Burkholderia phytofirmans PsJN acclimates grapevine to cold by modulating carbohydrate metabolism.

Abstract

Low temperatures damage many temperate crops, including grapevine, which, when exposed to chilling, can be affected by symptoms ranging from reduced yield up to complete infertility. We have previously demonstrated that Burkholderia phytofirmans PsJN, a plant growth-promoting rhizobacteria (PGPR) that colonizes grapevine, is able to reduce chilling-induced damage. We hypothesized that the induced tolerance may be explained at least partly by the impact of bacteria on grapevine photosynthesis or carbohydrate metabolism during cold acclimation. To investigate this hypothesis, we monitored herein the fluctuations of photosynthesis parameters (net photosynthesis [P(n)], intercellular CO(2) concentration, stomatal conductances, ΦPSII, and total chlorophyll concentration), starch, soluble sugars (glucose, fructose, saccharose, mannose, raffinose, and maltose), and their precursors during 5 days of chilling exposure (4°C) on grapevine plantlets. Bacterization affects photosynthesis in a non-stomatal dependent pattern and reduced long-term impact of chilling on P(n). Furthermore, all studied carbohydrates known to be involved in cold stress tolerance accumulate in non-chilled bacterized plantlets, although some of them remained more concentrated in the latter after chilling exposure. Overall, our results suggest that modification of carbohydrate metabolism in bacterized grapevine plantlets may be one of the major effects by which this PGPR reduces chilling-induced damage.

Full text

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496 / Molecular Plant-Microbe Interactions
MPMI Vol. 25, No. 4, 2012, pp. 496–504. http://dx.doi.org/10.1094/MPMI-09-11-0245. © 2012 The American Phytopathological Society
Burkholderia phytofirmans PsJN Acclimates Grapevine
to Cold by Modulating Carbohydrate Metabolism
Olivier Fernandez,1 Andreas Theocharis,1 Sophie Bordiec,1 Regina Feil,2 Lucile Jacquens,1
Christophe Clément,1 Florence Fontaine,1 and Essaid Ait Barka1
1Université de Reims Champagne Ardenne, Unité de Recherche Vignes et Vins de Champagne–Stress et Environnement
(EA 2069), UFR Sciences Exactes et Naturelles, BP 1039, 51687 Reims Cedex 2, France; 2Max Planck Institute of Molecular
Plant Physiology, Am Mühlenberg 1, 14424 Potsdam, Germany
Submitted 21 September 2011. Accepted 16 November 2011.
Low temperatures damage many temperate crops, includ-
ing grapevine, which, when exposed to chilling, can be
affected by symptoms ranging from reduced yield up to
complete infertility. We have previously demonstrated
that Burkholderia phytofirmans PsJN, a plant growth-pro-
moting rhizobacteria (PGPR) that colonizes grapevine, is
able to reduce chilling-induced damage. We hypothesized
that the induced tolerance may be explained at least
partly by the impact of bacteria on grapevine photosyn-
thesis or carbohydrate metabolism during cold acclima-
tion. To investigate this hypothesis, we monitored herein
the fluctuations of photosynthesis parameters (net photo-
synthesis [Pn], intercellular CO2 concentration, stomatal
conductances,  PSII, and total chlorophyll concentration),
starch, soluble sugars (glucose, fructose, saccharose, man-
nose, raffinose, and maltose), and their precursors during
5 days of chilling exposure (4 C) on grapevine plantlets.
Bacterization affects photosynthesis in a non–stomatal
dependent pattern and reduced long-term impact of chill-
ing on Pn. Furthermore, all studied carbohydrates known
to be involved in cold stress tolerance accumulate in non-
chilled bacterized plantlets, although some of them re-
mained more concentrated in the latter after chilling
exposure. Overall, our results suggest that modification of
carbohydrate metabolism in bacterized grapevine plant-
lets may be one of the major effects by which this PGPR
reduces chilling-induced damage.
Abiotic stresses are responsible for 50% of crop loss world-
wide (Boyer 1982; Bray et al. 2000; Nagarajan and Nagarajan
2010). Among them, exposure to low temperatures is one of
the most damaging environmental factors affecting plants
(Boyer 1982; Nagarajan and Nagarajan 2010). Upon chilling
(low temperature above 0C), common temperate crops such
as maize, potato, and grapevine can suffer severe damage (Ait
Barka et al. 2006; McKersie and Leshem 1994; Nagarajan and
Nagarajan 2010; Ruelland et al. 2009). Nevertheless, at chill-
ing temperatures, plants may enter a process called acclima-
tion, which improves tolerance and reduces damage due to
long-term chilling exposure (Ruelland et al. 2009). Cold accli-
mation involves several metabolic adjustments such as i) induc-
tion of cold-specific gene expression (Chinnusamy et al. 2007;
Nakashima and Yamaguchi-Shinozaki 2006; Thomashow 2010),
ii) synthesis of cold-related proteins (Janská et al. 2010; Moffatt
et al. 2006; Peng et al. 2008; Ruelland et al. 2009; Sasaki et al.
2007), and iii) accumulation of osmolytes such as proline
(Kamata and Uemura 2004; Kaplan et al. 2007; Ruelland et al.
2009).
Additionally, accumulation of carbohydrates is one of the
most important metabolic adjustments by which plants achieve
low-temperature tolerance throughout cold acclimation (Janská
et al. 2010; Kaplan et al. 2004; Ruelland et al. 2009). Soluble
sugar concentrations (mostly sucrose, glucose, fructose, man-
nose, and raffinose) drastically increase upon chilling exposure
(Kaplan et al. 2004) and correlate with low-temperature toler-
ance in Arabidopsis thaliana (Rohde et al. 2004) as well as
many crops (Gusta et al. 2004; Kamata and Uemura 2004;
Koster and Lynch 1992). Soluble sugar increases during cold
acclimation may originate from starch degradation (Kaplan et
al. 2004) and from accumulation of several precursors such as
galactinol and phosphated (G6P, G1P, F6P, and others) and nu-
cleotided (UDPG) sugars (Kaplan et al. 2004; Ruelland et al.
2009). In contrast to many crops under cold acclimation,
grapevine is characterized by simultaneous soluble sugar and
starch accumulation (Ait Barka et al. 2006).
Sugars play multiple roles in low-temperature tolerance. As
typical compatible osmolytes, they contribute to the preserva-
tion of water within plant cells, reducing thereby water avail-
ability for ice nucleation in the apoplast (Ruelland et al. 2009;
Steponkus 1984). Sugars might protect plant cell membranes
during cold-induced dehydration, replacing water molecules in
establishing hydrogen bonds with lipid molecules (Ruelland et
al. 2009; Uemura et al. 2003). Sugar glass-forming properties
offer additional protection against cold damage, especially in
woody species that can tolerate temperatures down to –20C
(Hirsh 1987). Finally, sugars might exhibit reactive oxygen
species (ROS)-scavenging properties (Bogdanović et al. 2008;
Nishizawa et al. 2008).
Related to carbohydrate metabolism, photosynthesis is
strongly affected by low temperatures. In A. thaliana, the
maximum quantum efficiency of PSII (Fv/Fm) is reduced by
20% (Zhang and Scheller 2004). Additionally, some photosyn-
thesis-related genes are affected by chilling (Janská et al.
2010). In grapevine, Fv/Fm, net photosynthesis (Pn), and
stomatal conductances (gs) are reduced under chilling tempera-
tures (Bertamini et al. 2005, 2006; Hendrickson et al. 2004).
In temperate climates, several wine-growing areas can be
subjected to low temperatures during the growing season that
can compromise grapevine productivity. Recently, the use of
beneficial microorganisms such as plant growth-promoting
rhizobacteria (PGPR) has emerged as a potential new solution
O. Fernandez and A. Theocharis have contributed equally to this work.
Corresponding author: E. A. Barka; E-mail: ea.barka@univ-reims.fr

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Vol. 25, No. 4, 2012 / 497
to reduce abiotic stress-induced damage, including chilling
damage (Yang et al. 2009). Some of these PGPR classified as
endophytes are able to transcend the endodermis barrier, cross-
ing from the root cortex to the vascular system, and subse-
quently thrive as endophytes in stem, leaves, tubers, and other
organs (Compant et al. 2005a). The extent of endophytic colo-
nization of host plant organs and tissues reflects the ability of
bacteria to selectively adapt to these specific ecological niches
(Gray and Smith 2005). As a result, intimate associations be-
tween bacteria and host plants can be formed without harming
the plant (Compant et al. 2005a; Gray and Smith 2005;
Lodewyckx et al. 2002). PGPR colonize the plant rhizosphere
and positively affect their growth and responses to stress
(Compant et al. 2008b; Yang et al. 2009). Surprisingly, very
little is known of their influence on plant photosynthesis and
primary metabolism. We have previously demonstrated that the
PGPR Burkholderia phytofirmans PsJN colonizes grapevine
(Compant et al. 2008a and b) and enhances several traits corre-
lated with low-temperature tolerance (CBF gene expression,
accumulation of proline, and phenolic compounds) (Ait Barka
et al. 2006; Theocharis et al. 2012). We also observed that B.
phytofirmans PsJN induces starch synthesis and enhances pho-
tosynthetic capacity after 2 weeks of chilling (Ait Barka et al.
2006). Moreover, B. phytofirmans PsJN strongly reduces long-
term chilling damage (Ait Barka et al. 2006; Theocharis et al.
2012).
In order to understand how B. phytofirmans helps grapevine
to withstand chilling damage, we suggest that the bacterium
modulates the photosynthetic capacity or accumulation of cryo-
protective soluble sugars in parallel with higher starch synthesis
before or during cold acclimation. Thus, we investigated the
fluctuations in photosynthesis parameters (Pn, intercellular
CO2 concentration [Ci], gs, PSII, and total chlorophyll con-
centration) as well as variations of the major carbohydrates
within the first 5 days of chilling exposure.
RESULTS
Changes of photosynthesis parameters
and pigment content.
Bacterized (B) plantlets at 26C showed 25% lower Pn (Fig.
1A) and slightly higher Ci compared with nonbacterized (NB)
ones (Fig. 1B). In contrast, gs displayed similar values in both
NB and B plantlets (Fig. 1C). The same trend was found for
PSII in NB and B plantlets (Fig. 1D). Regarding pigment
concentration, both chlorophyll and carotenoid concentrations
were lower in leaves of B plantlets (Fig. 2).
Pn was higher in leaves of nonbacterized chilled (NBC)
plantlets after 1 and 2 days at 4C but, 5 days later, leaves of
bacterized chilled (BC) plantlets exhibited a higher Pn (Fig.
1A). In both NBC and BC plantlets, Ci increased while gs re-
mained stable during the experiment, and PSII decreased
(Fig. 1C and D). Additionally, chilled plantlets were character-
ized by a reduction of pigment concentration (Fig. 2). However,
no difference was found between NBC and BC plantlets in any
of these parameters upon chilling (Figs. 1 and 2).
Changes of starch content.
Under normal conditions (26C), leaves of B plantlets con-
tained higher concentrations of starch compared with NB ones

Fig. 1. Photosynthesis parameters in bacterized (B) and nonbacterized (NB) plantlets during 5 days of chilling exposure. A, Net photosynthesis (Pn); B, internal
CO2 stomatal concentration (Ci); C, stomatal conductance (gs); D, effective PSII quantum yield (PSII). Values are means ( standard error) of two inde-
pendent experiments. If noted, asterisks (*) show significant difference (P < 0.05) between NB and B (day 0) or NB chilled and B chilled (days 1, 2, and 5)
plantlets.

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498 / Molecular Plant-Microbe Interactions
(Fig. 3A). Upon chilling, starch concentration increased in both
NBC and BC plantlets but faster and with higher levels in BC
plantlets. After 5 days of chilling exposure, BC plantlets still
contained a higher concentration of starch, close to 30% (Fig.
3A).
Quantitative and qualitative analysis of soluble sugars.
At 26C, total soluble sugars (TSS) concentration was almost
twofold higher in leaves of B plantlets when compared with
NB ones (Fig. 3B). Qualitatively, all soluble sugars measured
were present in a higher concentration in leaves of B plantlets
(Fig. 4A), with the most striking differences affecting maltose,
raffinose, and glucose (10-, four-, and twofold higher, respec-
tively, than the control).
Upon chilling (4C), TSS increased in leaves of NBC and
BC plantlets (Fig. 3B). During the first 2 days of chilling ex-
posure, leaves of BC plantlets displayed higher soluble sugar
concentration. After 5 days, leaves of NBC and BC plantlets
tended to have equal TSS concentrations (Fig. 3B). Qualita-
tively, sugars were differentially affected by chilling exposure.
To simplify analysis, data were expressed in concentration
ratio of NBC versus BC (or NB versus B for day 0) (Fig. 5A)
plantlets in addition to raw data (Table 1). Ratios of galactinol,
raffinose, and, to a lesser extent, mannose remained steady
during the first 2 days following chilling exposure (45, 25, and
60% respectively) and suddenly increased after 5 days, as a
consequence of their late increase in leaves of NBC plantlets
(Fig. 5; Table 1). Furthermore, raffinose concentration re-
mained higher in leaves of BC plantlets after 5 days (Fig. 5A;
Table 2). Glucose and fructose ratios were gradually enhanced
upon chilling (Fig. 5A), representing a faster increase in leaves
of NBC plantlets (Table 1), and reached similar levels in leaves
of NBC and BC plantlets after 5 days (Fig. 5A). Within 1 day,
the maltose ratio was higher than 100%, as a consequence of a
fast increase of maltose in leaves of NBC plantlets (Fig. 5A;
Table 1). Five days later, sucrose concentration was similar in
leaves of NBC and BC plantlets but maltose concentration,
unlike all other metabolites, remained significantly higher in
NBC ones (Table 1).
Quantitative and qualitative analysis of phosphated
and nucleotined sugars.
Phosphated and nucleotined sugars were analyzed to further
study the dynamics of carbon metabolism in leaves before or
during chilling exposure.
Concentrations of G6P, F6P, and S6P were similar in leaves
of NB and B plantlets before exposure to chilling (26C) (Fig.
4B). In contrast, levels of G1P, M6P, UDPG, and, to a lesser
extent, ADPG were lower in leaves of B plantlets (Fig. 4B).
Fluctuations of these compounds were followed during 5
days of chilling and represented as concentration ratio in
leaves of NBC versus BC plantlets (NB and B plantlets for day
0) (Fig. 5B) and raw data (Table 1). All metabolites were sub-
jected to an increase upon chilling exposure, ranging from 1.5-
fold (UDPG) to more than 60-fold (ADPG) after 5 days (Table
1). Interestingly, the ratio strongly increased within the first
day of exposure in leaves of NBC plantlets as a consequence
of a faster increase of these metabolites in NB plantlets (Fig.
5B, Table 1). After 2 days, all metabolites measured still dis-
played higher concentrations in leaves of NBC plantlets (Table
1). Such differences were no longer detected after 5 days, ex-
cept for M6P (Fig. 5B; Table 1).

Fig. 2. Pigment concentration in bacterized (B) and nonbacterized (NB)
plantlets during 5 days of chilling exposure. A, Chlorophyll a (Chla) and b
(Chlb) concentration; B, carotenoids concentration. Values are means (
standard error) of two independent experiments. If noted, asterisks (*)
show significant difference (P < 0.05) between NB and B (day 0) or NB
chilled and B chilled (days 1, 2, and 5) plantlets.

Fig. 3. A, Starch and B, total soluble sugars (TSS) in bacterized (B) and
nonbacterized (NB) plantlets during 5 days of chilling exposure. TSS con-
centration was calculated as the sum of glucose + fructose + maltose + su-
crose + raffinose + mannose. Values are means ( standard error) of three
independent experiments. If noted, asterisks (*) show significant differ-
ence (P < 0.05) between NB and B (day 0) or NB chilled and B chilled
(days 1, 2, and 5) plantlets.

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Vol. 25, No. 4, 2012 / 499
Glycolysis fluctuations.
To gain insight into the bacterization effect on glycolysis be-
fore or during chilling treatment, the levels of phosphoenolpy-
ruvate (PEP) and pyruvate were monitored (Fig. 6). Addition-
ally, we used pyruvate/PEP and F6P/fructose 1,6-bisphosphate
ratios (displayed as pyruvate kinase [PK] and phosphofructo-
kinase [PFK] ratios) to estimate the activities of PK and PFK
(Table 2).
Fig. 5. Concentration ratio of A, sugars and galactinol and B, phosphated and nucleotined intermediates in bacterized (B) and nonbacterized (NB) plantlets
during 5 days of chilling exposure (4C). At day 0, ratio is calculated as concentration of sugars in NB versus B plantlets and at days 1, 2, and 5 as concen-
tration of sugars in NB chilled versus B chilled plantlets. Black line indicates ratio of 100%.

Fig. 4. A, Sugars and galactinol and B, phosphated and nucleotined intermediates concentration in bacterized (B) and nonbacterized (NB) plantlets before
chilling exposure. Left axis indicates concentration for A, mannose, glucose, fructose, and sucrose and B, G1P, G6P, F6P, M6P, and UDPG; right axis indi-
cates concentration for A, galactinol, raffinose, and maltose and B, S6P and ADPG. Values are means ( standard error) of three independent experiments. If
noted, asterisks (*) show significant difference (P < 0.05) between NB and B plantlets.

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500 / Molecular Plant-Microbe Interactions
At 26C, pyruvate and PEP concentration were higher in
leaves of NB plantlets (Fig. 6). After chilling treatment (4C),
these metabolites behaved differently. Pyruvate concentration
drastically increased in leaves of NBC and BC plantlets but
faster in NBC (Fig. 6A). Nevertheless, the level of pyruvate
became similar after 5 days of chilling exposure. In contrast,
PEP concentration decreased in both conditions after 2 days of
4C exposure (Fig. 6B).
Regarding both PK and PFK ratios, the first one increased
faster in NBC and then was overtaken by BC plantlets (Table
2). The PFK ratio decrease was more pronounced in leaves of
NBC plantlets after 5 days at 4C (Table 2).
DISCUSSION
In a previous study, we showed that B. phytofirmans was
able to enhance tolerance of grapevine to low temperatures by
cold acclimation (Ait Barka et al. 2006). To explore the mecha-
nisms underlying bacterial-induced cold tolerance, we investi-
gated the fluctuations of both photosynthesis and carbohydrate
metabolism during acclimation.
Photosynthesis parameters are modified
in grapevine plantlets by both chilling and bacterization.
Both Pn and ΦPSII were markedly reduced in NBC and BC
plantlets without disruption of Ci and gs. This is not surprising
because a decrease in Pn and ΦPSII is a classical physiological
response of plants to chilling stress (Bertamini et al. 2005;
Hendrickson et al. 2004). The cessation of growth resulting
from cold stress reduces the capacity for energy utilization
which, in turn, probably results in feedback inhibition of pho-
tosynthesis (Ruelland and Zachowski 2010).
B. phytofirmans PsJN reduced photosynthesis in a non-
stomatal-dependent pattern. This is demonstrated by the fact that
B plantlets displayed a lower Pn but similar Ci and gs relative to
NB plantlets (Fig. 1A to C). Furthermore, bacterization affected
chlorophyll concentrations but not the quantum yield of photo-
system II (Figs. 1D and 2). Effects on photosynthesis parameters
have been described in the literature for other beneficial plant–
microbe interactions. In contrast with our results, long-term ex-
Table 2. Pyruvate kinase (PK) and phosphofructo-kinase (PFK) ratios in bacterized (B) and nonbacterized (NB) plantlets during 5 days of chilling exposurea

NBC (duration of cold exposure)

BC (duration of cold exposure)
Ratios NB 1 day 2 days 5 days B 1 day 2 days 5 days
PK
PFK
0.45 1.33 1.62 3.04 0.49 0.95 1.27 3.7
1,655 3,189 1,283 261 1,717 3,363 1,887 631
a NB and B plantlets at 26C, NBC = NB chilled plantlets (4C) and BC: B chilled plantlets (4C). PK and PFK ratios were calculated as
pyruvate/phosphoenolpyruvate and F6P/fructose 1,6-bisphosphate, respectively, in order to represent an estimate of PK and PFK enzyme activities.
Table 1. Analysis of sugars, galactinol, and phosphated and nucleotined intermediates in bacterized and nonbacterized plantlets during 5 days of chilling
exposurea

Concentration ( mol g–1 FW)

NBC (duration of cold exposure)

BC (duration of cold exposure)
Metabolites NB 1 day 2 days
3.48a  0.18
19.01a  1.27
18.32  0.78
13.82  0.17
0.4a  0.08
0.22a  0.07
4.26  0.55
362b  19
809b  45
182b  10
276b  23
20.6b  0.4
7.3b  1.3
96b  3
5 days
3.69  0.14
31.99  0.26
31.30  0.44
18.39  0.56
0.69  0.11
0.57a  0.12
7.90b  0.59
352  42
839  50
201  11
266b  36
22.8  1.8
12.8  2.8
108  1
B 1 day 2 days
5.23b  0.34
26.82b  0.71
20.52  1.4
14.53  0.52
0.99b  0.07
0.99b  0.15
3.10  0.07
170a  43
507a  76
118a  19
129a  18
14.1  1.7
2.6a  0.9
51a  12
5 days
4.9  0.73
32.72  0.55
29.73  0.66
16.41  1.14
0.88  0.16
1.27b  0.10
5.87a  0.57
269  69
699  127
163  28
202a  53
17.9  1.9
9.3  2.7
91  17
Mannose
Glucose
Fructose
Sucrose
Galactinol
Raffinose
Maltose
G1P
G6P
F6P
M6P
S6P
ADPG
UDPG
3.29  0.15
5.75  1.06
4.49  0.95
5.46  0.46
0.11  0.02
0.11  0.05
0.01  0.002
139  5
277  14
69  5
85  4
2  0.1
0.2  0.02
77  3
3.52  0.61
11.53a  1.23
10.26  0.86
12.84  0.63
0.23a  0.06
0.09a  0.04
1.43  0.28
337b  35
847b  70
183b  16
267b  27
17.2b  1.1
4.8b  1.2
80b  4
5.53  0.36
12.75  1.45
7.44  0.61
9.46  0.65
0.21  0.04
0.42  0.07
0.05  0.01
86  3
280  11
72  3
57  4
2.2  0.1
0.1  0.02
48  3
5.56  0.52
19.84b  1.02
12.91  1.01
11.80  0.98
0.47b  0.04
0.57b  0.13
1.01  0.18
109a  19
391a  60
89a  13
93a  16
8.5a  0.5
1.3a  0.2
41a  7

a NB = nonbacterized plantlets (26C), B = bacterized plantlets (26C), NBC = NB chilled plantlets (4C), and BC = B chilled plantlets (4C). Values are
means ( standard error) of three independent experiments. If noted, letters (a or b) indicate different means (P < 0.05) between BC and NBC plantlets.

Fig. 6. Concentration of A, pyruvate and B, phosphoenolpyruvate in
bacterized (B) and nonbacterized (NB) plantlets during 5 days of chill-
ing exposure. Values are means ( standard error) of four independent
experiments. If noted, asterisks (*) show significant difference (P < 0.05)
between NB and B (day 0) or NB chilled and B chilled (days 1, 2, and 5)
plantlets.

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Vol. 25, No. 4, 2012 / 501
posure of Arabidopsis to the beneficial soil bacterium Bacillus
subtilis (GB03) enhances plant photosynthetic capacity by in-
creasing photosynthetic efficiency and chlorophyll content
(Zhang et al. 2008). Such an increase in Pn was also reported in
maize plants colonized by the mycorrhiza Glomus mossea and
has been linked to salt stress resistance (Sheng et al. 2008). On
the other hand, gs was higher in squash colonized by the my-
corrhiza Glomus intraradices (Augé et al. 2008). The observed
increase, associated with a higher plant water potential, was
interpreted as resulting from the symbiont improving plant gas
exchange. Furthermore, the level of gs was higher in soybean
than cowpea following colonization by Bradyrhizobium japoni-
cum (Matiru and Dakora 2005). Overall, the impact of plant-
beneficial microorganisms on plant photosynthesis parameters
seems to be dependent on each associated partner.
Bacterization might help the plant to maintain a higher Pn
upon chilling. After 5 days of exposure to 4C, we observed
that Pn was higher in BC plantlets. This is in agreement with
previous data showing higher Pn in BC plantlets after 2 weeks
of chilling exposure (Ait Barka et al. 2006). Overall, our
results suggest that the presence of Burkholderia phytofirmans
may help plantlets to withstand chilling damage by maintain-
ing a higher Pn when plantlets undergo a long chilling expo-
sure and that the lower Pn measured in B plantlets does not
compromise chilling-induced tolerance by the bacterium.
Carbohydrates and related metabolites
in response to low temperatures in NBC plantlets.
Grapevine NBC plantlets accumulate soluble sugars and
starch. After 5 days of chilling, the TSS concentration increased
to fivefold higher than the initial concentration (Fig. 3B). TSS
are known to accumulate at low temperatures, leading to
improved plant tolerance to cold in several woody plants
(Leborgne et al. 1995a and b; Travert et al. 1997). Among sug-
ars, the largest contribution was due to glucose, fructose, and
sucrose. Interestingly, a shift from the production of sucrose to
monosaccharides as well as production of raffinose with its
precursor galactinol were detected in NBC plantlets. Similar
trends occurred naturally in vineyards at the onset of cool
autumn temperatures (Ait Barka and Audran 1996; Hamman et
al. 1996). Only mannose remained quite stable despite the
increase of its precursor (M6P) concentration. It might be pos-
sible that mannose was used as a precursor to ascorbate syn-
thesis (Zhang et al. 2009).
In grapevine NBC plantlets, TSS and starch accumulated
simultaneously. During 5 days of cold exposure, starch accu-
mulated, as did its precursor ADPG and the first derivative
sugar, maltose. Other reports have documented an increase in
soluble sugars concomitant with enhanced starch concentra-
tion during cold acclimation. This phenomenon has been de-
scribed in some plants such as cabbage seedlings (Sasaki et al.
1996), spinach (Guy et al. 1992), and grapevines grown in
vineyards (Ait Barka and Audran 1996). In contrast, in several
other species, starch is converted into soluble sugars during
cold exposure, which results in a decline in starch concentra-
tion (Fischer and Höll 1991; Greer et al. 2000; Ögren 1997;
Pollock and Lloyd 1987). However, the observed continuous
synthesis of starch might be explained by the uptake of sucrose
from the media in our experimental system.
Bacterization with B. phytofirmans mimics some aspects
of cold acclimation.
To our knowledge, this is the first study monitoring carbohy-
drate concentrations in a plant colonized by PGPR during ex-
posure to low temperature (4C).
B plantlets displayed a carbohydrate balance favorable to
low-temperature tolerance. Bacterization resulted in a twofold
increase in TSS and a 1.2-fold increase in starch, in agreement
with our previously published results (Ait Barka et al. 2006).
These features generally correlate with low-temperature toler-
ance observed in A. thaliana (Rohde et al. 2004). Thus, B
plantlets are characterized by a higher concentration of com-
patible osmolytes, a key element of low-temperature tolerance
(Ruelland et al. 2009). Qualitatively, all sugars known to be
involved in low-temperature tolerance displayed higher concen-
trations in B plantlets (Fig. 4A), especially glucose, sucrose,
and raffinose with its precursor, galactinol.
Sucrose is known to be an osmoprotectant, stabilizing cellular
membranes and maintaining turgor in plant cells (Castonguay
et al. 1995). Sucrose is one of the most easily detected sugars
in cold-tolerant species (Bohnert and Sheveleva 1998). Sucrose
and glucose are known to be important in low-temperature
tolerance because the sfr4 A. thaliana mutant, which exhibits
reduced accumulation of both sugars, is cold sensitive (Uemura
et al. 2003). Both sucrose and glucose may play a role in regu-
lation of cold-induced expression of genes in plants (Tabaei-
Aghdaei et al. 2003).
Raffinose is associated with low-temperature tolerance in
Medicago sativa (Castonguay et al. 1995). Raffinose, with its
precursor galactinol, could thus participate in limiting oxida-
tive damage in B plantlets when submitted to chilling, as sug-
gested recently in A. thaliana (Nishizawa et al. 2008).
The carbohydrate balance of B plantlets is also character-
ized by a shift from disaccharides to monosaccharides (glu-
cose and fructose), a feature naturally occurring in vineyards
during the autumn (Ait Barka and Audran 1996; Hamman et
al. 1996; Uemura et al. 2003).
Bacterization might induce a change in carbohydrate balance
to mimic the effect of cold acclimation. In accordance, the lev-
els of starch, TSS, galactinol, glucose, and, to a lesser extent,
fructose and sucrose were equal in B plantlets compared with
1-day-chilled NBC plantlets (Figs. 3 and 4; Table 1). This the-
ory is supported by several points issued from this study, in-
cluding i) the rapid increase in maltose content in NBC plant-
lets at 4C, which could be related to a demand for starch
degradation to provide precursors for other soluble sugars
(Fig. 5A; Table 1); ii) the faster and higher increase in all
phosphated and nucleotided intermediates in NBC plantlets,
suggestive of a stronger requirement for carbohydrate synthe-
sis during cold acclimation relative to BC plantlets (Fig. 5B;
Table 1); and iii) the faster stimulation of glycolysis in NBC
plantlets, represented by a faster increase in PK activity and
pyruvate accumulation, which suggests higher energy require-
ments for NBC plantlets when exposed to 4C.
Nevertheless, some aspects of carbohydrate changes induced
by B. phytofirmans are not in agreement with this “preacclima-
tion” hypothesis. For instance, the higher concentration of sug-
ars in B plantlets does not compromise further accumulation of
some carbohydrates following cold exposure. For example,
concentrations of starch, raffinose, and mannose remained
higher in BC plantlets even after 5 days of exposure to 4C
(Figs. 3 and 5). This could be due to a specific stimulation of
this pathway by B. phytofirmans PsJN. Particularly, mannose
(Streb et al. 2003; Zhang et al. 2009) as well as fructose
(Bogdanović et al. 2008; Nishizawa et al. 2008), could be re-
lated to a general stimulation of ROS-scavenging properties by
this bacteria via ascorbate synthesis. Improved starch synthesis
without compromising synthesis of other sugars could be ex-
plained by a stimulator effect of the bacteria on both starch
synthesis and degradation pathways. To date, no data are avail-
able to confirm or deny this hypothesis.
Overall, B. phytofirmans influences grapevine carbohydrate
metabolism with both acquired and inducible parameters that
are somehow comparable with cold acclimation and low-tem-

Page 7

502 / Molecular Plant-Microbe Interactions
perature tolerance. Therefore, this phenomenon may partly ex-
plain how the B. phytofirmans strain improves tolerance to
chilling.
MATERIALS AND METHODS
Plant material and in vitro growth conditions.
Plantlets of Vitis vinifera ‘Chardonnay’ were micropropa-
gated by nodal explants grown on 15 ml of agar medium in 25
mm-culture tubes as described earlier (Ait Barka et al. 2006).
Cultures were performed in a growth chamber at constant tem-
perature of 26C under white fluorescent light (200 mol m–2
s–1) with 16 h of light per day.
Bacterial inoculum and plant bacterization.
The bacterial inoculum was produced by transferring two
loops of B. phytofirmans PsJN tagged with green fluorescent
protein (Sessitsch et al. 2005) to 100 ml of King’s B liquid
medium in a 250-ml Erlenmeyer flask incubated at 20C at
150 rpm for 48 h. Bacteria were collected by centrifugation
(3,000 × g for 15 min) and washed twice with phosphate-
buffer saline (PBS; 10 mM, pH 6.5). The pellet was resus-
pended in PBS and used as inoculum. The bacterial concentra-
tion was estimated by spectrophotometry (600 nm) and adjusted
to 3 × 108 CFU ml–1 with PBS (Pillay and Nowak 1997). Roots
of 2-week-old plantlets were immersed in bacterial inoculum
(3 × 108 CFU ml–1) or PBS (control) for 10 s. After inocula-
tion, plantlets were grown as described above for 4 weeks be-
fore chilling treatment.
Chilling treatment.
Half of B and NB plantlets were transferred to a cold growth
chamber maintained at 4C under 16 h of light (white fluores-
cent light, 200 mol m–2 s–1) and 8 h of darkness, whereas the
control plantlets were maintained at 26C. Each treatment was
replicated at least three times and each replicate consisted of
six plantlets.
Leaves from NB and B plantlets were sampled at 0, 1, 2,
and 5 days after chilling exposure. The four conditions for
plantlet treatments were abbreviated as follows: NB plantlets
at 26C (NB), B plantlets at 26C (B), NB plantlets exposed to
4C (NBC), and B plantlets exposed to 4C (BC).
Leaf gas exchanges.
The Pn, gs, and Ci were measured with an open gas exchange
system (LI-6400; Li-Cor, Lincoln, NE, U.S.A.) using equations
developed by Caemmerer and Farquhar (1981). The infrared
gas analysis system was equipped with a clamp-on leaf cuvette
that exposed 6 cm2 of leaf area. Air temperature and humidity
were maintained at 25C and 30%, respectively. The level of
photosynthetically active radiations provided by a red and blue
light-emitting diode (Li-6400-02, Li-Cor) was fixed to 1,500
mol m–2 s–1. CO2 concentration was maintained at a constant
level of 400 mol liter–1 using a CO2 injector with a high-
pressure liquid CO2 cartridge source (LI-6400-01; Li-Cor).
Gas exchange measurements were performed on six plantlets
per condition and on three leaves per plantlet.
Chlorophyll fluorescence.
Chlorophyll fluorescence reflects the functionality of the
photosynthetic apparatus because it results from absorbed
light. Chlorophyll fluorescence was evaluated simultaneously
with gas exchange measurement at ambient CO2 concentration
and temperature. Leaf chlorophyll fluorescence was quantified
using a pulse-modulated fluorometer (FMS2; Hansatech,
King’s Lynn, U.K.). The measuring system applies an array of
blue light-emitting diodes, adjusted to 10 Hz (peak wavelength
at 470 nm), for saturating light pulses. During a saturation
pulse application (5,000 mol m–2 s–1), the fluorescence yield
(F) and the maximum fluorescence yield (Fm) were assessed
in leaves exposed to light. The efficiency of PSII (PSII) rep-
resents the number of electrons transported by a PSII reaction
center per mole of quanta absorbed by PSII. PSII was calcu-
lated as the ratio (Fm – F)/Fm according to Genty and associ-
ates (1989).
Photosynthetic pigments.
Leaf slices were dissected and pigments were extracted
overnight at 4C under continuous agitation in 80% (vol/vol)
acetone amended with 0.5% (wt/vol) MgCO3 to prevent
chlorophyll acidification. Crude extract was centrifuged at
10,000 × g for 10 min at 4C and the supernatant was used to
estimate pigment concentrations spectrophotometrically accord-
ing to the absorbance coefficients determined by Lichtenthaler
(1987). Results were expressed in milligrams per gram of fresh
weight (FW).
Soluble sugars extraction.
Soluble sugars were extracted according to Lunn and associ-
ates (2006). Frozen leaf powder (20 mg) was mixed in a 2-ml
tube (precooled with liquid N2) with 350 l of ice-cold
CHCl3/CH3OH (3:7 [vol/vol]), vigorously vortexed, and incu-
bated at –20C for 2 h. Afterward, 350 l of ice-cold ultrapure
water (resistivity: 18.2 M) was added in the tube. Samples
were vortexed and next centrifuged for 10 min at 14,000 × g at
4C. Water phase was collected in a 2-ml screw-cap tube.
Then, water extraction was repeated with 300 l of ice-cold
ultrapure water. Fractions were combined and evaporated to
dryness using a vacuum dryer and then redissolved in 350 l
of ultrapure water (resistivity: 18.2 M). Before further analy-
sis, samples were membrane-filtered (Multiscreen Ultracel-10;
Millipore, Bedford, MA, U.S.A.).
Sugars and primary metabolites analysis.
Soluble sugars and galactinol. Soluble sugars and sugar alco-
hols were analyzed using HPAE-PAD, with a Dionex DX-500
(Dionex, Sunnyvale, CA, U.S.A.), consisting of an autosampler
AS50, a gradient pump GP50, and an electrochemical detector
ED50 working in pulsed amperometric mode for sugar detec-
tion. Sugars were separated using an analytical Carbo Pac PA1
column (250-by-2-mm internal diameter, 10-m pellicular resin
size) (Dionex) equipped with a guard column (50-by-2-mm
internal diameter, 10-m pellicular resin size) (Dionex). The
column was equilibrated with 100 mM NaOH (eluent A). Then,
25 l of standard or sample was injected onto the column and
the carbohydrates were separated as follows: 0- to 1-min iso-
cratic run with 100 mM NaOH, 1- to 3.5-min linear gradient
from 100 to 140 mM NaOH, 3.5- to 8-min isocratic run with
140 mM NaOH, 8- to 19-min linear gradient from 100 to 180
mM NaOH, 19- to 20-min linear gradient from 180 to 200 mM
NaOH, 20- to 30-min isocratic run with 200 mM NaOH (eluent
B), 30- to 31-min linear gradient from 200 to 100 mM NaOH,
and 31- to 40-min isocratic run with 100 mM NaOH. Flow rate
was set to 0.3 ml min–1 and assays were performed at room tem-
perature. Soluble sugar standards (Sigma, St. Louis) were used
for external calibration. Data were collected and processed using
chromeleon software, version 6.8 (Dionex). Results were ex-
pressed in micromoles per gram FW.
Other metabolites. Phosphated and nucleotined sugars, as
well as pyruvate and PEP, were assayed in leaf extract using a
Dionex high-performance liquid chromatography system cou-
pled to a Finnigan TSQ Quantum MS-Q3 apparatus (Thermo-
Finnigan, Waltham, MA, U.S.A.) according to Lunn and asso-
ciates (2006).

Page 8

Vol. 25, No. 4, 2012 / 503
Starch extraction and analysis.
Leaf powders of the different conditions were resuspended
at 4C in a mortar containing 0.1 M phosphate buffer, pH 7.5.
The homogenates were centrifuged at 12,000 × g for 15 min,
and the pellets were used for starch analysis. The collected
pellets were resuspended in dimethyl sulfoxide–8 M hydro-
chloric acid (4:1 [vol/vol]). Starch was dissolved over 30 min
at 60C under continual agitation (60 rpm), then centrifuged
for 15 min at 12,000 × g. Supernatant (100 l) was mixed with
100 l of iodine-HCl solution (0.06% KI and 0.003% I2 in
0.05 M HCl) and 1 ml of distilled water. The absorbance was
read at 600 nm after 15 min of incubation at room tempera-
ture. Results were expressed in milligrams per gram of dry
weight (DW).
Statistical analysis.
Results were replicated in two or three independent experi-
ments for photosynthesis measurement or carbohydrate analysis,
respectively. Standard analysis of the variance (t test) and a
Tukey test were performed with Kyplot software to assess the
significance of the treatment means at P < 0.05 level.
ACKNOWLEDGMENTS
We thank Europol Agro for funding this work, B. Courteaux for help in
maintaining grapevine plantlets culture, and J. E. Lunn for allowing us to
use the MPI-MP Gölm facility.
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