Figure 1.
Phyllobacterium brassicacearum STM196 enhances A. thaliana survival under severe water deficits.
A) Average soil relative water content and B) survival percentage of non-inoculated (NI) and inoculated plants (I) in five watering scenarios including constant well-watered conditions (35% g H2O−1 dry soil; 35%), water withdrawing from the two firsts leaves (L2) followed by constant moderate water deficit (20% g H2O−1 dry soil; 20%c), punctual severe water deficits with rewatering after 10 days at 10% g H2O−1 dry soil (10% g H2O−1 dry soil; 10%p-10d) or after1 day (10%p), and after 1 day at 7% g H2O−1 dry soil (7%p). C) Average soil relative water content and D) survival percentage of NI (closed symbols) and I (open symbols) plants in two watering scenarios including constant well-watered conditions (35% g H2O−1 dry soil; 35%), and water withdrawing from the four-leaves stage (L4) followed by rewatering after 1 day at 6% g H2O−1 dry soil (6%p). Asterisks indicate significant differences following Chi2 test between NI and I plants (***: P<0.001).
Table 1.
Description of watering scenarios and design of experiments.
Figure 2.
P. brassicacearum STM196 does not affect A. thaliana mortality threshold but delays and reduces mortality rate during soil drying.
A) Whole-rosette Fv/Fm just before rewatering (i.e., 0.06 g H20−1 dry soil) of non-inoculated (NI) and inoculated (I), and surviving (▴; n = 19–36) and perishing (▾; n = 16–29) plants as observed at the end of the experiment conducted with water withdrawing followed by rewatering at 6% g H2O−1 dry soil (6%p stress in Figure 1). B) Relationships between Fv/Fm and survival probability (same data as in A); the 90%-mortality threshold (Fv/Fm = 0.398) is shown. C) Fv/Fm false-colour images (left) and visible images (middle) of vegetative rosettes before rewatering and of surviving flowering and perishing plant at the end of the experiment (right). D) Mortality rate of stressed NI (closed symbols) and I (open symbols) plants during soil drying and rewatering, as estimated from the 90%-mortality threshold. Asterisks indicate significant differences following Chi2 test between NI (n = 28–242) and I (n = 16–187) plants (**: P<0.01; ***: P<0.001).
Figure 3.
P. brassicacearum STM196 improves A. thaliana tolerance to higher levels of leaf photosynthetic damages under severe water deficit (WD; 6%p).
Whole-rosette Fv/Fm of A) surviving and B) perishing non-inoculated (NI; nsurviving = 7–147; nperishing = 6–137) and inoculated (I; nsurviving = 10–152; nperishing = 6–78) plants during soil drying and during rewatering. Dashed lines show the 90%-mortality threshold and arrows indicate the beginning of soil rewatering. Grey points represent individuals for each condition (NI; closed symbols and I; open symbols).
Figure 4.
P. brassicacearum STM196 induces a delayed dehydration of tissues and increases tolerance to severe water deficit.
A) Leaf relative water content and B) relationship between whole-rosette Fv/Fm and leaf relative water content of non-inoculated (NI; closed symbols) and STM196-inoculated (I; open symbols) plants under well watered (WW) and water deficit (WD; 6%p) during soil drying (35%, 20%, 10% and 6%) and after rewatering (20%r, 35%r and 35%r at flowering). Arrow in A indicates the beginning of soil rewatering. Dashed line in B represents the 90%-mortality threshold. Surviving plants with mean Fv/Fm values above the threshold, are represented by triangles (▴; n = 3–10 and n = 3–19 for NI and I plants, respectively) and perishing plants, with mean Fv/Fm below the threshold, are represented by upside-down triangles (▾; n = 3–9 and n = 3 for NI and I plants, respectively). Asterisks indicate significant differences following Kruskal-Wallis tests between NI and I plants (*: P<0.05 and ***: P<0.001).
Figure 5.
P. brassicacearum STM196 increases plant survival to severe leaf dehydration.
Relationship between plant survival (estimated from whole-rosette Fv/Fm values; Figure 1B) and leaf relative water content of non-inoculated (NI; closed circle; n = 36) and STM196-inoculated (I; open circle; n = 44) under severe water deficit (6%p). Insert represents fitting of logistic regression at very low leaf relative water content (solid and dashed lines for NI and I plants, respectively).
Figure 6.
P. brassicacearum STM196 increases growth rate of surviving plants after rewatering.
A) Total projected leaf area of non-inoculated (NI; closed symbols) and STM196-inoculated (I; open symbols) plants under well-watered condition (WW) and severe water deficit (WD; 6%p in Figure 1) as a function of days after four-leaves stage. Arrow in A indicate the beginning of soil rewatering. Insert in A represents the maximum rate of leaf expansion (Rmax) after rewatering of surviving stressed plants. Area of individual leaves of I and NI plants under B) WW condition and C) WD. Insert in C shows total leaf area of surviving plants at flowering. Data are means (±SE) of 11–27 plants. Different letters indicate significant differences between means following Kruskal-Wallis tests (P<0.05).