Page 51 - BCI Bonsai & Stone Appreciation Q1 2025

it, rapid leaf expansion can negatively affect adaptation
to low water availability. Another process modified is
root growth. Water availability affects the relation-
ship between the growth of the aerial part and the
root; the root continues its development while the
aerial part stops growing because of stress. Thus, plants
can continue root development in search of water in
deeper soil zones. Calcium (Ca2+) acts as a signal in
the response to ABA. Recent studies identified the role
of phosphorylation in response to ABA by finding that
AtCPK32 regulates ABA-related transcription factors
(ABRE BINDING FACTOR) such as 4/ABSCISIC
ACID RESPONSIVE ELEMENT BINDING PROTEIN
(AREB). ABA also activates mitogen-activated protein
kinase (MAPK) that phosphorylates many ABA- and
stress-responsive transcription factors necessary
for stress response function. In the transcriptional
regulation of genes by ABA, both cis- and trans-acting
factors have been identified. Currently, more than 20
functional ABRE-like elements have been found in
promoters of ABA-responsive genes. The element is
defined as an 8-10 base pair sequence with an ACGT
sequence, which corresponds to the core or center of
the cis-transcriptional factor known as the G-box. The
sequences flanking the ACGT core are important for
the in vivo function of this element. Expression studies
indicate that the (C/T)ACGTGGC sequence is a strong
ABRE; however, other sequences are equally functional.
In plants, several transcription factors have been cloned
in trans with basic, leucine zipper-like (bZip) domains
that bind to the ABRE as dimers.
Although the expression of many genes that are
induced during water stress is significantly increased
by ABA applications, there is no consistent correla-
tion between mRNA levels and ABA levels under
these conditions. This evidence strongly suggests that
the expression of some genes during dehydration is
totally or partially independent of ABA. Based on these
data, Shinozaki and Yamaguchi-Shinozaki, in 1996,
proposed the existence of at least four independent
pathways of gene regulation during water stress: two
ABA-dependent (one dependent on protein synthesis
and the other independent of protein synthesis) and two
ABA-independent, through DRE/CTR-type elements.
It has been shown that the cold-independent pathway is
also regulated by DRE/CTR-type elements that respond
exclusively to cold.
Macromolecules Involved
in Waterlogging
Plants respond to waterlogging stress by regulating
their morphological structure, energy metabolism,
endogenous hormone biosynthesis, photosynthesis,
respiration, and the concentration of different reactive
oxygen species (ROS). Although excess ROS are det-
rimental to plant cells, ROS can also act as signaling
molecules in plant cells under stress. NADPH oxidase
is a key enzyme in ROS production and plays a vital
role in ROS-mediated signal transduction. Expression
of the NADPH oxidase-related gene Atrboh D, a gene
TABLE 1
ABA: Abscisic Acid
ACS: Salicylic Acid
Response genes that
improve growth and
help deter stress in
plants.
ADH: Alcohol
Dehydrogenase,
facilitates fermentation
and produces a small
amount of energy as
ATP.
APX: Ascorbate
Peroxidase, another
enzyme that mitigates
ROS by converting H₂O₂
into H₂O.
ARs/Aerenchyma:
Formation of adven-
titious roots as a
mechanism to absorb
water when normal
roots are compromised,
or conditions elsewhere
on the tree are more
suitable for root growth.
ATP: Adenosine
Triphosphate
Auxin: First plant
growth hormone
BR: Brassinosteroids,
a hormone involved
in many pathways
in plants, including
development, stress
response, and growth.
CAT: Catalase, an anti-
oxidant enzyme that
prevents PCD.
ElL1A: Ethylene
Insensitive Protein,
helps cells block the
ethylene pathway that
would otherwise lead
to PCD.
ET: Ethylene, a plant
hormone that signals
several stress pathways.
GA: Gibberellic Acid, a
plant hormone involved
in numerous processes,
including stress
responses.
GA2Ox7: A gibberellin-
synthesizing gene and
a positive regulator of
stress response.
JA: Jasmonic Acid, a
plant hormone that
regulates growth
and participates
in numerous plant
functions, including
stress responses.
LDH: Lactate
Dehydrogenase, an
enzyme involved in oxy-
gen-deficient conditions
(e.g., stagnant water),
producing chemicals to
prevent cell death.
MT: Melatonin, an
antioxidant and stress
reliever.
PAS: Phenolic Acid,
causes dark pigmen-
tation and signals
responses to PCD.
PCD: Programmed Cell
Death, a last resort to
contain dying cells,
prevent infection, and
recover some cellular
chemicals for the plant.
RBOH: Respiratory
Burst Oxidase Homolog,
regulates growth by
producing secondary
metabolites such as
vitamin E, carotenoids,
and ascorbate, helping
to manage ROS.
ROS: Reactive Oxygen
Species, undesirable by-
products of metabolism
produced in various
cellular compartments,
especially under unfa-
vorable environmental
conditions.
SA: Salicilic Acid
SD1: The semidwarf1
gene, functions to
change leaf angles
to improve light
penetration.
SK1/SK2: Snorkel 1 and
2, proteins that induce
GA synthesis.
SLR1: Slender Rice1,
a plant protein first
identified in rice that
accumulates GA.
SOD: Superoxide
Dismutase, an enzyme
in the first line of
defense against ROS.
Sub1A: Ethylene
response transcription
factor that produces
proteins receptive to
ethylene during plant
stress to avoid PCD.
associated with ROS production, is induced by water-
logging and positively regulates H2O2 production and
increased ADH1 gene expression. Therefore, this signal
enhances ethanol fermentation ability and increases
the survival rate of plants under waterlogging.
Plants may rely on antioxidant enzyme systems
and other active antioxidants to maintain a dynamic
ROS balance, thereby reducing the extent of oxidative
damage. Waterlogging treatment results in an increase
in catalase, ascorbate peroxidase, and superoxide
dismutase activities, as well as polyphenol oxidase.
Rapid ethylene (ET) accumulation is an important
way plants respond to waterlogging. Endogenous ET
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