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Plant Physiology 133:1654-1655 (2003) © 2003 American Society of Plant Biologists Zeanthin is the Blue Light Photoreceptor in StomataGenetic, biochemical, and physiological studies have identified the chloroplastic carotenoid, zeaxanthin, as the blue light photoreceptor in guard cells. The observation showing that stomata from npq1, an Arabidopsis mutant that fails to accumulate zeaxanthin, lacks a specific blue light response supported the zeaxanthin hypothesis. That conclusion, however, was challenged by two subsequent studies showing that npq1stomata opened when irradiated with blue light applied under a subsaturating red light background. Moreover, one of these studies reported that stomata from a phototropin-deficient double mutant (phot1 phot2) of Arabidopsis lack a blue light response, supporting the hypothesis that phototropin is the main photoreceptor mediating blue light responses in guard cells. Talbott et al. (pp. 1522-1529) argue that it is not possible to clearly interpret these results because of the interactions between the specific blue light response of guard cells and guard cell photosynthesis, which is also stimulated by blue light. Talbott et al., therefore, initiated a detailed characterization of the light responses of npq1 stomata under blue and red light irradiation, which would allow an unambiguous interpretation of the photobiological properties of this mutant. The authors report that blue light-stimulated opening in npq1 was reversed by far-red but not green light, indicating that npq1 has a phytochrome-mediated response and lacks a blue light-specific response. The absence of a blue light-specific response and the far-red reversal of the opening observed under low light fluences indicate that the zeaxanthin-less npq1 mutant is a true null function mutant for the specific blue light response of stomata. They also report that the stomata of the phot1 phot2 double mutant open in response to blue light. This opening was green light reversible and far-red light insensitive, indicating that the stomata of the phot1 phot2 double mutant have, contrary to previous reports, a detectable blue light-specific response. Phytochrome B and Potato (Solanum tuberosum) Yield in the Field As sunlight penetrates the canopy of field crops, the levels of photosynthetically active light and the ratio of red light (R) to far-red light (FR) decrease. The perception of this altered R:FR ratio by phytochromes initiates a number of responses such as increased stem extension growth, reduced branching and accelerated leaf senescence. Although the responses to low R:FR ratios are adaptive in nature, they may decrease yields under field conditions. In this issue, Boccalandro et al. (pp. 1539-1546) report that the ectopic expression of the Arabidopsis phytochrome B gene (PHYB), a photoreceptor involved in detecting red to far-red light ratio associated with plant density, can increase tuber yield in field-grown transgenic potato crops (Fig. 1). The PHYB transgenics showed higher maximum photosynthesis in leaves throughout the canopy, and this effect was largely due to increased leaf stomatal conductance. Surprisingly, the yield-enhancing effects of increased PHYB expression were larger at high densities, possibly because the increased PHYB expression not only altered the ability of plants to respond to light signals but also, due to the reflection of FR light, modified the light environment itself. This combination resulted in larger effects of enhanced PHYB expression on tuber number and crop photosynthesis at higher planting densities. The authors propose that enhanced PHYB expression could be used in breeding programs seeking to shift optimum planting densities to higher levels.
Phytochrome Modulation of Chloroplast Movement Chloroplast movements are light-directed responses that occur in a number of diverse plant groups including algae, moss, ferns, and angiosperms. Exposure to dim-light causes chloroplasts to accumulate along cell walls oriented perpendicular to the incident light, whereas strong illumination causes chloroplasts to migrate to the anticlinal walls parallel to the incident light. Light-induced chloroplast movements provide an adaptive function allowing for more efficient photon absorption when light is scarce and protection against photodamage via self-shading when light intensities are too high. Blue light (BL)-induced chloroplast movement in angiosperms is mediated by the phototropins. Phot induces the high light response, whereas phot1 and phot2 act redundantly to mediate the low light response. In lower plants, however, it is quite common for red and blue light to act synergistically in effecting these movements, Therefore, DeBlasio et al. (pp. 1471-1479) sought to determine whether phytochrome could be involved in these phototropin-mediated pathways in Arabidopsis. They analyzed BL-induced chloroplast movements in various phytochrome-deficient mutants. Their results indicate that phyA, B, and D are not required for induction of chloroplast movements in Arabidopsis, but deficiencies in phyA or phyB result in an enhancement of the high light response. The effects were most pronounced when leaves were exposed to an intermediate fluence rate of BL. These results suggest that phytochromes may play a role in the fine-tuning of light transmittance properties of leaves for maintenance of maximal photosynthetic productivity. Internal Oxygen Tensions and Oil Synthesis in Seeds The percentage of reduced carbon stored as oil in seeds varies between species from 1% to 60% of total dry weight. Although our understanding of the pathways involved in Suc to oil conversion in oil-storing seeds has improved in recent years, little is known about how the flux of carbon through these pathways is controlled, or how the partitioning of carbon between oil and other storage products is determined. Vigeolas et al. (pp. 2048-2060) investigated whether endogenous restrictions in oxygen supply are limiting for oil metabolism in developing rape (Brassica napus) seeds. They measured the effects of altered ambient levels of oxygen on energy status as well as oil and starch production. Their results show that lipid biosynthesis is clearly restricted by the prevailing oxygen concentrations within seeds, and can be increased by increasing oxygen supply. In contrast, starch synthesis was not significantly increased by increased oxygen levels. These findings suggest possible strategies to increase yield in oil-seed crops, including effecting changes in the expression of oxygen-binding proteins, the gas permeability of the seed, and the photosynthetic capacity of the fruits. Peroxisomes are Essential
Peroxisomes serve many functions in plant cells. In addition to their well-established roles in the mobilization of seed lipid via the peroxisomal pathways of Increasing the Salt Tolerance of Arabidopsis Enhancing the efficiency of Na+ extrusion from plant cells is one approach to increasing a plant's salt tolerance. The yeast (Schizosaccharomyces pombe) gene SOD2 encodes for a plasma membrane Na+/H+ anti-porter. Gao et al. (pp. 1873-1881) report that the ectopic expression of SOD2 in Arabidopsis has no obvious morphological or developmental effects, but does markedly enhances the tolerance of Arabidopsis to high salinity. The overexpression of SOD2 in Arabidopsis improved seed germination and seedling salt tolerance. The photosynthetic rate and the fresh weight of the transgenic lines were also higher than that of wild-type plants under high NaCl conditions. Analyses of Na+ and K+ contents of the symplast and apoplast in the root cortex and spongy mesophyll revealed that the transgenic lines accumulated less Na+ and more K+ in the symplast than did wild-type plants. Diphtheria Toxin-Mediated Cell Ablation Fertilization of the female gametophyte in angiosperm plants initiates a process of coordinated development of embryo, endosperm, and seed coat that ensures the production of a viable seed. Mutant analysis has suggested that communication between the endosperm and the seed coat is an important determinant in this process. Cell groups within the embryo, derived from the apical and from the basal cell, respectively, act in concert to establish a functional root meristem, whereas cells in the apical region of the embryo are hypothesized to repress cell divisions in the basal cell-derived suspensor. The available evidence for these interregional communication events mostly relies on the analysis of mutant phenotypes. To provide independent and direct evidence for communication events, Weijers et al. (pp. 1882-1892) used conditional domain-specific expression of the highly toxic diphtheria toxin A chain (DTA) in developing Arabidopsis seeds. By using a collection of cell- or tissue-type-specific promoters to express DTA in different tissues, they show that the expression of DTA in the protoderm of the embryo proper led to excessive proliferation of suspensor cells, sometimes resulting in the formation of secondary embryos. In contrast, the endosperm-specific expression of DTA caused complete cessation of seed growth, followed by pattern defects in the embryo and embryo arrest. These results demonstrate the importance of interregional communication in embryo and seed development and demonstrate the usefulness of cell ablation by diphtheria toxin expression as a complementary method to the phenotypic analysis of developmental mutants.
Department of Natural Sciences Mercy College Dobbs Ferry, New York 10522 FOOTNOTES www.plantphysiol.org/cgi/doi/10.1104/pp.900095. Related articles in Plant Physiol.:
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-oxidation and the glyoxylate cycle, and the salvage of carbon via the photorespiratory cycle, new functions are still being discovered. In recent years it has become apparent that peroxisomes also play important roles in reactive oxygen metabolism, including the formation and turnover of the signaling molecules nitric oxide and H2O2, as well as in the biosynthesis of indoleacetic acid and jasmonic acid. In mammals, defects in peroxisome biogenesis result in multiple system abnormalities, severe developmental delay, and death, whereas in unicellular yeasts, peroxisomes are dispensable unless required for growth on specific substrates. PEX10 encodes an integral membrane protein required for peroxisome biogenesis in mammals and yeast. Mutants in Pex10p have a defect in transporting matrix proteins containing peroxisomal-targeting signals. To investigate the importance of PEX10 in plants, Sparkes et al. (pp.