There is plenty of evidence for improved nutrient acquisition by ectomycorrhizas in trees; however, their role in water uptake is much less clear. In addition to experiments showing improved performance during drought by mycorrhizal plants, there are several studies showing reduced root hydraulic conductivity and reduced water uptake in mycorrhizal roots. The clearest direct mechanism for increased water uptake is the increased extension growth and absorbing surface area, particularly in fungal species with external mycelium of the long-distance exploration type. Some studies have found increased aquaporin function and, consequently, increased root hydraulic conductivity in ectomycorrhizal plants while other studies showed no effect of ectomycorrhizal associations on root water flow properties. The aquaporin function of the fungal hyphae is also likely to be important for the uptake of water by the ectomycorrhizal plant, but more work needs to be done in this area. The best-known indirect mechanism for mycorrhizal effects on water relations is improved nutrient status of the host. Others include altered carbohydrate assimilation via stomatal function, possibly mediated by changes in growth regulator balance; increased sink strength in mycorrhizal roots; antioxidant metabolism; and changes in osmotic adjustment. None of these possibilities has been sufficiently explored. The mycorrhizal structure may also reduce water movement because of different fine root architecture (thickness), cell wall hydrophobicity or the larger number of membranes that water has to cross on the way from the soil to the xylem. In future studies, pot experiments comparing mycorrhizal and nonmycorrhizal plants will still be useful in studying well-defined physiological details. However, the quantitative importance of ectomycorrhizas for tree water uptake and water relations can only be assessed by field studies using innovative approaches. Hydraulic redistribution can support nutrient uptake during prolonged dry periods. In large trees with deep root systems, it may turn out that the most important function of mycorrhizas during drought is to facilitate nutrient acquisition.

Cryptochromes (CRYs) and UV RESISTANCE LOCUS 8 (UVR8) photoreceptors perceive UV-A/blue (315–500 nm) and UV-B (280–315 nm) radiation in plants, respectively. While the roles of CRYs and UVR8 have been studied in separate controlled-environment experiments, little is known about the interaction between these photoreceptors. Here, Arabidopsis wild-type Ler, CRYs and UVR8 photoreceptor mutants (uvr8-2, cry1cry2 and cry1cry2uvr8-2), and a flavonoid biosynthesis-defective mutant (tt4) were grown in a sun simulator. Plants were exposed to filtered radiation for 17 d or for 6 h, to study the effects of blue, UV-A, and UV-B radiation. Both CRYs and UVR8 independently enabled growth and survival of plants under solar levels of UV, while their joint absence was lethal under UV-B. CRYs mediated gene expression under blue light. UVR8 mediated gene expression under UV-B radiation, and in the absence of CRYs, also under UV-A. This negative regulation of UVR8-mediated gene expression by CRYs was also observed for UV-B. The accumulation of flavonoids was also consistent with this interaction between CRYs and UVR8. In conclusion, we provide evidence for an antagonistic interaction between CRYs and UVR8 and a role of UVR8 in UV-A perception.

No abstract

Summary• Changes in secondary metabolites in silver birch ( Betula pendula ) seedlings are presented in response to ultraviolet-B radiation (UV-B) and nutrient addition.• Concentrations of individual secondary metabolites and nutrients were measured in leaves of greenhouse-grown silver birch seedlings exposed to five time-integrated irradiances of biologically effective UV-B () and fertilized with two relative nutrient addition rates.• Several phenolics were not only induced by UV-B, but their concentration was dependent on UV-B daily time-integrated irradiance. Relative nutrient addition rate also affected the concentration of phenolics but had little effect on the UV-B daily time-integrated irradiance-responses. The mineral nutrient concentration of leaves was affected by nutrient addition but not by increasing . Possible thresholdexposures for the accumulation of secondary metabolites or nutrients were not detected.• UV-B and relative nutrient addition rate have mainly an additive (rather than synergistic) effect in silver birch, and leaves respond to increasing UV-B by synthesizing metabolites (e.g. flavonols), which are important UV-B filters. This study reports the first UV-B-exposure-response curves for individual secondary metabolites and nutrients in leaves of a woody tree species.

The following Supporting Information is available for this article: Figure S1. Solar spectrum at different times of the day when plants were moved outdoors. Figure S2. Photon irradiance for different wavebands in solar radiation.Figure S3. Multidimensional scaling of RNA-seq data.Figure S4. Comparison between RNA-seq and qRT-PCR data.Figure S5. Venn diagrams showing the number differentially expressed genes in RNA-seq data.Figure S6. Enrichment of KEGG pathways in RNA-seq data.Figure S7. In vitro absorption spectra of Arabidopsis UVR8 protein.Figure S8. Position weight matrices of the enriched DNA-binding motifs.Figure S9. Transcript abundance of seven genes measured using qRT-PCR.Table S1. Information of primers used and genes assessed in qRT-PCR. Table S2. Summary of the ANOVA of the qRT-PCR data.Methods S1. Description of the filters and the waveband contrasts.Dataset S1. Outcome of differential gene expression analysis for the three genotypes and multiple waveband contrasts combination. The dataset is included as a separate file in .Rda format and can be read using R.