Plant Physiology 135:1-2 (2004)
© 2004 American Society of Plant Biologists
ON THE INSIDE
Arabidopsis near Chernobyl
Ionizing radiation has dose-dependent effects on plant growth and development, ranging from stimulatory effects at very low doses and increasingly harmful effects for vegetative growth at intermediate levels to pronounced decreases in reproductive fitness and yields at high radiation levels. Previously, most studies concerning the effects of ionizing radiation on plants have examined a single generation of laboratory plants. Studies of the effects of chronic ionizing radiation over the course of multiple generations were rarely undertaken due to difficulties in creating a suitable model environment. Nearly 18 years have passed since the tragic nuclear accident at Chernobyl that resulted in the release of large amounts of radiation into the nearby environment. Interestingly, plants have continued to grow even in the most radioactively contaminated areas near Chernobyl. These plant populations provide a unique opportunity to study how plants adapt to chronic ionizing radiation over many generations. In this issue, Kovalchuk et al. (pp. 357–363) report that the progeny of the "Chernobyl" Arabidopsis plants are more resistant to high concentrations of chemical mutagens than are wild-type plants. In regard to the possible molecular mechanisms underlying their resistance, the authors report that these plants have a much lower frequency of extrachromosomally regulated homologous recombination, significant differences in the expression of radical scavenging and DNA repair genes upon exposure to mutagens, and a higher level of genomic methylation. These data suggest that adaptation to ionizing radiation is a complex process involving epigenetic regulation of gene expression and genome stabilization.
An Intermediate Type of Plastid Inheritance
Plastid genomes are maternally inherited in about 85% of all angiosperm species; the rest exhibit the potential for biparental inheritance. In maternal inheritance systems, paternal transmission of plastids is impeded during either the first pollen mitosis via unequal plastid distribution or by plastid degeneration. Therefore, the generative and sperm cells in mature pollen are generally free of plastids. The angiosperm Chlorophytum comosum is unusual in that different pollen grains show two different modes of plastid inheritance. About 50% of these pollen grains exhibit the potential for biparental plastid inheritance, whereas the rest exhibit maternal plastid inheritance. There is no morphological difference between these two types of pollen. Liu et al. (pp. 193–200) report that plastid localization is polarized in pollen grains and that plastids become excluded from the generative cell during the first pollen mitosis. However, this exclusion is incomplete in 50% of the pollen grains, and the few plastids distributed to the generative cells do divide actively after mitosis and contain large amounts of DNA. The authors propose that C. comosum is a transitional type with a mode of plastid inheritance that is genetically intermediate between the maternal and biparental modes.
Auxin Transport and the Polarization of Brown Algae Embryos
Light gradients and a variety of other chemical, mechanical, and electrical signals are effective in establishing developmental polarity in brown algae zygotes. The initial asymmetric cell division of the zygote leads to the formation of apical and basal daughter cells that are precursors to thallus and rhizoid tissues, respectively. This initial algal division resembles the first asymmetric division in land plant embryos that leads to the formation of apical and basal cells. It is known that land plants orient their growth relative to light and gravity through complex mechanisms that require auxin redistribution. This raises the question of whether auxin is also involved in the development of polarity in brown algae embryos. In this issue, Sun et al. (pp. 266–278) report that indole-3-acetic acid (IAA), and auxin efflux inhibitors, such as naphthylphthalamic acid (NPA), reduced polarization in early embryos of the brown algae Fucus distichus in response to gravity and light vectors. The effects of IAA and NPA on gravi- and photopolarization occurred maximally within 2.5 to 4.5 hours after fertilization, a period that corresponded to rearrangements in the cytoskeleton, most notably the development of actin patches on the shaded side. Treatment with NPA reduced the polar localization of actin patches but not patch formation itself. The microfilament inhibitor Latrunculin B prevented polarization and also altered auxin transport. Together, these results indicate a role for auxin in the establishment of developmental polarity and suggest interactions between actin and auxin transport in F. distichus embryos.
Tomato (Lycopersicon esculentum) Responses to Spider Mite-Infestation
When herbivores feed on a plant, the mechanical damage that results produces an aspecific wound response. However, herbivores, through species-specific peculiarities of their salivas and regurgitants, can also produce cues that enable plants to respond defensively in a much more targeted fashion. Previously, it has been established that the two-spotted spider mite (Tetranychus urticae) induces a rapid jasmonate-regulated direct defense-response in tomato. In nature, however, there can also be an indirect defense response, the recruitment of the predatory mite Phytoseiulus persimilis, an enemy of T. urticae that is commonly used to control spider mites on tomato. It has been established that such predatory species discriminate between prey-infested and uninfested plants on the basis of odors. In this issue, Kant et al. (pp. 483–495) analyze, through a combined metabolomics and transcriptomics approach, the events that occur during the first five days of infestation of tomato plants with two-spotted spider mites. This approach has enabled the authors to simultaneously assess the temporal progress of leaf damage, spider mite performance, transcriptome changes, changes in volatile emission, and the attraction of predatory mites. The researchers report that although the spider mites had caused little visible damage to the leaves after one day, induced direct defense responses were already evident. For example, proteinase inhibitor activity had doubled and the transcription of genes involved in jasmonate-, salicylate-, and ethylene-regulated defenses was enhanced. Although transcriptional up-regulation of the enzymes involved in the biosynthesis of monoterpenes and diterpenes was evident on day one, a significant increase in the emission of volatile terpenoids was delayed until day four. This increase in volatile production coincided with the increased olfactory preference of predatory mites for infested plants. These results indicate that tomato activates its indirect defenses (volatile production) to complement the direct defense response against spider mites.
Overexpression of GA Oxidases
The final steps in GA biosynthesis are regulated by cytosolic 2-oxoglutarate-dependent dioxygenases, namely GA 20-oxidase (GA20ox), GA 3-oxidase (GA3ox), and GA 2-oxidase (GA2ox). GA20ox and GA3ox are involved in the production of bioactive GA species, whereas GA3ox plays a role in the deactivation of bioactive GA species. In this issue, two groups of researchers have independently examined the effects of overexpressing various types of GA oxidases on plant function. In the first paper, Biemelt et al. (pp. 254–265) investigated the impact of overexpressing either GA20-oxidase (AtGA20-ox) or GA2-oxidase (AtGA2-ox) gene from Arabidopsis on the growth and metabolism of tobacco (Nicotiana tabacum) plants. The biomass of the transgenic tobacco plants was increased or decreased in AtGA20-ox or AtGA2-ox plants, respectively. These changes in biomass were positively correlated with the rate of photosynthesis at the whole plant level. The authors suggest that differences in dry matter accumulation were most likely due to changes in lignin deposition. Altered lignification of transgenic plants was paralleled by up- or down-regulation of the expression of lignin biosynthetic genes. Further analysis revealed a differential effect of GA on the formation of xylem and pith cells. The number of lignified vessels was increased in AtGA20-ox plants, suggesting a stimulation of xylem formation, whereas the number of pith cells declined, indicating a negative regulation.
Israelsson et al. (pp. 221–230) describe the cloning and characterization of a functional GA3ox isolated from aspens (Populus tremula x Populus tremuloides). The ectopic expression of an Arabidopsis GA3ox in hybrid aspen had relatively minor effects on GA1 and GA4 homeostasis, although tissue-dependent differences were observed. Their results suggest that GA 20-oxidation is much more important as a rate-limiting step than GA 3 -hydroxylation in GA-controlled shoot elongation. They also present evidence that GA4 is the predominant bioactive GA in aspens.
Is Auxin Involved in Reaction Wood Formation?
Angiosperm and gymnosperm trees differ in the nature of their reaction woods. In angiosperm trees, such as poplar (Populus tremula), reaction wood is called tension wood and forms on the upper side of stems that have been displaced from the vertical. Tension wood characteristically has few, small vessels and fibers with an inner gelatinous cell wall layer that consists of almost pure cellulose with microfibrils that are parallel to the long cell axis. In gymnosperms, such as pine (Pinus sylvestris), reaction wood is called compression wood and forms at the lower side of displaced stems. Compression wood is characterized by short, rounded tracheids that have thick walls with increased lignin content and increased microfibril angles. Numerous experiments involving applications of IAA or IAA-transport inhibitors have suggested that reaction wood is induced by a redistribution of IAA around the stem. The most popular model of reaction wood formation postulates that tension wood requires a difference in auxin concentration around the stem and forms in the region deficient in IAA, whereas compression wood is induced by an increase in auxin concentration. Hellgren et al. (pp. 212–220) have analyzed endogenous IAA distribution across the cambial region tissues in both Populus and Pinus trees forming reaction wood, using tangential cryosectioning combined with sensitive gas chromatography-mass spectrometry analysis. Surprisingly, their analysis of endogenous IAA concentrations demonstrates that reaction wood is formed without any obvious alterations in IAA balance. Moreover, they provide evidence that cambial growth on the tension wood side was stimulated without an increase in IAA. Taken together, these results suggest a role for signals other than IAA in the reaction wood response.
Peter V. Minorsky
Department of Natural Sciences, Mercy College, Dobbs Ferry, New York 10522
FOOTNOTES
www.plantphysiol.org/cgi/doi/10.1104/pp.900109
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