Sugars are molecules of fundamental importance for life on earth. Sugars act as primary carriers of captured energy from the sun. Sugars not only fuel cellular carbon and energy metabolism but also pay pivotal role as signaling molecules and sugar status modulates & coordinates internal regulators that govern growth and development. The genes involved in production of carbon from photosynthesis with its utilization, mobilization and allocation in various tissues at different developmental stages are highly regulated by sugars. In most plants, sucrose (Suc) is the end product of photosynthesis for translocation from the source to heterotrophic sinks through the sieve element/companion cell complex of the phloem.

1. SUGAR SENSING AND SIGNALING IN
PLANTS
Anjali Dahiya
Ph.D Scholar

2. INTRODUCTION
 Sugars are not only important energy sources and structural components;
they are also central regulatory molecules controlling physiology,
metabolism, cell cycle, development, and gene expression in prokaryotes
and eukaryotes.
 In higher plants, sugars affect growth and development throughout the life
cycle, from germination to flowering to senescence.
 Sugars are physiological signals repressing or activating plant genes
involved in many essential processes, including photosynthesis, glyoxylate
metabolism, respiration, starch and sucrose synthesis and degradation,
nitrogen metabolism, pathogen defense, wounding response, cell cycle
regulation, pigmentation, and senescence.
 Plant sugar regulation is mediated by diverse sugar signals, which are
generated at different locations depending on environmental conditions and
developmental stage. Sucrose transport and hydrolysis play key regulatory
roles in sugar signal generation

3. SUCROSE SYNTHESIS AND PHLOEM
LOADING IN SOURCE LEAVES
 Suc is synthesized in cytosol by
two enzymes: Suc-phosphate
synthase and Suc-phosphate
phosphatase.
 SPS uses UDP-Glc and Fru-6-
phosphate as substrates to
synthesize Suc-6-phosphate,
whereas SPP releases
orthophosphate (Pi) from Suc-6-
phosphate, yielding Suc
Ruan (2014)

4. SUCROSE UNLOADING, TRANSPORT
AND METABOLISM IN SINK TISSUES
 Upon translocation through the phloem to sinks, Suc is degraded by either
invertase (INV) or Suc synthase (Sus) into Hexes or their derivatives, which
are then used in diverse ways.
 INV hydrolyzes Suc into Glc and Fru, whereas Sus degrades Suc in the
presence of UDP into UDP-Glc and Fru.
 INVs are classified as apoplasmic (cell wall), vacuolar, or cytoplasmic
isoforms according to their optimum pH and subcellular locations.
 These forms are referred to as cell wall INV (CWIN), vacuolar INV (VIN),
and cytoplasmic INV (CIN).

5. WHAT HAPPENS IN THE SINK ???
 In sink tissues, sucrose can be imported into cells through
plasmodesmata (symplastic transport) or the cell wall (apoplastic
transport).
 Intracellular sucrose is cleaved by cytoplasmic INV (C-INV) or by
sucrose synthase (SUS).
 Sucrose can also be imported and stored in the vacuole and vacuolar
INV (V-INV) is a major intracellular source of hexoses in
expanding tissues.
 In the apoplast, extracellular sucrose is hydrolyzed by CWINV, a
major driving force in sugar unloading and gradient maintenance
and therefore sink strength.
 These enzymes generate high levels of extracellular glucose and
fructose that are taken up by hexose transporters, which
coexpressed and coordinately regulated with CW-INV.

6. SUCROSE UNLOADING, TRANSPORT, AND
METABOLISM IN SINK TISSUES

7. INVERTASE INHIBITORS
 INHs are small proteins that have molecular masses ranging from 15
to 23 kDa and are characterized by four conserved cysteine residues.
 Research in Arabidopsis has identified a small set of conserved
residues of INH that are required to interact with CWIN in a pH-
dependent manner, with an optimal pH of 4.5.
 The requirement of acid pH for the interaction may explain why
INHs target CWIN or VIN but not CIN.
 Alternatively, compared with the apoplasm and vacuole, the cytosol
offers many other means to regulate sugar status.
 Hence, it may be unnecessary for INH to modulate CIN activity.
 Work in tomato has shown that a large proportion of CWIN activity
is capped by its INH in vivo and that removal of this inhibition
delays leaf aging and improves fruit and seed yield

8. DEVELOPMENTALLY PROGRAMMED AND ABA-INDUCED LEAF SENESCENCE
IS BLOCKED BY ENHANCEMENT OF CWIN ACTIVITY THROUGH SILENCING
ITS INHIBITOR IN TOMATO
Ruan et al.( 2010)

9. DISRUPTION OF SUCROSE METABOLISM AND
SIGNALING CAUSING REPRODUCTIVE FAILURE UNDER
ABIOTIC STRESS
 Abiotic stress blocks Suc import, represses invertase (INV) and
Suc synthase (Sus) activities, and depletes starch reserves.
 This leads to dramatic reduction of hexoses (Hexes), especially
glucose (Glc), in reproductive organs and ultimately to their
abortion.
 low-Glc pool may (i) directly inhibit cell cycle gene expression
and hence cell division and (ii) reduce the metabolic activity of
hexokinase (HXK) associated with the mitochondrial outer
membrane, hence decreasing ATP use and the regeneration of
ADP required for ATP synthesis.
 This could disrupt the energization status of the respiratory
electron transport chain, leading to the overproduction of
reactive oxygen species (ROS) and hence oxidative damage and
even programmed cell death (PCD).
 In parallel, the low availability of Suc may activate Suc non-
fermenting related kinase 1 (SnRK1) to repress growth. The
reduction of INV and Sus expression may occur before the rise
in abscisic acid (ABA) level, but, reciprocally, the stress-
induced increase in ABA can reduce the expression of INV and
Sus.
Ruan (2014)

10. SUCROSE METABOLISM & SUGAR
SIGNALING
Braun et al. (2014)

11. YEAST AS A MODEL SYSTEM
Rolland et al. (2006)

12. SUGAR SENSING MECHANISM IN PLANTS
Smeeken (2000)
Sugar sensing is the
interaction between a sugar
molecule and a sensor protein
in such a way that a signal is
generated.
The signal then initiates signal
transduction cascades that result
in cellular responses such as
altered gene expression and
enzymatic activities.
Sugars, like hormones, can act
as primary messengers and
regulate signals that control the
expression of various genes
involved in sugar metabolism.

13. SUGAR SENSING IN HIGHER
PLANTS
 Sugar repression of photosynthetic genes is likely a central control
mechanism mediating energy homeostasis in a wide range of algae and
higher plants. It overrides light activation and is coupled to developmental
and environmental regulations.
 To establish that glucose represents a physiological regulator, they tested the
effect of lower concentrations of glucose.
 Three photosynthetic fusion genes, cabZm5cat,rbcSZml-cat,and
C4ppdkZml-cat, were used as reporters to monitor repression by measuring
the chloramphenicol acetyltransferase (CAT) activity that is not affected by
various sugar treatments.
Jang & Sheen ( 2004)

14. GLUCOSE ELICITS PHOTOSYNTHETIC GENE
REPRESSION AT PHYSIOLOGICAL
CONCENTRATIONS
 Glucose at 1 to 10 mM was enough to
cause fourfold repression of the
cabZm5 promoter activity.
 Little repression could be triggered by
the glucose analog 3-O-methylglucose
(3-OMG) at the same concentration,
indicating that repression was not the
result of osmotic change.
 Sucrose at 10 mM had much less
effect, suggesting that glucose was
likely the direct signal.
 The glucose repression of cabZm5-cat
was similar in green and greening
protoplasts, but green protoplasts were
more sensitive to glucose.

15. CONT…..
Jang & Sheen ( 2004)
Maize protoplasts transfected with
rbcSZm7-cat & effect of Glucose on the
expression of rbcSZml-cat in Greening
and Green Maize Protoplasts
Maize protoplasts transfected with
C4ppdkZml-cat & effect of Glucose on the
expression of C4ppdkZm7-cat in Greening
and Green Maize Protoplasts

16. GLUCOSE REDUCES THE ACCUMULATION
OF THE CABZM5-CAT TRANSCRIPT
 Glucose represses the transcription of
cabZm5-cat and C4ppdkZml-cat.
 To show directly that glucose affects
transcript accumulation, a sensitive
reverse transcriptase-polymerase chain
reaction (RT-PCR) assay was used to
determine the steady state mRNA levels
in electroporated protoplasts.
 Figure shows that the CAT mRNA level
controlled by the cabZmS promoter was
significantly reduced by 10 mM glucose
but not by sucrose.
 The reduction of CAT mRNA was
specific because the GUS mRNA level
regulated by the nos promoter remained
constant with 10 mM glucose

17. CAN REPRESSION BE TRIGGERED BY
OTHER SUGARS?
 To investigate the signal specificity
of sugar repression in higher plants,
they tested the effect of other sugars.
 Greening protoplasts
coelectroporated with cabZm5-cat
and nos-gus were incubated.with
mono-, di-, and trisaccharides at 10
mM.
 Figure shows that hexoses, such as
galactose and fructose, caused
repression similar to that of glucose.
 Mannose was very potent and
specific, triggering more than 50-
fold repression.
 Approximately a twofold reduction
of cabZmdcat expression in the
presence of sucrose and lactose but
not in the presence of the
trisaccharide raffinose.

18. SUGAR PHOSPHATES DO NOT
TRIGGER REPRESSION
Effect of G-6-P and glucose.
 Reporter genes (cabZm5-cat
and 35sgus) and 20 mM G-6-P
or 20 mM glucose (G) were
delivered into cells by
electroporation.
 Transfected maize greening
protoplasts were incubated for 4
hr before CAT and GUS assays
were performed.
Jang & Sheen ( 2004)

19. FACTORS THAT DETERMINE THE LEVEL OF
SUGAR REPRESSION
 Physiological and metabolic status and photosynthetic capacity of the
leaf determine the level of sugar repression.
 Sucrose does not trigger the same level of repression as
glucose at 10 mM. The effectiveness of sucrose in causing repression is
presumably dependent on its hydrolysis to hexose sugars, which act as
direct signals.
 Sugar repression of photosynthetic genes can also be triggered by other
extrinsic or intrinsic stimuli.
 For example, wounding and bacterial infection cause rapid induction of
extracellular invertase expression. the hydrolysis of apoplastic sucrose by
this elevated invertase may lead to a higher influx of glucose and fructose,
which in turn triggers the repression of photosynthetic genes .This sugar
regulation mechanism may be used as a gene expression switch that
facilitates the cell defense response.

20. MODEL FOR SUGAR REPRESSION OF
PHOTOSYNTHETIC GENE TRANSCRIPTION IN HIGHER
PLANTS

21. HEXOKINASE AS A SUGAR SENSOR
 In the Arabidopsis genome, six HXK and HXK-like (HKL) genes can be
found, serving a variety of physiological functions and likely controlled by
tissue-specific expression patterns, subcellular localization and protein
complex formation.
 Proteomic and GFP fusion analyses indicate that several plant HXKs are
located on the outer mitochondrial membrane, where a completely
functional glycolytic metabolon can be found .
 In addition, plant HXKs are found in plastids.
 Unique in moss, the chloroplast stromal HXK accounts for 80% of the total
hexose kinase activity and is responsible for glucose-mediated growth .

22. SUBSTRATE FOR HEXOKINASE
Smeeken (2000)

23. MORE FACTS CONT.…
 HXK1 is also found in high molecular weight complexes in the nucleus (Cho et al.,
2006b).
 Apparently, HXK1 regulates transcription by direct binding to the promoter of
glucose-repressed genes.
 Chromatin immunoprecipitation experiments with HXK1 show that this complex
specifically binds to cis-regulatory elements upstream of the CAB2 (chlorophyll a/b
binding protein 2) and CAB3 coding regions.
 This nuclear activity requires two HXK1 unconventional partners, HUP1 and HUP2,
components shown to be part of the plant vacuolar H+-ATPase (VHA-B1) and the
19S regulatory particle (RPT5B) of the proteasome complexes, respectively .
 Finally, glucose-dependent gene repression an ChIP experiments in intact plants
suggest that HXK1 and VHA-B1/RPT5B are in contact with specific target gene
promoters and directly regulate glucose-mediated transcription repression. The
studies support a novel concept that a key metabolic enzyme can form complexes
with other conserved proteins to play unique roles and directly control gene
expression in the nucleus, thus uncoupling its signaling activities from metabolism.

24. HEXOKINASE AS A SUGAR SENSOR IN
HIGHER PLANTS
 First enzyme in the hexose assimilation pathway, hexokinase (HXK), acts as
a sensor for plant sugar responses.
 Transgenic Arabidopsis plants expressing antisense hexokinase (AtHXK)
genes are sugar hyposensitive, whereas plants overexpressing AtHXK are
sugar hypersensitive.
 The transgenic plants exhibited a wide spectrum of altered sugar responses
in seedling development and in gene activation and repression.
 Furthermore, overexpressing the yeast sugar sensor YHXK2 caused a
dominant negative effect by elevating HXK catalytic activity but reducing
sugar sensitivity in transgenic plants.
 The result suggests that HXK is a dual-function enzyme with a distinct
regulatory function not interchangeable between plants and yeast.
Sheen et al. (2007)

25. ATHXK AS A SUGAR SENSOR IN PLANTS
(A) and (B) Transgenic plants overexpressing sense (left) and antisense (middle) AtHXKI were grown on plates
containing 6% glucose or 2-dGlc, respectively. Wild-type control plants (right) are shown for comparison
(C) and (D) Sense-AtHXK! (left), anti-AtHXK1 (middle), and control (right) plants were grown on plates
containing 6% mannitol or 6% 3-O-methylglucose, respectively.
(A) (B)
(C) (D)

26. ATHXK MEDIATES SUGAR EFFECTS ON
HYPOCOTYL ELONGATION
 The length of the hypocotyl of
sense-AtHXK1 plants was
reduced 90% when grown on 6%
glucose plates compared with
those grown on 2% glucose plates.
 In contrast, the reduction of
hypocotyl elongation in anti-
AtHXK1 plants was only near
50% when plants were grown on
6% glucose plates compared with
those grown on 2% glucose plates

27. CELL SURFACE RECEPTORS
 In yeast, extracellular glucose and sucrose are detected by the Gpr1-Gpa2 system,
one of only two GPCR systems, the other one being involved in pheromone
detection.
 In striking contrast to animals, where GPCRs constitute one of the major
mechanisms for extracellular signal detection, plants apparently contain only one
canonical G-protein α-subunit (encoded by GPA1 and RGA1 in Arabidopsis and
rice, respectively).
 These proteins and the associated β and γ subunits have been implicated in a wide
variety of developmental, light, phospholipid, and hormone responses oxidative
stress response, and fungal disease resistance.
 GPA1 interacts with two putative receptor proteins: G protein coupled receptor1
(GCR1), a seven transmembrane domain protein with some homology to classical
GPCRS, and Regulator of G-protein signaling1 (RGS1), an unusual hybrid seven-
transmembrane domain protein with a C-terminal RGS-box.
 Based on the use of different sugars and sugar-analogs, it is suggested that AtRGS1
functions in an HXK independent glucose signaling pathway.

28. CONT….
 Another potential extracellular glucose or sucrose detection system in plants
may involve proteins analogous to the yeast glucose transporter-like
sensors, Snf3 and Rgt2.
 An atypical sucrose transporter SUT2/SUC3 was proposed to act as a sensor
I analogy to SNF3 and RGT2 glucose sensors in yeast .
Hanson & Smeeken (2009)

29. D-GLUCOSE SENSING BY A PLASMA
MEMBRANE REGULATOR OF G SIGNALING
PROTEIN, ATRGS1
 Plants use sugars as signaling molecules and possess mechanisms to detect
and respond to changes in sugar availability, ranging from the level of
secondary signaling molecules to altered gene transcription.
 G-protein-coupled pathways are involved in sugar signaling in plants. The
Arabidopsis thaliana Regulator of G-protein Signaling protein 1 (AtRGS1)
combines a receptor-like seven transmembrane domain with an RGS
domain, interacts with the Arabidopsis Gα subunit (AtGPA1) in a D-
glucose-regulated manner, and stimulates AtGPA1 GTPase activity.
 AtRGS1 interacts with additional components, genetically defined here, to
serve as a plasma membrane sensor for D-glucose. This interaction between
AtRGS1 and AtGPA1 involves, in part, the seven-transmembrane domain of
AtRGS1.
Grigston et al. (2008)

30. MODEL FOR A ATRGS1-G-PROTEIN
SUGAR SENSOR
Grigston et al. (2008)

31. D-GLUCOSE DOSE & TIME DEPENDENCE OF
AT4G01080 TRANSCRIPT INCREASE BY D-
GLUCOSE
D-glucose dose-dependence of At4g01080
transcript increase by D-glucose. Wild-type and
Atrgs1–2 7 day old seedlings were treated with
various concentrations of D-glucose
Time dependency of At4g01080
transcript levels in response to treatment
with D-glucose or mannitol.
At4g01080 transcript level increase in response to a range of sugars and
sugar analogues
Grigston et al. (2008)

32. GENERATING SUGAR SIGNALS
 As photoautotrophic organisms, plants generate their own sugars through the
process of photosynthesis. During the day, photosynthetic source tissue
converts CO2 and water to carbohydrates and oxygen, using sunlight as an
energy source.
 Carbohydrates, generated in the chloroplasts, are then exported to the cytosol,
mainlyas triose-phosphates, where they can be converted to hexose phosphates
or sucrose for local use or storage in the vacuole.
 In addition, sucrose is transported to non-photosynthetic sink tissues. There,
sucrose is taken up and converted to different hexoses by invertases and
sucrose synthases or stored in vacuoles and in amyloplasts as starch for longer
term storage.

33. SUGAR SIGNALS IN PLANTS
Rolland et al. (2006)

34. CONT…..
Rolland et al. (2006)

35. THE ROLE OF HEXOKINASE IN PLANT
SUGAR SIGNAL TRANSDUCTION AND
GROWTH AND DEVELOPMENT
 Arabidopsis thaliana hexokinases (AtHXK1 and AtHXK2) have a central
role in the glucose repression of photosynthetic genes and early seedling
development.
 However, it remains unclear whether HXK can modulate the expression of
diverse sugar-regulated genes.
 On the basis of the results of analyses of gene expression in HXK transgenic
plants, suggests that three distinct sugar signal transduction pathways exist
in plants:
(i) The first is an AtHXK1-dependent pathway in which gene expression is
correlated with the AtHXK1- mediated signaling function.
(ii) The second is a glycolysis-dependent pathway that is influenced by the
catalytic activity of both AtHXK1 and the heterologous yeast Hxk2.
(iii) The last is an AtHXK1-independent pathway in which gene expression
is independent of AtHXK1.
 Further investigation of HXK transgenic Arabidopsis discloses a role of
HXK in glucose-dependent growth and senescence.
Xiao et al. (2011)

36. THE EXPRESSION OF PHOTOSYNTHETIC GENES
IS MEDIATED BY ATHXK1-DEPENDENT
PATHWAY
 To determine the role of AtHXK1 in glucose regulated
photosynthetic gene expression, they examined the
effect of exogenous glucose on the expression of CAB1
(chlorophyll a/b-binding protein), PC (plastocyanin),
and rbcS (ribulose-1,5-bisphosphate carboxylase small
subunit) genes in the HXK transgenic and the wild-type
plants.
 The expression levels of CAB1, PC, and rbcS were
very low in the wild-type plants as photosynthetic
genes are repressed by glucose which overrides light
activation.
 The expression of these genes was further reduced in
35S-AtHXK1 plants, indicating that they are
hypersensitive to glucose.
 In contrast, no repression was observed in 35S-
antiAtHXK1 plants.
 Interestingly, the glucose repression of these
photosynthetic genes was also diminished in two
independent lines of 35S-YHXK2 plants, although it
was shown that their HXK activities were similar to
those in 35S-AtHXK1 plants

37. THE EXPRESSION OF PR1 AND PR5 IS
DEPENDENT ON GLYCOLYSIS
 Glucose induction of PR1 and PR5 expression was
higher in 35S-AtHXK1 plants than in the wild-type
plants.
 Loss of PR gene induction in 35S-antiAtHXK1
plants indicated the requirement of AtHXK1.
 However, unlike the regulation of CAB1, PC and
rbcS, the induction of PR1 and PR5 was
exaggerated to the same extent in two independent
35S-YHXK2 lines as in 35S-AtHXK1.
 These results suggest that PR1 and PR5 induction
by glucose may depend on HXK catalytic activity
but not the signalling function of AtHXK1.
 In this case, the signal(s) may not be glucose itself
but rather an unknown metabolite(s) downstream
in the glycolytic pathway.
Xiao et al. (2011)

38. EXPRESSION OF GENES MEDIATED BY ATHXK1
INDEPENDENT SIGNALING PATHWAYS
 AGPase, CHS, and PAL1 were up-
regulated, whereas AS1was
repressed by glucose.
 These results are consistent with
previous studies conducted in
diverse plant species and under
different growth conditions.
 Interestingly, the effect of glucose
on the expression of these genes
was independent of the 3 types of
transgenic plants.
 The expression of CIN1 and PAL3
was not regulated by glucose
under this specific condition.
Xiao et al. (2011)

39. THREE SUGAR SIGNAL TRANSDUCTION
PATHWAYS IN PLANTS
Xiao et al. (2011)

40. PATHWAYS BY WHICH SUGARS METABOLISM
PERFORMS ITS SIGNALING ROLES IN
DEVELOPMENT
Ruan (2014)

41. KIN10/11 ROLE
Sheen et al. (2008)

42. STRESS REGULATED GENE EXPRESSION THROUGH
SUCROSE CONTROLLED BZIP TRANSCRIPTION
FACTORS
Hanson & Smeeken (2009)

43. GLUCOSE/SUCROSE FACILITATES THE JUVENILE
TO ADULT PHASE CHANGE IN ARABIDOPSIS BY
REPRESSING MICRORNA (MIRNA) 156 EXPRESSION
Yang et al. (2013)

44. ROLE OF TREHALOSE
 Trehalose, a non-reducing disaccharide of glucose, is known as a reserve
metabolite in yeast and fungi.
 Trehalose metabolism, a small side-branch of the major carbon flux in
bacterial, yeast, and plant cells, has recently drawn a lot of attention because
of its intriguing regulatory effects on plant growth, development, and stress
resistance.
 The disaccharide trehalose is typically synthesized in a two-step reaction:
 T6P is first synthesized from G6P and UDP-Glc by TPS and then
dephosphorylated to trehalose by a T6P phosphatase (TPP).
 In Arabidopsis, addition of even fairly low amounts of external trehalose to
the growth medium results in a significant inhibition of seedling root
elongation.
 Transgenic Arabidopsis plants overexpressing the E.coli trehalose-6-
phosphate synthase (TPS; OtsA), the first enzyme of trehalose metabolism,
exhibit better growth than wild-type seedlings on media supplemented with
sugars.
 Overexpression of the E.coli trehalose-6-phosphate phosphatase (TPP;
OtsB), the second enzyme in trehalose metabolism, causes a total arrest of
seedling growth on sugar media, suggesting an important role for T6P in
sugar utilization.

45. CONT….
 Plants are unique in that they can synthesize both of the nonreducing
disaccharides found in nature, sucrose and trehalose. The sugars have
divergent roles in plants; sucrose is found at high concentrations and
trehalose is found in trace abundance in most species.
 Trehalose pathway genes have proliferated in plants [2 classes of trehalose
phosphate synthases (TPS), 11 genes in A. thaliana; 10 A. thaliana trehalose
phosphate phosphatase (TPP) genes; but only 1 trehalase gene]. The genes
are under purifying selection. The acquisition of bacterial TPP may have
driven the creation of a new role for class II TPSs, which have both
synthase and phosphatase domains but no demonstrated catalytic activity.
 TPS1 (class I) synthesizes trehalose 6-phosphate (T6P), class II TPSs may
have a regulatory function, and TPPs are catalytically active as
phosphatases. TPS1 is constitutively expressed. Class II TPSs and TPPs are
regulated transcriptionally by carbon status and stress. Class II TPSs are
phosphorylated and interact with 14-3-3 proteins.
 Its role may lie in its proximity to UDPG and hexose phosphate pools.
UDPG is important in cell wall synthesis and hence cell and organ growth
and development. The role of trehalose is less clear but it may regulate
starch breakdown

46. SUGAR SIGNALLING PATHWAYS INTERCONNECT
AND CONTROL PLANT GROWTH
Lastdrager et al., 2014

47. ARABIDOPSIS GLUCOSE-SIGNALING
NETWORKS
Sheen (2014)

48. MODEL OF SUGAR-SENSING & SIGNALING
MECHANISMS IN PLANTS
Rolland et al. (2006)