Ectomycorrhizas (EMs) alleviate stress tolerance of host plants, but the underlying molecular mechanisms are unknown. To elucidate the basis of EM-induced physiological changes and their involvement in stress adaptation, we investigated metabolic and transcriptional profiles in EM and non-EM roots of gray poplar (Populus 3 canescens) in the presence and absence of osmotic stress imposed by excess salinity. Colonization with the ectomycorrhizal fungus Paxillus involutus increased root cell volumes, a response associated with carbohydrate accumulation. The stress-related hormones abscisic acid and salicylic acid were increased, whereas jasmonic acid and auxin were decreased in EM compared with non-EM roots. Auxin-responsive reporter plants showed that auxin decreased in the vascular system. The phytohormone changes in EMs are in contrast to those in arbuscular mycorrhizas, suggesting that EMs and arbuscular mycorrhizas recruit different signaling pathways to influence plant stress responses. Transcriptome analyses on a whole genome poplar microarray revealed activation of genes related to abiotic and biotic stress responses as well as of genes involved in vesicle trafficking and suppression of auxin-related pathways. Comparative transcriptome analysis indicated EM-related genes whose transcript abundances were independent of salt stress and a set of salt stress-related genes that were common to EM non-salt-stressed and non-EM salt-stressed plants. Salt-exposed EM roots showed stronger accumulation of myoinositol, abscisic acid, and salicylic acid and higher K + -to-Na + ratio than stressed non-EM roots. In conclusion, EMs activated stress-related genes and signaling pathways, apparently leading to priming of pathways conferring abiotic stress tolerance.Under natural conditions, many economically important tree species including fast-growing poplars (Populus spp.) form ectomycorrhizas (EMs) between roots and EM fungi. Colonization with EM fungi leads to profound changes in root architecture and morphology. Usually, EM roots are strongly ramnified and EM root tips show a bulb-like appearance (Smith and Read, 2008). In EMs, plants and fungi interact mutualistically: while the plant receives mineral nutrients and water through the fungus, the fungus is supplied with carbohydrates by its host (Smith and Read, 2008). To fulfill these functions, specific anatomical structures are established, which involve the formation of a hyphal mantle ensheathing the root tip with hyphae emanating into soil for nutrient uptake. Inside the mantle, hyphae grow between, but not inside, the cortex cells within the cell wall, forming an interface called the Hartig net for bidirectional nutrient exchange.The establishment of EMs requires a coordinated developmental program in both partners of the symbiosis. Transcriptional changes during initial stages of host recognition and colonization have been the focus of several recent studies (Johansson et al., 2004;Duplessis et al., 2005;Le Queré et al., 2005;Morel et
Many forest ecosystems have evolved at sites with growth-limiting nitrogen (N) availability, low N input from external sources and high ecosystem internal cycling of N. By contrast, many poplar species are frequent constituents of floodplain forests where they are exposed to a significant ecosystem external supply of N, mainly nitrate, in the moving water table. Therefore, nitrate is much more important for N nutrition of these poplar species than for many other tree species. We summarise current knowledge of nitrate uptake and its regulation by tree internal signals, as well as acquisition of ammonium and organic N from the soil. Unlike herbaceous plants, N nutrition of trees is sustained by seasonal, tree internal cycling. Recent advances in the understanding of seasonal storage and mobilisation in poplar bark and regulation of these processes by temperature and daylength are addressed. To explore consequences of global climate change on N nutrition of poplar trees, responses of N uptake and metabolism to increased atmospheric CO(2) and O(3) concentrations, increased air and soil temperatures, drought and salt stress are highlighted.
Summary• Salinity causes osmotic stress and limits biomass production of plants. The goal of this study was to investigate mechanisms underlying hydraulic adaptation to salinity.• Anatomical, ecophysiological and transcriptional responses to salinity were investigated in the xylem of a salt-sensitive (Populus · canescens) and a salt-tolerant species (Populus euphratica).• Moderate salt stress, which suppressed but did not abolish photosynthesis and radial growth in P. · canescens, resulted in hydraulic adaptation by increased vessel frequencies and decreased vessel lumina. Transcript abundances of a suite of genes (FLA, COB-like, BAM, XET, etc.) previously shown to be activated during tension wood formation, were collectively suppressed in developing xylem, whereas those for stress and defense-related genes increased. A subset of cell wall-related genes was also suppressed in salt-exposed P. euphratica, although this species largely excluded sodium and showed no anatomical alterations. Salt exposure influenced cell wall composition involving increases in the lignin : carbohydrate ratio in both species.• In conclusion, hydraulic stress adaptation involves cell wall modifications reciprocal to tension wood formation that result in the formation of a novel type of reaction wood in upright stems named 'pressure wood'. Our data suggest that transcriptional co-regulation of a core set of genes determines reaction wood composition.
The storage and mobilization of nutrients in wood and bark tissues is a typical feature of trees. Sulfur can be stored as sulfate, which is transported from source to sink tissues through the phloem. In the present study two transcripts encoding sulfate transporters (SULTR) were identified in the phloem of grey poplar (Populus tremula x P. alba). Their cell-specific expression was analyzed throughout poplar in source tissues, such as mature leaves, and in sink tissues, such as the wood and bark of the stem, roots and the shoot apex. PtaSULTR1;1 mRNA was detected in companion cells of the transport phloem, in the phloem of high-order leaf veins and in fine roots. PtaSULTR3;3a mRNA was found exclusively in the transport phloem and here in both, companion cells and sieve elements. Both sulfate transporter transcripts were located in xylem parenchyma cells indicating a role for PtaSULTR1;1 and PtaSULTR3;3a in xylem unloading. Changes in mRNA abundance of these and of the sulfate transporters PtaSULTR4;1 and PtaSULTR4;2 were analyzed over an entire growing season. The expression of PtaSULTR3;3a and of the putative vacuolar efflux transporter PtaSULTR4;2 correlated negatively with the sulfate content in the bark. Furthermore, the expression pattern of both PtaSULTR3;3a and PtaSULTR4;2 correlated significantly with temperature and day length. Thus both SULTRs seem to be involved in mobilization of sulfate during spring: PtaSULTR4;2 mediating efflux from the vacuole and PtaSULTR3;3a mediating loading into the transport phloem. In contrast, the abundance of PtaSULTR1;1 and PtaSULTR4;1 transcripts was not affected by environmental changes throughout the whole season. The transcript abundance of all tested sulfate transporters in leaves was independent of weather conditions. However, PtaSULTR1;1 abundance correlated negatively with sulfate content in leaves, supporting its function in phloem loading. Taken together, these findings indicate a transcriptional regulation of sulfate distribution in poplar trees.
Seasonal tree-internal nitrogen cycling is an important strategy for trees to achieve high efficiency in the use of nitrogen (N). Key processes of this N redistribution are autumnal leaf senescence and storage of released N as bark storage proteins (BSP) in perennial tissues. While the regulation of leaf senescence has been intensively analysed in trees, the coordination of the complementary storage processes is still poorly understood. Therefore, we ascertained relationships between physiological-level and molecular-level processes and environmental factors under natural conditions in the bark of Populus x canescens. We analysed amino-N concentrations, total soluble protein concentration and transcript abundances of BSP genes in the bark of field-grown P. x canescens harvested during two annual growth cycles. By correlation analysis and linear modelling, we assessed interactions between biological data and meteorological conditions. Day length correlated with BSP expression, and air temperature correlated strongly with total protein concentration (r = -0.92), gamma-aminobutyric acid (GABA; r = 0.76) and arginine (r = -0.70). GABA and arginine also correlated significantly with total protein concentration and transcript abundances of BSP genes. We conclude that GABA and arginine potentially contribute to adjust storage processes in the bark of poplar trees to seasonal changes in environmental conditions.
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