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Crop yields are frequently lowered by biotic and abiotic stresses, and one
of the most effective strategies to improve agricultural output is to breed
or engineer plants tolerant or resistant to stress. Initial research on
molecular manipulation focused on production of enzymes that detoxify
reactive oxygen species, such as superoxide dismutase. Reactive oxygen
species are induced by most types of stresses. Since then, many approaches
have been pursued to produce plants tolerant to a broad range of stresses.
In a paper published in the Plant Journal, March 2005, researchers from
Bayer Biosciences report that breeding or engineering for high energy-use
efficiency under stress conditions is a valuable approach to enhance overall
stress tolerance of crops1, June 2005 by Tawanda Zidenga.


When a plant is exposed to stress, survival mechanisms turn on to reduce
damage. The balance between stress and survival signals determines the level
of damage suffered by the plant. Among key molecules studied in relation to
stress are reactive oxygen species (ROS), which, due to their high
reactivity and therefore toxicity, have been called the unwelcome companions
of aerobic life. Under stress conditions, the steady state level of ROS
usually increases, and it has been hypothesized that ROS (specifically
hydrogen peroxide) might also act as messengers, turning on stress-related
genes5. Recent progress in plant signal transduction reveals that ROS are
more than just the misfortunes of an aerobic life. They also play a
regulatory role in the plant's physiology (for a review please see ref. 2).

Engineering indifference to stress!

Since it is not very practical to completely avoid growing crops in
conditions that are stressful, scientists focus on engineering plants to be
indifferent to stress. Marc DeBlock and colleagues report a strategy of
improving stress tolerance in plants by maintaining the plant's energy
homeostasis under stress1. When plants are exposed to a stress signal, they
expend a lot of energy and exhibit enhanced respiration rates. This is
partially due to a breakdown in the NAD+ pool caused by the enhanced
activity of poly(ADP-ribose) polymerase (PARP), which uses NAD+ as a
substrate to synthesize polymers of ADP-ribose. This poly(ADP ribosyl)ation
is a posttranslational modification of nuclear proteins that seems to be
initiated by oxidative and other types of DNA damage. Stress-induced
depletion of NAD+ results in a similar depletion of energy, since ATP
molecules are required to resynthesize the depleted NAD+. In this paper1,
DeBlock and coworkers show that plants with lowered poly(ADP ribosyl)ation
activity appear tolerant to multiple stresses.

First, the researchers demonstrated that inhibiting PARP activity via
chemical inhibitors (3-methoxybenzamide, nicotinamide, and isonicotinamide)
protects plants from oxidative stress. To produce transgenic stress tolerant
plants, they transformed Arabidopsis and oilseed plants with double-stranded
RNA constructs of the parp1 and parp2 genes. Use of double-stranded RNA is
now a widely accepted strategy for silencing endogenous genes. In this way,
the group downregulated PARP1 and PARP2. Arabidopsis and oilseed plants were
transformed with dsRNA constructs containing the 5' end of Arabidopsis
AtParp1 (hairpinAtParp1 or hpAtParp1) or AtParp2 (hpAtParp2) in the stem
structure. The AtParp lines showed enhanced tolerance to stress, such as
heat and drought. The figure below shows pictures of transgenic, azygous,
and control plants after exposure to heat and drought stress.

Managing the economics of energy under stress

Most stresses interfere with mitochondrial function1. Stress deregulates the
physiology of the plant and causes NAD+ breakdown, ATP overconsumption, and
enhanced respiration. These reactions deplete the energy of the plant, cause
the production of reactive oxygen species, and consequently induce cell
death3. According to the paper1, inhibiting PARP prevents energy
overconsumption under stress, allowing normal mitochondrial respiration.

To demonstrate the high energy efficiency of hpParp lines, the researchers
cultured hypocotyl explants of control and hpParp plants on medium
containing 0.06 M glucose as the sole carbon source (concentrations higher
than 0.1 M glucose have to be used to allow a good callus induction and
growth). While the control plants were necrotic and showed poor callus
formation, hpParp lines remained green and showed vigorous callus.
Germination on medium containing 2 - 6% glucose resulted in no difference in
seedling growth between the hpAtParp lines and the non-transgenic control
line. This indicates that the differential callus formation and survival of
the explants on glucose medium between the stress-tolerant hpAtParp lines
and the control line is not due to an altered sugar sensing, but rather due
to efficient energy metabolism in the hpParp lines1.

In summary, this paper provides a potential approach for engineering for
broad stress tolerance in crop plants. However, poly(ADP ribosyl)ation has
been reported, at least in animal systems, to be a cellular response to
oxidative damage4. Poly(ADP-ribosyl)ation plays an important role in the
recovery of proliferating cells from certain types of DNA damage, and this
has been linked mechanistically with an involvement in DNA base-excision
repair.

It is therefore not clear whether plant parp genes have a similar function
as animal parps, in terms of maintenance of genomic stability. If they do,
downregulating them may have negative effects to the growth of the plant.
The researchers report that they have not found any negative effect of
downregulating parp genes on the growth of plants. They suggest that since
the activity is not completely downregulated, perhaps the remnant activity
is enough to carry out DNA repair. More work on this area will probably be
required to show that this approach works without shortchanging the plant.

References

1. De Block M et al. (2005) Poly(ADP-ribose) polymerase in plants affects
energy homeostasis, cell death and stress tolerance. The Plant Journal 41,
95â??106

2. Ron Mittler et al. (2005) Reactive oxygen gene network of plants. Trends
in Plant Science 9, (10)490-498

3. De Block M et al. (2004) Pflanzenschutz-Nachrichten Bayer 57/2004, 1,
105-110

4. Burkle A (2000) Poly(ADP-ribosyl)ation: a posttranslational protein
modification linked with genome protection and mammalian longevity.
Biogerontology 1(1), 41-6

5. Plant Physiology Online. [www.plantphys.net]

[www.isb.vt.edu]

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