Abstract
Abstract: A comprehensive investigation was carried out to determine the changes that occurred in water-stressed cucumber (Cucumis sativus L.) in response to melatonin treatment. We examined the potential roles of melatonin during seed germination and root generation and measured its effect on reactive oxygen species (ROS) levels, antioxidant enzyme activities, and photosynthesis. Melatonin alleviated polyethylene glycol induced inhibition of seed germination, with 100 μm melatonin-treated seeds showing the greatest germination rate. Melatonin stimulated root generation and vitality and increased the root:shoot ratio; therefore, melatonin may have an effect on strengthening cucumber roots. Melatonin treatment significantly reduced chlorophyll degradation. Seedlings treated with 100 μm melatonin clearly showed a higher photosynthetic rate, thus reversing the effect of water stress. Furthermore, the ultrastructure of chloroplasts in water-stressed cucumber leaves was maintained after melatonin treatment. The antioxidant levels and activities of the ROS scavenging enzymes, i.e., superoxide dismutase, peroxidase, and catalase, were also increased by melatonin. These results suggest that the adverse effects of water stress can be minimized by the application of melatonin.
Introduction
Melatonin (N-acetyl-5-methoxytryptamine) is a naturally
occurring compound in plants and has been detected in the
roots, leaves, fruits, and seeds of a considerable variety of
plant species [1, 2]. Melatonin is a ubiquitous and highly
conserved molecule in plant and animal kingdoms [3, 4] and
has been identified in insects, arthropods, planarians,
mollusks [5], dinoflagellates [6], and brown algae [7]. The
known physiological functions of melatonin in animals
include timing circadian rhythms, signaling environmental
changes, cancer inhibition and detoxification of free radicals,
and other reactive oxygen species (ROS) and related
products [8–13].
There are reports demonstrating the ability of melatonin
to alleviate the effects of abiotic stresses such as low
temperature, copper stress, and light conditions during seed
germination [14–17], but little is known about its comprehensive
effects in plants. Evidence suggests the role of
melatonin in seed germination, and plant survival may be
related to melatonin-induced changes in membrane and
protein peroxidation [16, 18, 19]. In addition to its
antioxidant and growth-regulating functions, melatonin
may play a role in regulating photoperiod and circadian
rhythms in plants [20, 21]. Furthermore, melatonin may
play a role in protecting tissues during flower and seed
development in Datura metel [22]. Recently, it has been
reported that apple leaves treated with melatonin clearly
exhibited a delayed senescence process [23].
Many studies on melatonin in plants have focused on its
phytochemical characteristics, but whole plants have been
less frequently systematically analyzed, and growth conditions
have not always been taken into consideration [24].
Many investigations have examined the effects of melatonin
on in vitro organogenesis, such as improving cotyledon
expansion [25], promoting hypocotyl and coleoptile growth
[26], and preventing apoptosis during cold-treatment in
Daucus carota cell suspensions [27]. However, in vitro
tissues do not truly reflect the physiological state of a
complete plant because of a lack of material transportation
and long distance signal transduction.
Osmotic stress and water deficit can reduce the ability of
plants to take up water. Leaves and roots of herbaceous
plants commonly consist of more than 80% water when
turgid. Cucumber plants require a substantial amount of
water during its growth period, making it an ideal species to
study the variation in traits related to drought tolerance. To
accurately and efficiently control water potential,
Abstract: A comprehensive investigation was carried out to determine the
changes that occurred in water-stressed cucumber (Cucumis sativus L.) in
response to melatonin treatment. We examined the potential roles of
melatonin during seed germination and root generation and measured its
effect on reactive oxygen species (ROS) levels, antioxidant enzyme activities,
and photosynthesis. Melatonin alleviated polyethylene glycol induced
inhibition of seed germination, with 100 lm melatonin-treated seeds showing
the greatest germination rate. Melatonin stimulated root generation and
vitality and increased the root:shoot ratio; therefore, melatonin may have an
effect on strengthening cucumber roots. Melatonin treatment significantly
reduced chlorophyll degradation. Seedlings treated with 100 lm melatonin
clearly showed a higher photosynthetic rate, thus reversing the effect of water
stress. Furthermore, the ultrastructure of chloroplasts in water-stressed
cucumber leaves was maintained after melatonin treatment. The antioxidant
levels and activities of the ROS scavenging enzymes, i.e., superoxide
dismutase, peroxidase, and catalase, were also increased by melatonin. These
results suggest that the adverse effects of water stress can be minimized by the
application of melatonin.
Na Zhang1
, Bing Zhao1
, Hai-Jun
Zhang1
, Sarah Weeda2
, Chen
Yang1
, Zi-Cai Yang1
, Shuxin Ren2
and Yang-Dong Guo1
1
College of Agriculture and Biotechnology,
China Agricultural University, Beijing, China;
2
School of Agriculture, Virginia State
University, Petersburg, VA, USA
Key words: antioxidation, cucumber,
germination, lateral root, melatonin,
photosynthesis, water stress
Address reprint requests to Yang-Dong Guo,
College of Agriculture and Biotechnology,
China Agricultural University, No. 2 Yuanmingyuan
West Road, Beijing 100193, China.
E-mail: yaguo@cau.edu.cn
Shuxin Ren, School of Agriculture, Virginia
State University, PO Box 9061, Petersburg,
VA 23806, USA.
E-mail: sren@vsu.edu
Received March 27, 2012;
Accepted May 25, 2012.
J. Pineal Res. 2013; 54:15–23
Doi:10.1111/j.1600-079X.2012.01015.x
2012 John Wiley & Sons A/S
Journal of Pineal Research
15
Molecular, Biological, P
h
ysiological and Clinical Aspects of Mel
atoni
n
polyethylene glycol (PEG), a nonionic, long chain, nonpenetrating,
inert polymer [28], was used to maintain
rooting media at predetermined ww values [28–31].
An oxidative burst is an intrinsic feature of plant
response to biotic and abiotic stress. It is known that the
deleterious effects resulting from the cellular oxidative state
may be alleviated by the enzymatic and nonenzymatic
antioxidant systems [32]. Plants respond and adapt to water
stress by altering their cellular metabolism and invoking
various defense mechanisms [33]. The addition of this
indoleamine treatment enables plants to survive under
environmental stresses by enhancing recovery potential
[1, 34].
In the present study, we focused on the alleviation effect
of melatonin on PEG-stressed cucumber seed germination
and seedling growth. Our study demonstrates, for the first
time in plants, the relationship between changes in melatonin
levels and water stress during growth.