Plant Physiology 133:1-2 (2003)
© 2003 American Society of Plant Biologists
ON THE INSIDE
Mycorrhizal Uptake of Phosphate
Arbuscular mycorrhizal (AM) fungi are vital components of almost all terrestrial ecosystems, forming mutualistic symbioses with the roots of around 80% of all vascular plants. The main physiological basis for this mutualism is bi-directional nutrient transfer. Plants supply the AM fungi with sugars, and the fungi enhance the ability of the plants to scavenge for scarce and immobile nutrients, particularly P. In those plants that form mycorrhizal associations, there are two potential pathways of P uptake: P can bypass the mycorrhizae and be absorbed directly at the soil-root interface through root epidermis and root hairs, or P can enter through the 'mycorrhizal' pathway via external AM hyphae. In those cases where there is no response to symbiosis in terms of P uptake or plant growth, it has commonly been assumed that the mycorrhizal uptake pathway is not functioning and that all the plant P is being absorbed directly through P transporters in the root epidermis and root hairs. Contrary to these assumptions, Smith et al. (pp. 16-20) present evidence that AM fungi can provide the dominant route for plant P supply, even in those cases where the overall growth or P uptake remain unaffected by the development of mycorrhizal associations. Their results clearly indicate a loss of function of the direct uptake pathway in roots colonized by AM fungi; in some cases, this loss of function is complete.
Measuring Mitochondrial Ca2+ Dynamics
Plants that have been transgenically engineered to express the Ca2+-sensitive photoprotein aequorin (AEQ) have provided unprecedented insights into the Ca2+ dynamics of plant cells. By targeting AEQ to specific organelles such as the nucleus, the ER, the vacuole and the chloroplast, it has been possible to attain data concerning the dynamic changes in the compartmentalization of Ca2+ ions within plant cells. In this issue, Logan and Knight (pp. 21-24) describe the first successful targeting of AEQ to plant mitochondria and provide the first direct measurements of the elevations in [Ca2+]mit that arise from various types of stimulation. The authors engineered a chimeric construct comprised of cDNAs for AEQ, green fluorescent protein (GFP) and the  -subunit of the mitochondrial F1 ATPase. An analysis of the positive transformants by epifluorescence microscopy demonstrated that this construct was stably inherited and correctly targeted to the mitochondria of Arabidopsis. The average resting [Ca2+]mit measured in vivo was 208 nm, which is almost twofold higher than the resting concentration of Ca2+ in the cytoplasm. Challenging Arabidopsis seedlings by rapid cooling or osmotic shock stimulated rapid increases in [Ca2+]mit peaking at 526 nm and 504 nm, respectively. In both cases, the peak responses of [Ca2+]mit were approximately half the [Ca2+]cyt peak, although the temporal kinetics (signature) from the two compartments were otherwise very similar. In contrast, experiments involving mechanical (touch) and oxidative (hydrogen peroxide) stimulation indicate that mitochondria are not simply passively "sampling" the local [Ca2+]cyt conditions but are capable of generating unique (relative to [Ca2+]cyt) calcium signatures.
GoldenRice (Oryza sativa): A Progress Report
One of the great successes of plant biotechnology to date has been the creation of "GoldenRice," a genetically modified rice line engineered to synthesize and accumulate provitamin A ( -carotene) in the endosperm. This technology has the potential to improve the health of an estimated 250 million people who suffer from Vitamin A deficiency. To achieve practical success, the GoldenRice technology has to be introduced into many local rice varieties. Although the first GoldenRice lines were Japonica, most of the world's mal-nourished population lives mainly on Indica rice. Here, Hoa et al. (pp. 161-169) report that they have successfully transferred the trait to several Indica varieties. They also report that substantial progress has been made in resolving several regulatory issues that have slowed the widespread implementation of GoldenRice technology. Since their initial publication, the creators of GoldenRice have minimized extraneous DNA in the vector backbone, investigated the absence of beyond-border-transfer, and have used Agrobacterium-mediated transformation to obtain defined integration patterns. To avoid an antibiotic selection system, they now rely exclusively on phospho-Man isomerase as the selectable marker. In their initial studies, the most golden (carotenoid-rich) line they produced suffered from having undergone multiple integrations and recombination events, making it less than ideal as the basis for obtaining deregulation. Single integrations are now given a preference to minimize potential epigenetic effects in subsequent generations. The novel GoldenRice lines described here, now in the T3 generation, are expected to more readily receive approval for follow-up studies, such as nutritional and risk assessments.
Respiratory Supercomplexes in Plants
Five protein complexes provide the structural basis for oxidative phosphorylation in mitochondria. These complexes include NADH dehydrogenase (complex I), succinate dehydrogenase (complex II), cytochrome c reductase (complex III, a functional dimer), cytochrome c oxidase (complex IV), and ATP synthase (complex V). According to the popular "liquid state" model, these protein complexes are randomly arranged in the membrane and freely diffuse within the lateral plane of the inner mitochondrial membrane. Other results, however, indicate that these protein complexes form ordered associations of larger structure. These so-called "supercomplexes" have been described for bacteria and the mitochondria of yeast and mammals. In this issue, Eubel et al. (pp. 274-286) present initial results from their systematic investigation of respiratory supercomplexes in plant mitochondria. They have identified three high molecular mass complexes of 1,100, 1,500, and 3,000 kD. The 1,100 kD complex represents dimeric ATP synthase and only is stable under very low concentrations of detergents. In contrast, the 1,500 complex is stable at medium and even high concentrations of detergents and includes the complexes I and III2. Depending on the plant species, 50% to 90% of complex I forms part of this supercomplex if solubilized with digitonin. The 3,000 kD complex, which also includes the complexes I and III, is of low abundance and most likely has a III4I2 structure. The complexes IV, II, and the alternative oxidase were not part of supercomplexes under all conditions applied. The authors speculate that supercomplex formations between complexes I and III limit access of alternative oxidase to its substrate ubiquinol and possibly regulate alternative respiration.
Plant Proteases Impede Foreign Protein Accumulation
Equistatin is a protease inhibitor from the sea anemone Actinia equina that has previously been found to be highly active against the gut proteases of Colorado potato beetle (Leptinotarsa decemlineata). Artificial diets that included the protein caused complete growth inhibition and mortality among the beetle larvae. In an attempt to obtain potato (Solanum tuberosum) plants resistant to Colorado potato beetle larvae, Outchkourov et al. (pp. 379-390) tried expressing equistatin under the control of various promoters, and targeting it to the secretory pathway with and without the endoplasmic reticulum retention signal. All constructs yielded similar stepwise protein degradation patterns, which considerably reduced the amount of active inhibitor in planta and resulted in insufficient levels for resistance against Colorado potato beetle larvae. This problem of the proteolytic degradation of heterologously expressed proteins is widespread and not just limited to equistatin. Much more needs to be learned about the specific plant proteases involved in this process. The proteases involved in the equistatin degradation were characterized with synthetic substrates and inhibitors. The results indicate that Arg/Lys-specific and legumain-type Asn-specific Cys proteases seriously impede the functional accumulation of recombinant equistatin in planta. Kininogen domain 3, an inhibitor of Cys proteases, completely inhibited equistatin degradation in vitro. The authors propose that the co-expression of Cys protease inhibitors such as kininogen domain III may potentially be an effective strategy for increasing the stability of foreign proteins in plants.
A General Developmental Response to Pathogen Infection
Korves and Bergelson (pp. 339-347) show that susceptible Arabidopsis plants accelerate their reproductive development and alter their shoot architecture in response to infection by three different pathogen species, including the bacteria Pseudomonas syringae and Xanthomonas campestris, and the oomycete Peronospora parasitica. Infection with each of the three pathogens reduced time to flowering and the number of aerial branches on the primary inflorescence. The authors propose that these changes in flowering time and branch architecture constitute a general developmental response to pathogen infection that may affect tolerance of and/or resistance to disease. Conceivably, the developmental changes with infection might enhance a plant's ability to cope with pathogen infection in several ways. First, faster development may lead to the early development of age-related resistance. Second, some of the developmental changes with infection may increase early seed production, which could enhance the fitness of plants that may be prematurely killed by the pathogen. Finally, the increases in the number of basal branches may compensate for the production of fewer aerial branches. Further work is necessary to determine whether this general developmental response to pathogen infection enhances the plants' abilities to escape from, resist, or compensate for disease, and what molecular pathways are involved in these developmental responses.
A Mutant with Enhanced Polar Auxin Transport
The phenotype of the polycotyledon mutant of tomato (Lycopersicon esculentum) is pleiotropic and includes, to name a few, the formation of extra cotyledons, changes in leaf shape, and faster and more dramatic gravitropism. Al-Hammadi et al. (pp. 113-125) report that this mutant exhibits a 2.5 to 3-fold enhancement of its polar auxin transport rate. This increase in polar auxin transport did not appear to be caused by a decrease in flavonoids, as the mutant had normal flavonoid levels. Application of 2,3,5-triiodobenzoic acid, an inhibitor of auxin polar transport, rescued many of the phenotypic abnormalities of the young seedlings. Thus, there appears to be an important but unidentified factor that negatively regulates polar auxin transport in tomato and whose dysfunction affects plant development and organogenesis.
Peter V. Minorsky
Department of Natural Sciences Mercy College Dobbs Ferry, NY 10522
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