 |
Mo Mustard Blues |
Mo is an essential micronutrient for
plants. The Mo content of plants is a direct reflection of the
availability of Mo in the soil. Plants growing in acidic soils often
suffer from Mo deficiency, whereas plants growing in basic soils with
poor drainage and high organic content often take up too much Mo. High
soil Mo can also result from environmental pollution, particularly from
mines and the stainless steel industry. Ruminants that consume plants
that are too Mo-rich often suffer from molybdenosis. With the aim of
developing Mo phytoremediation techniques, Hale at al. (pp.
1391-1402) have launched a project to elucidate the mechanisms
that control the accumulation, tolerance, and biotransformation of Mo
in Indian mustard (Brassica juncea), a species widely used in phytoremediation. They report that when Indian mustard or
Brassica rapa is supplied with (colorless) molybdate, the
plants accumulate a blue crystalline, Mo-containing compound in the
vacuoles of their epidermal cells (Fig.
1). An anthocyanin-lacking B. rapa failed to turn blue when treated with molybdate. Therefore,
Mo appears to be sequestered in the epidermal vacuoles of
Brassica spp. as a blue-colored Mo-anthocyanin
complex.
View larger version (96K):
[in this window]
[in a new window]
|
Figure 1.
Supraoptimal Mo causes a blue Mo-anthocyanin
precipitate to form in the epidermis of Indian Mustard as seen in this
petiole.
|
|
| |
Major Intrinsic Proteins (MIPs): Genomics and
Nomenclature |
MIPs, including aquaporins, are channel proteins that facilitate
the passive flow of water and other small polar molecules across the
membranes of a wide range of organisms. Gene sequencing has revealed
the MIPs to be an ancient, ubiquitous, and highly conserved family.
Because of the different research paths that have led to their
identification, the nomenclature for plant MIPs is unnecessarily
confusing. Johanson et al. (pp. 1358-1369) have used the
complete and sequenced set of Arabidopsis MIPs as a cladistic framework
for classifying the MIPs of all plants. According to the authors'
analysis, plant MIPs can be classified into four different subfamilies
based on sequence homologies. Two of the subfamilies
plasma membrane
intrinsic proteins and tonoplast intrinsic proteinsare named after
their primary location in the cells. A third subfamily consists of the
NOD26-like integral proteins, first discovered in the peribacteroid
membranes of N2-fixing soybean root nodules.
Recently, a fourth MIP subfamily, the small basic integral proteins,
has been identified. Thirty-five different MIPs have been identified in
the genome of Arabidopsis, including 13 plasma membrane intrinsic
proteins, 10 tonoplast intrinsic proteins, nine NOD26-like integral
proteins, and three small basic integral proteins. Current evidence
suggests that maize (Zea mays) has a similar number and
distribution of MIP subtypes.
| |
More Roles for H2O2 in Plant
Signaling |
The last decade has seen
H2O2 become established as
a bona fide secondary messenger in plant cells.
H2O2 generated during the
oxidative burst in pathogen-infected cells helps to orchestrate the
hypersensitive response and systemic plant immunity.
H2O2 also affects stomatal
aperture and induces transient increases in cytoplasmic
Ca2+. In this issue, Zhang et al. (pp.
1438-1448) report on pharmacological experiments pertaining to
the role of H2O2 in abscisic acid (ABA) regulation of stomatal aperture in fava bean (Vicia faba).
H2O2 was found to promote
stomatal closure, and ABA-induced stomatal closure was partially
abolished by the addition of exogenous catalase and diphenylene
iodonium (DPI), which are an
H2O2 scavenger and NADPH
oxidase inhibitor, respectively. The microinjection of ABA into guard
cells markedly induced H2O2
production as judged by a dichlorofluoroscein fluorescence assay, and
this effect was abolished by simultaneous injection of catalase or DPI.
This assay also revealed that ABA-induced
H2O2 production first
occurs in the region of the guard cell chloroplast (Fig. 2).
View larger version (161K):
[in this window]
[in a new window]
|
Figure 2.
Dichlorofluorescein assay reveals that ABA-induced
H2O2 production begins in
the chloroplasts of fava bean guard cells.
|
|
In another contribution in this issue, Desikan et al. (pp.
1579-1587) report on the differential effects of
H2O2 and harpin on two
mitogen-activated protein kinases in Arabidopsis leaves and suspension
cultures. Harpins are Gly-rich protein elicitors secreted by several
phytopathogenic bacteria, including Pseudomonas syringae pv
syringae. Harpin induces a number of defense responses in
Arabidopsis cell suspension cultures, including the generation of
H2O2, the activation of
defense gene expression, and programmed cell death. By means of
specific antibodies, the authors identify the harpin-activated
mitogen-activated protein kinases in Arabidopsis as AtMPK4 and
AtMPK6. H2O2 only activated
AtMPK6. Since treatments with catalase or DPI do not inhibit
harpin-induced activation of either AtMPK4 or AtMPK6, the activation of
AtMPK4 and AtMPK6 must occur independently of
H2O2 production. Harpin
apparently activates several signaling pathways, one leading to the
production of H2O2, and
others leading to the activation of AtMPK4 or AtMPK6 by alternative means.
| |
Increased Seed Lys in Arabidopsis |
Lys is an essential amino acid that is often present in limited
amounts in the seeds of crop plants. Both anabolism and catabolism regulate the level of Lys in seeds. In this issue, the physiological significance of Lys catabolism in regulating Lys titer in Arabidopsis seeds is reported upon by Zhu et al. (pp. 1539-1545), who constructed an Arabidopsis knockout mutant with a T-DNA inserted into
an exon of the gene encoding for Lys ketogluarate/saccharopine dehydrogenase (LKR/SDH). This enzyme, which catalyzes the first two
steps in Lys catabolism, is present as a single copy gene in
Arabidopsis. Although the gross phenotype of the LKR/SDH
knockout was indistinguishable from wild-type plants under normal
growth conditions, the mature seeds of the knockout mutant contained 6% more Lys than wild-type seeds. Such Lys catabolism knockout mutants
provide a new avenue of approach for raising Lys titer in
agriculturally important seed crops.
| |
Tyr Aminotransferase (TAT) and Plant Defense |
Octadecanoids, such as jasmonic acid and related compounds are
involved in plant defense reactions against microbial pathogens and
herbivores. Coronatine, a chlorosis-inducing phytotoxin produced by
P. syringae, is known to mimick the effects of jasmonic
acid, including the elicitation of proteinase inhibitors and
defense-related secondary metabolites. It is counterintuitive,
therefore, that coronatine has also been found to be an essential
factor in the early stages of infection of Arabidopsis by P. syringae. In this issue, Lopukhina et al. (pp.
1678-1687) identify one more piece of this puzzle. They report
that TAT is up-regulated in Arabidopsis after application of
octadecanoids, coronatine, or wounding. TAT catalyzes the conversion of
Tyr to 4-hydroxylphenylpyruvate, a precursor of homogentisate, itself a
precursor of both 
-tocopherols and plastoquinones. Either or both of
these end-products might act by scavenging the reactive oxygen species
produced during plant defense responses. Alternatively,
4-hydroxyphenylpyruvate might serve a crosslinking function in the cell
wall, thereby strengthening this physical barrier to pathogen invasion.
| |
Cation Transporter Families in Arabidopsis |
The uptake and translocation of cationic nutrients is essential to
many plant processes, including growth, nutrition, signal transduction
and development. Approximately 5% of the Arabidopsis genome appears to
encode for membrane transport proteins. These proteins are classified
in 46 unique families containing approximately 880 members. In
addition, several hundred putative transport proteins have not yet been
assigned to a family. In a paper based on the pooled data of six
research teams, Mäser et al. (pp. 1646-1667) report
upon the phylogenetic relationships of over 150 cation transport
proteins. Closely related isoforms and separate subfamilies exist
within many of these gene families, indicating specialized functions
and possible redundancies (a problem insofar as attaining knockout
mutants). One of the more startling findings of this monumental study
is that more than 40 genes encode for putative Na+/H+ antiporters in
Arabidopsis, a curious finding given that Na+ is
not even an essential nutrient in plants.
TOP
Mo Mustard Blues
Major Intrinsic Proteins...