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Pyrroloquinoline Quinone:
A New Plant Growth
Promotion Factor
Plant growth-promoting rhizobacteria (PGPR) are bacteria that colonize
plant roots and enhance plant growth
by a wide variety of mechanisms. The
use of PGPR in sustainable agriculture
is steadily increasing and offers an attractive way to replace chemical fertilizers, pesticides, and supplements. PGPR
have been applied to various crops to
enhance growth, seed emergence, crop
yield, and disease control, and some have
been commercialized. A PGPR called
Pseudomonas fluorescens B16 was previously isolated from the roots of graminaceous plants. The wild-type B16
colonizes the roots of various plants and
produces an antibacterial compound
that is effective against certain plant
root pathogens. This PGPR strain also
significantly promotes the growth of
cucumber (Cucumis sativus) and barley
(Hordeum vulgare) under greenhouse
and field conditions. In this issue, Choi
et al. (pp. 657–668) report that a key
factor involved in the promotion of plant
growth by P. fluorescens B16 is pyrroloquinoline quinone (PQQ). Analysis of
culture filtrates confirmed that the wildtype B16 produces PQQ, whereas the
mutants defective in plant growth promotion do not. Application of the wildtype B16 on tomato (Solanum esculentum)
plants cultivated in a hydroponic culture
system significantly increased the height,
flower number, fruit number, and total
fruit weight, whereas none of the strains
that were unable to produce PQQ promoted tomato growth. Furthermore, 5
to 1,000 nM of synthetic PQQ conferred
a significant increase in the fresh weight
of cucumber seedlings, confirming that
PQQ is a plant growth promotion factor. Treatment of cucumber leaf discs with
PQQ and the wild-type B16 resulted in
the scavenging of reactive oxygen species and H2O2, suggesting that PQQ
acts as an antioxidant in plants.
generate and maintain an internal temperature of about 20°C even when the
ambient air temperature drops below
0°C (Fig. 1). Traditionally, heat production in thermogenic plants has been
thought to be associated with an increase in the activity of the cyanideresistant electron transport pathway in
mitochondria. This pathway is mediated by an alternative oxidase (AOX)
that accepts electrons from the ubiquinone pool and uses them to reduce
oxygen to water. The free energy generated by the flow of electrons from
ubiquinol to AOX does not generate
ATP but instead is lost as heat. In the
case of mammals, however, mitochondrial uncoupling proteins (UCPs) have
been shown to play a crucial role in nonshivering thermogenesis. UCPs reside
in the mitochondrial inner membrane,
across which they dissipate energy from
the proton gradient that is built up by
the respiratory chain, and this results in
heat production. Although plants also
have UCPs, the roles of these UCPs in
thermogenic plants remain poorly understood. Onda et al. (pp. 636–645)
examine the question of whether UCPs
might also play a role in skunk cabbage
thermogenesis. Among genes that may
possibly mediate heat production in
skunk cabbage are the SrAOX gene that
encodes an AOX and two cDNAs that
encode UCPs, designated SrUCPa and
SrUCPb. To further clarify the physiological roles of AOX and UCP during
skunk cabbage thermogenesis, the authors identified the thermogenic cells
surrounding the stamens in the florets.
These putative thermogenic cells coexpressed transcripts for SrAOX and
SrUCPb. Moreover, in vitro biochemical
www.plantphysiol.org/cgi/doi/
10.1104/pp.104.900246
Transgenic Seeds with Low
Phytic Acid and High
Inorganic Phosphorus
Most of the phosphorus (P) in soybean (Glycine max) seeds exists in the
form of phytic acid (PA), a storage
compound that is indigestible by poultry and swine. PA also chelates divalent
cations, making them less nutritionally
available to livestock. Moreover, underutilized PA in animal feeds often leads
to water pollution and the eutrophication of lakes and ponds. Previously,
some of the authors of Bilyeu et al. (pp.
468–477) generated a transformed
Arabidopsis (Arabidopsis thaliana) line
with an embryo-expressed periplasmic phytase (APPA) encoded by the
Escherichia coli appA gene. To improve
APPA accessibility to seed PA in planta,
they initially employed a construct that
directed APPA to the vacuole. While
significant reductions in seed PA were
observed in two independent transgenic
lines, there was not a compensatory
increase in free inorganic phosphate. In
the present study, the authors employed
a cytoplasmic version of APPA in soybean. One of these new transgenic lines
exhibiting cytoplasmic expression of
APPA (CAPPA) exhibited high levels
of phytase activity, $90% reduction in
seed PA, and no loss of total seed P.
Because CAPPA also produces abundant active enzyme in mature seeds,
soymeal from this new, transgenic line
can also be used to convert the PA of
admixed meals, such as cornmeal, directly into utilizable inorganic P.
ABC Transporters and
Root Exudation
Thermoregulation in
Skunk Cabbage
The spadix of the Japanese skunk
cabbage (Symplocarpus renifolius) can
studies of isolated mitochondria also
suggested that UCPs were functional.
Thus, the results suggest that functional coexpression of AOX and UCP
underlies the molecular basis of heat
production in skunk cabbage.
Figure 1. Thermogenesis in the spadices of
Japanese skunk cabbage involve both alternative oxidase and uncoupling proteins. (Photo
by Masaki Sawamoto.)
Root secretions, or root exudates, help
roots penetrate the soil and orchestrate
rhizosphere interactions, including symbiotic, pathogenic, and allelopathic interactions. Hence, exudates play a central
role in plant growth in natural habitats.
Root exudates are composed of both
low and high molecular weight components, including primary and secondary
Plant Physiology, February 2008, Vol. 146, pp. 323–324, www.plantphysiol.org Ó 2008 American Society of Plant Biologists
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323
metabolites, proteins, and peptides. The
compounds secreted vary in quantity
and chemical structure depending on
the plant species, the developmental
stage, the interacting organism(s), and a
wide variety of environmental factors.
The ATP-binding cassette (ABC) transporters encompass a large protein family found in all phyla and utilize the
energy of ATP hydrolysis to translocate solutes across cellular membranes.
Given the known ATP-dependence of
root exudation, Badri et al. (pp. 762–
771) sought to examine the possible role
of ABC transport proteins in exudation
from Arabidopsis roots. Generally, the
root exudation of phytochemicals is assumed to occur near the root tip and
root elongation zone. Therefore, they
hypothesized that ABC transporter genes
specifically expressed in the epidermis
of these regions may likely be involved
in root secretion processes. They identified 25 ABC transporter genes that
were highly expressed in root cells. Of
these 25 genes, they selected six fulllength ABC transporters and a half-size
transporter for in-depth molecular and
biochemical analyses. They compared
the exuded root phytochemical profiles
of these seven ABC transporters mutants
to those of the wild type. There were
three nonpolar phytochemicals missing
in various ABC transporter mutants
compared to the wild type when the
samples were analyzed by HPLC-MS.
These data suggest that more than one
ABC transporter can be involved in the
secretion of a given phytochemical and
that a transporter can be involved in the
secretion of more than one secondary
metabolite. Thus, the release of phytochemicals by roots is partially controlled
by ABC transporters.
Vacuolar Peroxidases:
Targets for Increasing
Medicinal Alkaloids
Class III peroxidases are multifunctional enzymes that catalyze the oxida-
tion of small molecules at the expense
of H2O2. They are capable of recognizing a broad range of substrates and
exist in a high number of isoenzymes.
Class III peroxidases have mostly been
implicated in key processes determining the architecture and defense properties of the plant cell wall. Much less is
known about vacuolar peroxidases, although in vitro studies have revealed
that they use a number of vacuolar metabolites, such as phenols, flavonoids,
and alkaloids, as substrates. In this
issue, Costa et al. (pp. 403–417) report
the full gene structure and cDNA cloning of the major class III peroxidase isoenzyme (CrPrx1) present in the leaves
of Cantharanthus roseus, the Madagascar
periwinkle. This enzyme is thought to
be involved in the production of the
dimeric alkaloids vinblastine and vincristine, two drugs used widely in cancer
chemotherapy. These drugs accumulate naturally but only at low levels in
the leaves of C. roseus. Since approximately 450 kg of dry C. roseus leaves is
needed to obtain 1 g of vinblastine for
pharmaceutical production, there is a
strong incentive to increase the titer of
these drugs in C. roseus leaves. The results obtained in the present study provide further evidence in support of a
role for CrPrx1 in the vacuolar dimerization of the indole alkaloids vindoline
and catharanthine into the direct precursor of the anticancer indole alkaloids
vinblastine and vincristine, indicating
the potential of CrPrx1 as a target to increase alkaloid levels in the plant.
Does Photosynthesis Affect
Stomatal Conductance?
The fact that guard cells are generally
the only epidermal cells with prominent chloroplasts has always intrigued
plant physiologists, and many researchers have proposed hypothetical
mechanisms by which guard cell photosynthesis might directly influence
stomatal conductance. Stomatal opening in C3 species in response to light
is thought to be induced by distinct
mechanisms depending on the wavelength of incident light. Blue light is
perceived directly by phototropins and
activates a signaling cascade that results in fast stomatal opening. The opening response of stomata to red light
requires higher irradiance and is abolished by 3-(3,4-dichlorophenyl)-1,1dimethylurea, a PSII inhibitor. It has
also been suggested that the guard cell
response to red light is in part an indirect response to red-light-driven intercellular CO2 uptake in the mesophyll.
For example, chloroplast-containing
guard cells in albino sections of variegated leaves do not respond to photosynthetically active radiation, but are
sensitive to blue light and CO2. New
studies by Baroli et al. (pp. 737–747),
however, add to accumulating studies
that suggest that photosynthesis plays
no direct role in changing stomatal
conductance. The authors have contrasted the red light response of stomata of wild-type tobacco (Nicotiana
tabacum) with that of antisense plants
impaired in photosynthetic CO2 assimilation either by a decrease in chloroplast electron transport rate and ATP
synthesis or by a decrease in Rubisco
activity and ATP consumption. They
report that these impairments do not
affect stomatal conductance. To further
explore the relationship between photosynthesis and stomatal conductance,
they also examined the stomatal response of wild-type and antisense
Rubisco plants to growth irradiance.
Remarkably, despite the large difference
in photosynthetic rates, the transpiration
machinery of wild-type and antisense
Rubisco plants responded in the same
manner to the different light growth
conditions. Thus, the red light response
of stomatal conductance is apparently
independent of the concurrent photosynthetic rate of the guard cells or of
that of the underlying mesophyll.
Peter V. Minorsky
Division of Health Professions and Natural Sciences
Mercy College
Dobbs Ferry, New York 10522
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Plant Physiol. Vol. 146, 2008
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