The mechanisms of carbon starvation: how, when, or does it even occur at all?Recent observations of increasing vegetation mortality events appear to be a result of changing climate, in particular, an increase in the frequency, length and intensity of droughts (e.g. Allen et al., 2010). The threat of widespread increases in future mortality has rekindled interest in the mechanisms of plant mortality and survival because we do not yet understand them well enough to confidently model future vegetation dynamics (Sitch et al., 2008). In this issue of New Phytologist, provide a viewpoint on the 'carbon (C) starvation hypothesis ' (McDowell et al., 2008). Their viewpoint is invaluable for stimulating our field to explicitly refine our definitions and identify the key experiments needed to understand mechanisms of vegetation survival and mortality. Two important conclusions of their paper were that mortality can occur at nonzero carbohydrate levels and that careful experiments focused on the explicit mechanisms of C starvation, as well as on partitioning the roles of hydraulic failure and C starvation, are needed to understand the physiological underpinnings of how plants die. We applaud these conclusions, and agree that hasty acceptance of any hypothesis before adequate testing is foolish. In this commentary, we highlight some of the valuable ideas from Sala et al. and provide additional comments that we hope will prompt careful future tests on the mechanisms of plant mortality.When the C-starvation hypothesis was proposed (McDowell et al., 2008), it represented an attempt to summarize and interpret the existing literature on vegetation mortality, of which there was a wealth of indirect studies, but a paucity of true, mechanistic tests. The original formulation of the hypothesis suggested that stomatal closure minimizes hydraulic failure during drought, causing photosynthetic C uptake to decline to low levels, thereby promoting carbon starvation as carbohydrate demand continues for maintenance of metabolism and defense. The plant either starves outright, or succumbs to attack by insects or pathogens, whichever occurs first. By contrast, failure to maintain xylem water tension lower than its cavitation threshold results in embolisms, which, if unrepaired, can eventually lead to widespread hydraulic failure, desiccation and mortality. We hoped that the C-starvation and hydraulic failure hypotheses would generate discussion and new ideas; and 'The paucity of studies that quantified mortality forces scientists to use data from nonmortality studies to develop hypotheses … we do this at the risk of confusing stress responses with mortality mechanisms.' , as summarized by Sala et al., active discussion is taking place. A primary conclusion from the discussion is that we need clarification of the various mechanisms by which C starvation can occur, if it occurs at all.Plants maintain metabolism through respiratory processes that consume carbohydrates, and in doing so their C budgets must obey the law of conservation of energ...

Summary• Identification of ectomycorrhizal (ECM) fungi is often achieved through comparisons of ribosomal DNA internal transcribed spacer (ITS) sequences with accessioned sequences deposited in public databases. A major problem encountered is that annotation of the sequences in these databases is not always complete or trustworthy. In order to overcome this deficiency, we report on UNITE, an open-access database.• UNITE comprises well annotated fungal ITS sequences from well defined herbarium specimens that include full herbarium reference identification data, collector/source and ecological data. At present UNITE contains 758 ITS sequences from 455 species and 67 genera of ECM fungi.• UNITE can be searched by taxon name, via sequence similarity using BLAST n, and via phylogenetic sequence identification using galaxie. Following implementation, galaxie performs a phylogenetic analysis of the query sequence after alignment either to pre-existing generic alignments, or to matches retrieved from a BLAST search on the UNITE data. It should be noted that the current version of UNITE is dedicated to the reliable identification of ECM fungi.• The UNITE database is accessible through the URL http://unite.zbi.ee

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The mycobionts of Piceirhiza bicolorata, a distinct ectomycorrhizal morphotype of conifers and hardwoods, have been identified by internal transcribed spacer 1 (ITS1) nuclear ribosomal DNA (rDNA) sequence comparison of the fungi involved. Samples of Piceirhiza bicolorata were obtained from seedlings of Picea abies, Pinus sylvestris, Betula pubescens, Populus tremula, Quercus robur and Salix phylicifolia. In an initial screening, the fungus amplified with universal ITS primers from ectomycorrhizal root samples of P. bicolorata shared approx. 95% ITS1 sequence identity with the ericoid mycorrhizal fungus Hymenoscyphus ericae. A total of 77 out of 88 (l 87.5%) DNA samples (i.e. 52\56 root samples and 25\32 axenic culture isolates) of P. bicolorata were successfully amplified with a taxon-selective primer designed for exclusive amplification of H. ericae-like strains. Forty-seven amplicons were sequenced, yielding 15 different ITS1 genotypes that differed by 1-14 nucleotide character state changes. An inferred ITS1 phylogeny (maximum parsimony) showed that a single major evolutionary lineage of P. bicolorata embraced the historically important H. ericae isolates in a 100% bootstrap-supported clade. The 15 P. bicolorata genotypes were positioned in four subclades, roughly corresponding to morphological groups of P. bicolorata isolates observed in axenic culture. Culture isolates of H. ericae and P. bicolorata share some common morphological features including slow, dense growth and formation of short aerial hyphal aggregates. Our results suggest that members of the H. ericae aggregate participate in the formation of the distinct ectomycorrhizal morphotype P. bicolorata. This opposes the widely accepted discrimination of ericoid and ectomycorrhizal mycobionts of the boreal forest ecosystem. The high prevalence of the P. bicolorata morphotype on pioneer seedlings of P. sylvestris, B. pubescens and S. phylicifolia at a copper mine spoil was remarkable. Hypotheses of possible nutrient mobilization and detoxification potentials of the fungal associates of P. bicolorata are discussed. We hypothesize that ericoid and ectomycorrhizal plants may share mycobionts of the H. ericae aggregate.