Water is the key resource limiting world agricultural production. Although an impressive number of research reports have been published on plant drought tolerance enhancement via genetic modifications during the last few years, progress has been slower than expected. We suggest a feasible alternative strategy by application of rhizospheric bacteria coevolved with plant roots in harsh environments over millions of years, and harboring adaptive traits improving plant fitness under biotic and abiotic stresses. We show the effect of bacterial priming on wheat drought stress tolerance enhancement, resulting in up to 78% greater plant biomass and five-fold higher survivorship under severe drought. We monitored emissions of seven stress-related volatiles from bacterially-primed drought-stressed wheat seedlings, and demonstrated that three of these volatiles are likely promising candidates for a rapid non-invasive technique to assess crop drought stress and its mitigation in early phases of stress development. We conclude that gauging stress by elicited volatiles provides an effectual platform for rapid screening of potent bacterial strains and that priming with isolates of rhizospheric bacteria from harsh environments is a promising, novel way to improve plant water use efficiency. These new advancements importantly contribute towards solving food security issues in changing climates.

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SUMMARYA method for determining indole-3-acetic acid (IAA) production in mycorrhiza-forming fungi was developed. Culture medium and the fungal mycelium were, extracted with ethyl ether together with deuterated IAA as an internal standard, and the extracts thereafter evaporated to dryness. The dry extracts were then silylated in pyridine with BSTFA and analysed by gas chromatography-mass spectroscopy. Compared to other methods the procedure required a minimum of preparatory work, gave good reproducibility and a standard deviation of acceptable level for studies involving biological material.The method was used to study IAA production in 16 mycorrhiza-forming fungi. Results indicated large differences in the ability of the fungal strains to produce IAA. Pisolithus tinctorius 185, a strain previously shown by other workers to give a strong root infection in field experiments, produced the largest amount of IAA. Several other fungi showing high IAA values also infected plant roots with relative ease in laboratory experiments.

Fungi colonizing decayed roots of Pinus sylvestris and Picea abies seedlings were assessed by pure-culture isolation and direct sequencing of DNA extracted from roots collected from three environments: bare-root forest nurseries; afforested clear-cuts; and abandoned farmland. Pure-culture isolation from 1500 roots collected from 480 seedlings (240 of each tree species) yielded 1110 isolates which, based on mycelial morphology and ITS rDNA sequencing, were found to represent 87 distinct taxa. Direct ITS rDNA sequencing from decayed sections of 140 roots (70 of each tree species) yielded 160 sequences representing 58 taxa. Direct sequencing revealed a significantly higher fungal diversity per root segment than pure-culture isolation. Overall, a total of 131 taxa were found, 92 of which (70·2%) were identified at least to genus level. Only 14 taxa (10·7%) were detected by both methods, while 73 (55·7%) were detected exclusively by isolation and 44 (33·6%), exclusively by sequencing. The pathogens Fusarium oxysporum (25·6%) and Nectria radicicola (14·9%) were the most common isolates. In contrast, direct sequencing most frequently detected endophyte Phialocephala fortinii (33·1%) and Chalara sp. NS234A2 (10·0%). There were no significant differences in species richness between roots from the different environments, but there was a marked effect on fungal community structure. The results demonstrate that different fungi inhabit decayed roots of conifer seedlings in different environments, and that pure-culture isolation and direct sequencing are complementary methods that are both necessary for a complete description of the fungal communities colonizing diseased conifer roots.