 |
The 14-3-3 Gene Family in Arabidopsis Gets Larger |
Originally discovered as soluble proteins in brain tissues, 14-3-3 proteins appear to be ubiquitous in eukaryotic cells. The 14-3-3 proteins typically bind to the phosphorylated motifs of other proteins
and regulate their activity. In plants, 14-3-3 proteins have diverse
functions, including the inhibition of nitrate reductase and Suc
phosphate synthase and the stimulation of plasma membrane
H+-ATPases. The first 14-3-3 protein in
Arabidopsis was identified as part of a protein/G box complex and was
consequently named "G-box factor 14-3-3" or more succinctly,
"GF-14." Nine more GF-14 isoforms were subsequently discovered and
designated by Greek letters (GF-14
, etc.). Rosenquist et al.
(pp. 142-149) speculate that these different isoforms
may serve different functions in planta. In this issue, they report on
their discovery of two novel 14-3-3 proteins in Arabidopsis, namely
GF14o (omicron) and GF14
(iota). The GF14o-encoding gene
(grf11) is expressed in leaves, roots, and flowers, whereas
the GF14 gene (grf12) is expressed only in flowers. The
authors speculate that the exclusive expression of grf12 in
flowers may be related to GF14 iota's potential regulation of a
flower-specific H+-ATPase (encoded by
AHA9), which earlier studies have shown not to
bind to any of the previously described 14-3-3 proteins.
| |
Prokaryote-Like Protein Processing Enzyme in
Plastids |
An elegant piece of corroborating evidence for the endosymbiotic
theory of plastid evolution is that the initiation of protein synthesis
in plastids is essentially prokaryotic in nature. In both prokaryotes
and chloroplasts, peptide deformylases catalyze the removal of the
formyl group from N-formyl-Met (an initiating residue
during prokaryotic protein translation) at the amino termini of nascent
polypeptides. Peptide deformylases are essential enzymes: mutations
that disrupt peptide deformylase function are lethal to bacteria. In
this issue, Dirk et al. (pp. 97-107) report on their
identification of two putative peptide deformylase genes (AtDEF1 and AtDEF2) in Arabidopsis. Actinonin, a
specific inhibitor of peptide deformylase, was effective against both
forms of Arabidopsis peptide deformylase. Treatment of Arabidopsis and
other plants, including pea (Pisum sativum), with actinonin
caused chlorosis and severe inhibition of growth and development (Fig.
1). Thus, in plastids, as in bacteria,
peptide deformylase is essential for survival. The authors propose that
the overexpression of peptide deformylase in crop plants may render
them resistant to a new class of broad-spectrum herbicides based on
inhibitors of peptide deformylase such as actinonin. Such herbicides
would be completely non-toxic to other eukaryotic life forms.
View larger version (81K):
[in this window]
[in a new window]
|
Figure 1.
Actinonin, an inhibitor of the plastid protein
synthesis enzyme peptide deformylase, is toxic to peas and other plants
(left, actinonin-treated; right, control).
|
|
| |
MADS-Box Genes in Embryo Sacs of Maize (Zea
mays) |
Since the isolation of the first homeotic MADS-box
transcription factors AGAMOUS and DEFICIENS
in the early 1990s, numerous MADS-box genes have been isolated from a
wide variety of flowering plants. MADS-box genes are best known as key
players in floral development and differentiation. In this issue,
Heuer et al. (pp. 33-45) report on the isolation
of two novel MADS- box cDNAs
ZmMADS1 and
ZmMADS3from the egg cells and mature pollen of
maize. Both genes are expressed in egg cells before and after fertilization. Unlike ZmMADS3, whose expression in the
embryo sac is limited to the egg, ZmMADS1 is detectable in
all the cells of the embryo sac. During early somatic embryogenesis,
ZmMADS1 expression is restricted to cells with the
capacity to form somatic embryos and, at later stages, globular
embryos. Neither is expressed in non-embryogenic suspension cultures or
in isolated zygotic embryos. During floral development, both genes are
co-expressed in ear spikelet organ primordia, suggesting
that the non-expression of ZmMADS may be critical for the
function of organ identity genes. ZmMADS3 is also expressed
in stem nodes in a graded fashion, with the strongest expression
occurring in the uppermost node. Ectopic expression of
ZmMADS3 causes reduced height, reduced seed set, and male
sterility due to the absence of anthers (Fig.
2).
View larger version (69K):
[in this window]
[in a new window]
|
Figure 2.
Ectopic expression of the ZmMADS3 gene
causes the production of spikelets lacking anthers and having a
leaf-like inner glume (arrows).
|
|
| |
Anthranilate Synthase (AS) Goes Home |
The biosynthesis of almost all the essential amino acids of
higher plants, including Trp, takes place in the plastids.
Nearly all of the enzymes involved in these biosyntheses are encoded for by nuclear genes, synthesized in the cytoplasm and
imported into the plastids. Early in plant evolution, however, most of these enzymes were encoded for by the genome of cyanobacterial endosymbionts (the progenitors of modern chloroplasts). During the
course of plant evolution, however, most of the relevant genes left the
cyanobacterial genome and become integrated into the nuclear genome. In
this issue, Zhang et al. (pp. 131-141) report on their
success in sending the gene for AS, a major
feedback-sensitive control enzyme in Trp biosynthesis, back to the
plastid genome from where it came. More specifically, a cDNA encoding
for a feedback-insensitive subunit of AS (ASA2) was
successfully spliced into the plastid genome of tobacco
(Nicotiana tabacum). The transplastomic plants, which
appeared completely normal, exhibited higher AS activity, a reduction
in Trp-feedback inhibition, and a decreased sensitivity to the AS
inhibitor 5-methyl-Trp. Most importantly, there was a 10-fold increase
in Trp levels in the leaves. As expected, the transplastomic trait was
passed maternally to the next generation. This study is the first
demonstration of the introduction of a native nuclear gene of presumed
pre-endosymbiotic origin into plastids to modify an endogenous
biosynthetic pathway.
| |
Stress-Induced Patterns of Retrotransposon
Expression |
All plant retrotransposons characterized to date, including the
tobacco retrotransposon TnT1A, are expressed only under very precise
stress conditions. In this issue, Beguiristain et al. (pp.
212-221) report that all three of the TnT1 subfamilies differ
in their U3 sequences (wherein the promoters of the retrotransposon reside) and that all three are induced by stress in tobacco. The promoters of these subfamilies, however, respond differently to different stresses. For example, the induction of the TnT1A subfamily by elicitors (cryptogein) or methyl-jasmonate is especially strong, whereas the induction of the TnT1C subfamily is more sensitive to
salicylic acid and auxins. The direct relationship between U3-sequence
variability and differences in the stress-associated expression of the
Tnt1 elements present in a single host species supports the idea that
retrotransposons have adapted to their host genomes by the evolution of
highly regulated promoters that mimic those of
stress-induced plant genes. Such variability in expression would be
expected to have important consequences for retrotransposon evolution
since repeated exposure of the host to a particular stress situation
should favor amplification of a particular subfamily.
| |
BZIP Transcription Factors and Senescence |
The basic-region Leu-zipper (bZIP) proteins constitute
one of the major families of transcription factors. Arabidopsis, for example, possesses almost 100 bZIP-encoding genes. Within the bZIP-gene
family, the lip19 subfamily has been shown to be expressed in flowers and vascular tissue and also to be up-regulated by low
temperature. Recent studies have revealed that members of this family are also expressed during the aging of maize leaves. In
this issue, Yang et al. (pp. 23-32) extend these findings to tobacco. They report that the products of two tobacco genes, tbzF
and tbz17, show 73% identity and are located in the nucleus. The bZIP
proteins are shown to preferentially bind to DNA fragments spanning
A-box/G-box and C-box/G-box hybrid motifs and to function as
transcriptional activators. Transcripts of tbzF are present at high
levels in senescing tobacco leaves and flowers, whereas tbz17
accumulates only in aged leaves. In situ hybridization analysis revealed that transcripts of tbzF and tbz17 are localized predominantly in the guard cells and vascular tissue of senescing leaves. Stomata of
senescing leaves remain functional well after the mesophyll cells of
the leaf have turned yellow. It is possible that these bZIP proteins
may activate unidentified genes that function to retain cellular
activity in senescing guard cells and vascular tissue.
TOP
The 14-3-3 Gene Family...
Prokaryote-Like Protein...
