I. Waterlogging of plant roots causes oxygen deficiency, resulting in the production of ACC (1-aminocyclopropane-1-carboxylic acid) in the roots. ACC is then transported to the shoots where it is converted to ethylene. II. Ethylene induces epinasty in the shoots, causing downward leaf growth and drooping, which helps drain water from the flooded roots and allow respiration to resume. III. Plants have various metabolic adaptations for coping with low oxygen conditions, including shifting carbon metabolism to ethanol fermentation and producing compounds like alanine and succinate.

1. Some important slides
WATERLOGGING

3. A C C v ia x y le m
R o o ts , A n o x ia , h y p o x ia
O 2 d e fic ie n c y re s u lts in p ro d u c tio n o f
A C C (1 -a m in o a c y l p r o p a n e -1 -c a r b o x y lic a c id )
S h o o t A e r o b ic
A C C o x id a se
A C C E th y le n e
S h o o t A e r o b ic
E th y le n e re s p o n s iv e c e ll (a d a x ia l) g ro w
m o re th a n a b a x ia l. Its re s u lts in
E P I N A S T Y (d o w n w a r d le a f g ro w th ,
d r o o p in g b u t n o lo s s o f tu r g o r )
-Too much water or root flooding causes
anoxia or lack of oxygen in the roots
and thus the release of Ethylene.
-The ethylene induced epinasty helps
drain the roots of water allowing
respiration again.
-When root flooding is a permanent
condition, a plant might keep high levels
of SalicylicAcid (SA) around to keep
guard cells open and to cause
epinasty. It might use SAinstead of
Ethylene because SAis growth hormone
and Ethylene promotes senescence
which gets in the way of growth.
-Thirdly it might occur during
desiccation and induced be by ABA,
when water circulation is still needed
but transpiration is not available.
1
2
3

4. Figure 2. Pathways of carbohydrate metabolism
under oxygen deprivation. Plants have two routes
for the degradation of sucrose: the sucrose
synthase and invertase pathways. Formation of
phosphorylated hexose sugars by the sucrose
synthase pathway conserves one mole of ATP per
mole of sucrose compared with the invertase
pathway. Increases in an invertase inhibitor and
sucrose synthase under hypoxia would promote
degradation via the more energy efficient sucrose
synthase pathway. The recycling of pyrophosphate
(PPi) is required for the sucrose synthase and
glycolysis pathways [73]. As shown, four moles of
ATP and NADH are produced by glycolysis per
mole of sucrose metabolized. Anaerobic
fermentation regenerates NADC, which is essential
for glycolysis. The breakdown of starch to soluble
carbohydrates under hypoxia occurs in some
species [16].
initial decrease in pH
helps the plant to switch
from lactate to ethanol
fermentation by activation
of alcohol dehydrogenase
and inhibition of lactate
dehydrogenase
I. Shift from Lactate to ethanol

5. initial decrease in pH helps the plant to switch from lactate to ethanol fermentation
by activation of alcohol dehydrogenase and inhibition of lactate dehydrogenase

6. II. Plants continue to respire anaerobically (ADH increases)
(rate of glycolysis increases) and twoATPs per one hexose breakdown
are available
Hexoses  Pyruvic acid  Ethanol + 2ATP
III. Regeneration of NAD+ to continue glycolysis
PAAcetaldehyde Ethanol (NADH NAD+)

7. Crawford’s metabolic adaptation
IV.Avoid toxic ethanol production/alternative
fermentation pathway
sensitive plant: ethanol and acetaldehyde production and
tolerant plant: much safer compounds (OAA, Malate) are
produced
PEP  OAA  Malate (NADH NAD+)

8. A C C v i a x y l e m
R o o t s , A n o x i a , h y p o x i a
O 2 d e f i c i e n c y r e s u l t s i n p r o d u c t i o n o f
A C C ( 1 - a m i n o a c y l p r o p a n e - 1 - c a r b o x y l i c a c i d )
S h o o t A e r o b i c
A C C o x i d a s e
A C C E t h y l e n e
R O O T S
E t h y l e n e
H y d r o l a s e s l i k e c e l l u l a s e & p e c t i n a s e
H y d r o l y s i s o f c e l l w a l l s i n c o r t e x z o n e
1
2
3

9. Possible stages in aerenchyma formation in roots of Zea mays induced by partial oxygen shortage external to the
root and mediated by increased synthesis of ethylene that in turn induces a form of programmed cell death in
target cells of the cortex

10. Glycolysis and the tricarboxylic acid cycle are linked by alanine
aminotransferase during hypoxia induced by waterlogging of
Lotus japonicus.
The role of nitrogen metabolism in the survival of prolonged periods of waterlogging was
investigated in highly flood-tolerant, nodulated Lotus japonicus plants. Alanine production
revealed to be a critical hypoxic pathway. Alanine is the only amino acid whose
biosynthesis is not inhibited by nitrogen deficiency resulting from RNA interference
silencing of nodular leghemoglobin. The metabolic changes that were induced following
waterlogging can be best explained by the activation of alanine metabolism in
combination with the modular operation of a split tricarboxylic acid pathway. The sum
result of this metabolic scenario is the accumulation of alanine and succinate and the
production of extra ATP under hypoxia. The importance of alanine metabolism is
discussed with respect to its ability to regulate the level of pyruvate,
Marcio Rocha; Francesco Licausi; Wagner L Araújo; Adriano Nunes-Nesi; Ladaslav Sodek;
Alisdair R Fernie; Joost T van Dongen. Plant physiology, 152

12. Hormonal regulation during
hypoxia
• Ethylene is the principal mediator promoting the development of
aerenchyma in maize as well as other plants
– The formation of aerenchyma in rice roots has, however, been considered to be
a result of genetic control since the aerenchyma always forms in rice roots,
regardless of environmental conditions
• Auxins and gibberellins are prerequisites for ethylene action and play
triggering rather than regulatory functions
• ABA concentrations were found to increase in roots of pea plants during
the 2nd, 3rd, and 4th days of flooding, cansing stomata to partially close
and enriching the leaves with the hormone

13. Regulation of Gene Expression
• Plants also respond to anoxia by altering the pattern of protein
synthesis. The proteins which are synthesized as a specific response to
anaerobiosis are called the anaerobic polypeptides (ANPs)
• several of the genes encoding ANPs (Adh1, Adh2, Sh1, Ald) in maize,
pea, and Arabidopsis also share a consensus motif (ARE) in the
promoter regions
• Ca2+ is responsible for ANPs signal transduction. Ca2+ quickly rises
under low O2. which enhances genes for ADH (alcohol
dehydrogenase) and sucrose synthase