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Further Insights into a Gene for Big Tomatoes (Lycopersicon
esculentum) |
The quantitative trait locus
fw2.2 is responsible for approximately 30% of the
difference in fruit size between large-fruited, domestic varieties of
tomato and their small-fruited wild relatives. It had been demonstrated
previously that fw2.2 encodes for a RAS-like G protein
and is associated with altered cell division in ovaries. Although
increased fruit size is the most obvious phenotypic manifestation of
this quantitative trait locus, in this issue, Nesbitt and Tanksley (pp. 575-583) examine the possibility that the
gene may be pleiotropic, possibly effecting other changes in
plant morphology or overall fruit yield. Field observations on
near-isogenic lines (NILs) differing at the fw2.2
locus revealed that although a small-fruited NIL produced smaller
ovaries and fruits as expected, this was compensated for by a
larger number of inflorescences and fruits. Overall, however, there was
no net change in total fruit mass yield. In a flower removal
experiment that controlled for differences in inflorescence size
and number, the fruit size remained significantly different between the
NILs. These experiments reinforce the conclusion that the primary
effect of fw2.2 is in controlling ovary and fruit size,
and that other associated phenotypic effects, such as differences in
photosynthate partitioning between fruits, are secondary.
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Progress in the War on Limp Lettuce (Lactuca sativa) |
The wilting and yellowing that accompanies leaf senescence is
a major postharvest problem for leafy vegetables such as lettuce. In this issue, McCabe et al. (pp. 505-516) report
that a cytokinin-synthesizing gene (ipt) under the
control of the senescence-specific SAG12 promoter from Arabidopsis
(PSAG12-IPT) delays postharvest leaf senescence in mature
heads of transgenic lettuce homozygous for the transgene (Fig.
1). Apart from the retardation of
leaf senescence, the harvest-ready plants exhibit normal
morphologies and yields. Leaf viability was also maintained in plants
subjected to N starvation. Undesirable horticultural attributes,
however, became apparent during bolting and flowering. Transgenic
plants, possibly because of the maintained viability of their leaves, showed a slight delay in bolting (4-6 d), and a severe delay in flowering (4-8 weeks). A premature senescence of the upper leaves was
also evident and was correlated with elevated concentrations of
cytokinin and hexoses. The authors conclude that more research needs to
focus on the control of cytokinin-induced hexose accumulation before these research efforts become horticulturally viable.
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Figure 1.
Transgenic lettuce plants that have a cytokinin
synthesis gene under the control of a senescence promoter show delayed
postharvest senescence (bottom) compared with controls
(top).
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Water Movement in Early and Late Wood |
Although computed tomography (CT) has been used previously to
monitor the water distribution in a single tree over periods of days,
months, or years, the spatial resolution of these measurements was low.
In this issue, Fromm et al. (pp. 416-425) report on the use
of a new, high-resolution CT technique to measure the water contents in
spruce (Picea abies) and oak (Quercus robur) stems and branches. The spatial resolution (0.1225 mm3) of
this new technique is so acute that the water content differences within single annual rings can be studied. The authors report that tree
rings of the sapwood show steep water gradients from latewood to
earlywood, whereas those of the heartwood reflect water deficiency in
both species. Although it had previously been suggested that only the
youngest two annual rings of ring porous species are actually involved
in water transport, the high-resolution CT technique reveals similar
amounts of water in all the rings of oak sapwood. This indicates at the
very least that water storage is important in the entire sapwood (Fig.
2).
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Figure 2.
A transverse section of oak wood reveals the
tyloses (arrows) that typify the heartwood (bottom ring) are not
observed in the sapwood (top ring). Using CT, Fromm et al. demonstrate
that all of the rings in the sapwood have similar water
contents.
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Phosphate Acquisition and Acid Phosphatase |
P is among the most limiting factors for plant growth due to its
immobilization by inorganic and organic components in the soil.
Anywhere from 30% to 80% of soil P occurs in organic complexes. Under P-deficient conditions, plant roots typically increase the synthesis of acid phosphatases and their secretion into the soil. White
lupine (Lupinus albus) is an N2-fixing legume
that is highly efficient at acquiring soil P even though it lacks
mycorrhizal associations. One of the major adaptations that facilitate
its acquisition of P is the formation of proteoid roots (cluster roots) under low P conditions. Proteoid roots are specialized sites for the
production and secretion of both organic acids such as malate and
citrate, and acid phosphatase. In this issue, Miller et al. (pp.
594-606) characterize the secreted acid phosphatase and its gene
from white lupine. The secreted acid phosphatase is a glycoprotein with
broad substrate specificity. It appears to be encoded for by a
single gene containing seven exons. The putative 5'-upstream promoter
contains a 50-bp region having 72% identity to an Arabidopsis promoter
that is responsive to low P conditions. The authors propose that the
phosphatase promoter and targeting sequence may be useful tools for
genetically engineering important proteins from plant roots.
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Did Higher Plants Inherit Their Cellulose Synthase from
Cyanobacteria? |
Cyanobacterial extracellular polysaccharides are involved in
a wide range of functions including desiccation tolerance,
protection from UV light, adhesion to substrates, as well as motility.
Although cellulose biosynthesis among the bacteria has been suggested
previously, Nobles et al. (pp. 529-542) provide conclusive
evidence of the widespread occurrence of cellulose in the extracellular polysaccharides of these organisms. Based on the results of x-ray diffraction, electron microscopy, and cellobiohydrolase-gold labeling experiments, evidence for cellulose synthesis is reported in nine cyanobacterial species representing three of the five major groups of
cyanobacteria. The amino acid sequences of the cellulose synthase A
(CesA) enzymes from higher plants revealed greatest homology to
putative cellulose synthases from Anabaena sp. and
Nostoc punctiforme. Phylogenetic analyses indicate that the
cyanobacterial cellulose synthases share a common branch with the CesA
proteins of higher plants in a manner similar to the relationship
observed between cyanobacterial and chloroplast 16S rRNAs, indicating
that plants may have originally obtained CesA by lateral transfer from
cyanobacteria. Because no sequences with similarity to cellulose
synthase have been reported in the genomes of chloroplasts or cyanelles
to date, translocation of the gene to the nucleus must have occurred
relatively early in the evolution of the green algae.
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Genomics of Arabidopsis Ribosomal Protein Genes |
The eukaryotic ribosome is typically a complex structure composed
of four rRNAs and about 80 ribosomal proteins (r proteins). In contrast
to the information available on r proteins and their corresponding genes in prokaryotes and a few eukaryotic models (rats
and yeast), much less is known about the molecular biology of r
proteins in plants. In this issue, Barakat et al. (pp. 398-415) identify 249 genes (including some pseudogenes)
corresponding to 80 (32 small subunit; 48 large subunit) cytoplasmic
r-protein types in Arabidopsis. None of the r-protein genes are single
copy and most are encoded by three or four expressed genes, indicating substantial internal duplication of the Arabidopsis genome. An examination of the frequency of expressed sequence tags for the different r-protein gene family members and reverse transcriptase-PCR analysis of several r-protein gene families demonstrated differential patterns of gene expression with no clear relationship between expression levels and gene number. The identification of the r-protein genes and the determination of their primary structure and organization constitutes an important first step in determining their biological roles, the mechanisms controlling their expression, and the molecular structure of ribosomes in plants.
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