to be affected by drought/high temperature in the field (only coastal provenances) but not by drought only in experiments under controlled conditions (both, interior and coastal prov.). However, in an experimental approach, severe drought caused elevated isoprenoid pools in needles of coastal provenances. This work highlighted clear evidence that isoprenoid metabolism is influenced by drought stress in a provenance specific manner. It still has to be clarified if the compounds produced during drought exert stress protective functions in Douglas fir trees.
Publications
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(2012) A catalogue of putative unique transcripts from Douglas-fir (Pseudotsuga menziesii) based on 454 transcriptome sequencing of genetically diverse, drought stressed seedlings. BMC Genomics 13, 673
Müller, T., Ensminger, I., Schmid, K.J.
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(2013) Tree ring isotopic composition, radial increment and height growth reveal provenance-specific reactions of Douglas-fir towards environmental parameters. Trees - Structure and Function 27, 37-52
Jansen, K., Sohrt, J., Kohnle, U., Ensminger, I., Gessler, A.
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(2013). Suitability of Illumina deep mRNA sequencing for reliable gene expression profiling in a non-model conifer species (Pseudotsuga menziesii). Tree Genetics & Genomes, 9(6), 1513-1527
Hess M, Wildhagen H, Ensminger I
