Abstract
Abiotic stress has a growing impact on plant growth and agricultural activity worldwide. Specific plant growth promoting rhizobacteria have been reported to stimulate growth and tolerance to abiotic stress in plants, and molecular mechanisms like phytohormone synthesis and 1-aminocyclopropane-1-carboxylate deamination are usual candidates proposed to mediate these bacterial effects. Paraburkholderia phytofirmans PsJN is able to promote growth of several plant hosts, and improve their tolerance to chilling, drought and salinity. This work investigated bacterial determinants involved in PsJN stimulation of growth and salinity tolerance in Arabidopsis thaliana, showing bacteria enable plants to survive long-term salinity treatment, accumulating less sodium within leaf tissues relative to non-inoculated controls. Inactivation of specific bacterial genes encoding ACC deaminase, auxin catabolism, N-acyl-homoserine-lactone production, and flagellin synthesis showed these functions have little influence on bacterial induction of salinity tolerance. Volatile organic compound emission from strain PsJN was shown to reproduce the effects of direct bacterial inoculation of roots, increasing plant growth rate and tolerance to salinity evaluated both in vitro and in soil. Furthermore, early exposure to VOCs from P. phytofirmans was sufficient to stimulate long-term effects observed in Arabidopsis growth in the presence and absence of salinity. Organic compounds were analyzed in the headspace of PsJN cultures, showing production of 2-undecanone, 7-hexanol, 3-methylbutanol and dimethyl disulfide. Exposure of A. thaliana to different quantities of these molecules showed that they are able to influence growth in a wide range of added amounts. Exposure to a blend of the first three compounds was found to mimic the effects of PsJN on both general growth promotion and salinity tolerance. To our knowledge, this is the first report on volatile compound-mediated induction of plant abiotic stress tolerance by a Paraburkholderia species.
Highlights
Plants have evolved diverse mechanisms to cope with and survive environmental abiotic stresses such as salinity, including an early response to the short-term impact of high sodium concentrations, which is characterized by a rapid growth arrest by inhibition of cell growth and division (Munns and Tester, 2008), and the adjustment of osmotic potential by the cellular accumulation of compatible solutes (Ismail et al, 2014)
An estimation of salinity-induced mortality, senescence and tissue damage is necessary to ascertain if sodium exclusion or tissue tolerance mechanisms are being activated in the plants (Roy et al, 2014), and to determine if salt toxicity is effectively reduced by bacterial inoculation
Bacterial determinants of growth promotion and salinity tolerance in A. thaliana in response to PsJN inoculation have been studied in this work, based on previous results showing that short- and long-term changes in the response of inoculated plants are able to enhance their growth in the presence of salt (Pinedo et al, 2015)
Summary
Plants have evolved diverse mechanisms to cope with and survive environmental abiotic stresses such as salinity, including an early response to the short-term impact of high sodium concentrations, which is characterized by a rapid growth arrest by inhibition of cell growth and division (Munns and Tester, 2008), and the adjustment of osmotic potential by the cellular accumulation of compatible solutes (Ismail et al, 2014). There is a great variety in the extent of salinity tolerance among diverse land plants, there is substantial evidence on the activation of additional cell protection mechanisms, like ROS detoxification (Gill and Tuteja, 2010), sodium exclusion (Roy et al, 2014) and its storage within vacuoles (Fan et al, 2014), to achieve tolerance to saline stress in glycophyte plants (Munns and Tester, 2008) This includes most food crops, as well as the model plant, Arabidopsis thaliana (Hauser and Horie, 2010; Zhang and Shi, 2013). They represent attractive targets for enhancing productivity and growth of crop plants under field conditions (Farag et al, 2013)
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