Abstract
Several plant growth-promoting rhizobacteria (PGPR) are known to improve plant tolerance to multiple stresses, including low temperatures. However, mechanisms underlying this protection are still poorly understood. The aim of this study was to evaluate the role of the endophytic PGPR, Burkholderia phytofirmans strain PsJN (Bp PsJN), on Arabidopsis thaliana cold tolerance using photosynthesis parameters as physiological markers. Under standard conditions, our results indicated that Bp PsJN inoculation led to growth promotion of Arabidopsis plants without significant modification on photosynthesis parameters and chloroplast organization. However, bacterial colonization induced a cell wall strengthening in the mesophyll. Impact of inoculation modes (either on seeds or by soil irrigation) and their effects overnight at 0, -1, or -3°C, were investigated by following photosystem II (PSII) activity and gas exchanges. Following low temperatures stress, a decrease of photosynthesis parameters was observed. In addition, during three consecutive nights or days at -1°C, PSII activity was monitored. Pigment contents, RuBisCO protein abundance, expression of several genes including RbcS, RbcL, CBF1, CBF2, CBF3, ICE1, COR15a, and COR78 were evaluated at the end of exposure. To assess the impact of the bacteria on cell ultrastructure under low temperatures, microscopic observations were achieved. Results indicated that freezing treatment induced significant changes in PSII activity as early as the first cold day, whereas the same impact on PSII activity was observed only during the third cold night. The significant effects conferred by PsJN were differential accumulation of pigments, and reduced expression of RbcL and COR78. Microscopical observations showed an alteration/disorganization in A. thaliana leaf mesophyll cells independently of the freezing treatments. The presence of bacteria during the three successive nights or days did not significantly improved A. thaliana responses but prevented the plasmalemma disruption under freezing stress.
Highlights
Extreme environmental events such as prolonged drought, heavy rains or cold are likely to increase in the future due to climate change
In A. thaliana, Bp Burkholderia phytofirmans strain PsJN (PsJN) established root endophytic population independently of inoculation method, whereas, no bacteria were detected in leaves (Supplementary Figure S2)
Modification of Photosynthesis during and after a Cold Night In order to understand if bacterial colonization and growth promotion may help plant to resist to low temperatures Photosynthetic parameters were investigated
Summary
Extreme environmental events such as prolonged drought, heavy rains or cold are likely to increase in the future due to climate change. Plants percept cold and reply by multiple adjustments at physiological, biochemical, and molecular levels (Theocharis et al, 2012b) These include reduced membrane fluidity (Orvar et al, 2000), ultrastructural modifications in cell components, including plastids and mitochondria (Zhang et al, 2011), transiently increased cytosolic Ca2+ levels (Ruelland et al, 2009; Fanucchi et al, 2012), reprogramming of the transcriptome and the proteome (Thomashow, 1998; Chinnusamy et al, 2007; Miura and Furumoto, 2013). Three genes (CBF1/DREB1b, CBF2/DREB1c, and CBF3/DREB1a) have been well-studied in Arabidopsis Their transcription is activated only a few minutes after transferring plants to low temperature (Gilmour et al, 2004; Medina et al, 2011) and is followed by induction of CBF target genes, such as COR genes (Svensson et al, 2006). COR proteins may protect cells against environmental chilling stress or regulate gene expression during the adaptive response (Fowler and Thomashow, 2002)
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