Peter V. Minorsky

doi:10.1104/pp.124.3.921

Published November 2000. DOI: https://doi.org/10.1104/pp.124.3.921

Liverwort Isoprenoid Production

Among the Bryophytes, only the liverworts possess oil bodies containing large quantities of essential oils. Liverwort oil bodies are cytoplasmic structures bound by a single membrane that originates from the dilation of ER cisternae. They have no structural equivalent in the seed plants, the oil bodies of which accumulate mostly acyl lipids and are surrounded by a monolayer of phospholipids containing basic proteins. Biochemical studies have indicated that the isoprenoid biosynthetic pathways in liverworts are similar to those of the seed plants, but the isolation and purification of intact liverwort oil bodies is difficult and precludes direct analysis of their enzyme component. Suire et al. (pp. 971–978) have been able to circumvent this problem using a cyto-immunological approach employing antibodies for several key enzymes involved in isoprenoid biosynthesis. The immunolocalization of these enzymes in Marchantia polymorpha indicate that the enzymes equivalent to the plastid and cytosolic enzymes involved in isoprenoid biosynthesis in seed plants are found in the oil bodies of liverworts (Fig.1). The liverwort oil body, therefore, represents a unique structure in which isoprenoid production and storage are combined.

Fig. 1.

Oil bodies in liverworts are sites of isoprenoid biosynthesis.

Transgenic Proteins from the Tap

Transgenic protein isolation may someday be as easy as placing a beaker in a growth chamber, going home for a nice dinner, and returning the next morning to find the beaker filled with a simple solution rich in the protein of your choice. At least the possibility is raised by the results of Komarnytsky et al. (pp. 927–933), who report upon their initial success in getting transgenic tobacco plants to excrete three heterologous proteins by means of leaf guttation. The heterologous proteins (ER signal peptides fused to bacterial xylanase, jellyfish green fluorescent protein, or human placental alkaline phosphatase) accounted for almost 3% of the total soluble protein in the guttation fluid. This technology has the potential of enhancing the efficiency of recombinant protein production technology by increasing yield, abolishing extraction, and simplifying purification. Tobacco may not even be the best species for this purpose: there exist even weepier species in which collection might be more efficient.

Genomics of the Glutathione S-Transferase (GST) Gene Family

The GSTs are a huge multigene family present at every stage of plant development. Although they presumably function in nature to protect the cell from oxidative damage, they have been most intensively studied in relation to their ability to sequester xenobiotics. Several major classes of herbicides are rendered innocuous after conjugation with glutathione. In this issue, McGonigle et al. (pp. 1105–1120) report on their use of BLAST searches to identify 25 soybean and 42 maize GST sequences—probably the majority of GSTs expressed in these species. Plant GSTs are typically divided into three types. Types I and III are similar to the mammalian theta class and type II is similar to the mammalian zeta class. Type II GSTs turn out to be poorly represented both in variety of individual genes (two in maize and one in soybean) and in absolute expression levels. Sequence comparisons of 66 GST sequences from both maize and soybean revealed some notable features. There are three amino acids that are absolutely conserved in each case, most notably a Ser in the active site of the enzyme. Type III GSTs also have four conserved sequence patches mapping to distinct structural features (Fig. 2). Using DNA microarray analysis, the authors observed increased expression among the type III GSTs in maize in response to seven different inducer treatments. Different genes responded differently to different treatments. A GST active with one substrate generally exhibited some activity with all the others, suggesting broad individual enzyme substrate specificity.

Fig. 2.

Conserved sequence patches (colored ribbons) and active site (green) of maize GST.

Polyamine Regulation of Guard Cell K⁺Channels

Polyamines have been implicated in the responses of plants to a wide variety of abiotic stresses, including K⁺ deficiency, drought, salt stress, and air pollution. Despite numerous reports of a correlation between stress and polyamine titer, the physiological rationale for stress-induced polyamine accumulation remains unknown. In other organisms, there is accumulating evidence that polyamines may play a role in ion channel regulation. In animal cells, certain types of inward-rectifying K⁺ channels,N-methyl-d-aspartate receptors, and voltage-activated Ca²⁺ channels have been shown to be affected by intracellular polyamine levels. Recent studies have also implicated periplasmic polyamine levels in the regulation of porin activity in the outer membranes of Escherichia coli. In this issue, Liu et al. (pp. 1315–1325) extend these findings to include the voltage-dependent inward K⁺ channels in the plasma membrane of Vicia faba guard cells. Patch clamp recordings reveal that polyamines inhibit the guard cell K⁺ channel from the cytoplasmic side. Consistent with this finding, the authors report that polyamines prevent light-induced stomatal opening. The authors also found that polyamines block the inward K⁺ current induced in tobacco mesophyll cells by the expression of the Arabidopsis KAT1 gene (a putative guard cell K⁺ channel). This model system should prove most useful in elucidating the mechanism of polyamine inhibition.

Xylem Embolisms: An Un-Canny Interpretation of Cryo-Scanning Electron Microscopy (Cryo-SEM)

Not too along when plants were still essentially a “black box,” plant biologists could at least take some pride in the fact that they understood the ascent of sap. The “Cohesion-Tension” theory, first proposed in 1894, seemed almost unassailable. A century later, however, Martin Canny put forth an intrepid new hypothesis that threatened to overturn long-held notions concerning plant water relations. Much of Canny's arguments were based upon results garnered by two recently developed techniques: the direct measurement of sap pressure with xylem pressure probes and direct cryo-SEM observations of xylem vessels. Canny's hypothesis did what any good hypothesis does: it sent scientists back to the lab. Unfortunately, the new studies that have emerged have not been favorable to Canny's hypothesis. Previously, in this journal, Wei et al. (1999) provided evidence that called into question the anomalous xylem pressure probe studies. Now, in this issue, Cochard et al. (pp. 1191–1202) turn their attention to the anomalous cryo-SEM investigations. They confirm the earlier cryo-SEM findings that seemingly indicate, contrary to the predictions of the “Cohesion-Tension” theory, that a large percentage of vessels become transiently embolized during the diurnal peak in transpiration. Cochard et al., however, find no evidence for a corresponding diurnal trough in the hydraulic conductivity of the petioles that would be expected if these embolisms were real. One of the techniques has to be wrong. The authors present their case that artifactual cavitation may occur in vessels when their sap is frozen under high negative pressure such as occurs during cryo-SEM preparation.

Both Ethylene and ABA Involved in High Auxin Growth Inhibition

It has long been known that high concentrations of auxin, both natural and synthetic, give rise to enhanced ethylene production. Epinastic growth abnormalities and the inhibition of root and shoot growth are some of the symptoms induced by high auxin. Previously, Klaus Grossman and his colleagues have called attention to the fact that this auxin-induced increase in ethylene production is soon followed by a massive accumulation of ABA in the shoot, and that ethylene inhibitors prevent this increase in ABA. Since ethylene and ABA are both growth inhibitors, the question arises as to which of the hormones underlie the high auxin-induced inhibition of growth? In this issue, Hansen and Grossman (pp. 1437–1448) shed light on this question by comparing the effects of high auxin on the inhibition of shoot growth in two tomato mutants: the ethylene-perception mutant never-ripe and the ABA-synthesis mutant flacca. Both mutants showed less auxin-induced growth inhibition than did the wild types, suggesting a role for both hormones in auxin-induced growth inhibition. As expected, the two mutants also did not show the usual large increase in ABA or its precursor xanthoxal in response to high auxin treatment.

LITERATURE CITED

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1. Wei C,
2. Tyree MT,
3. Steudle E
(1999) Direct measurement of xylem pressure in leaves of intact maize plants: a test of the cohesion-tension theory taking hydraulic architecture into consideration. Plant Physiol 121:1191–1206.
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