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Plant Physiology 143:1439-1440 (2007) © 2007 American Society of Plant Biologists On the InsideA Mitochondrial Complex Involved in Gametophytic DevelopmentMitochondrial complex II (succinate dehydrogenase [SDH]) is part of both the tricarboxylic acid cycle and the respiratory electron transport chain. In Arabidopsis (Arabidopsis thaliana), the flavoprotein subunit of mitochondrial complex II is encoded by two nuclear genes, SDH1-1 and SDH1-2. The SDH1-2 gene is expressed weakly and only in roots, and its disruption has no effect on growth and development. In contrast, SDH1-1 transcripts are ubiquitously expressed, with highest expression in flowers. To gain insight into the physiological role of complex II, León et al. (pp. 1534–1546) have undertaken a reverse genetic analysis of the SDH genes. Heterozygous SDH1-1/sdh1-1 mutant plants showed normal vegetative growth but reduced seed set. This reduction in seed set stems from alterations in gametophytic development. Molecular and genetic analyses suggest that sdh1-1 is a gametophytic mutation affecting both male and female gametophyte development. SDH1-1 is essential for pollen development, and, without its proper activity, the asymmetric mitosis that normally results in bicellular pollen grains is blocked. SDH1-1 is also important for normal embryo sac development. Half the mutant embryo sacs showed arrested development, either at the two-nucleate stage or before polar nuclei fusion. The discovery that an apparent metabolic mutant has such severe developmental effects raises the possibility that some developmental regulators exert their effects through the modulation of basic metabolic processes. Metabolic regulation, therefore, may be a possible effector pathway in models of genetic regulation of development. A Nodulin-Like Protein in Phloem Identifying the proteins found in the sieve element-companion cell complex of the phloem is necessary to develop a full physiological understanding of this complex vascular tissue. Khan et al. (pp. 1576–1589) report that a monoclonal antibody line, selected from hybridomas raised against sieve elements isolated from Streptanthus tortuosus (Brassicaceae) tissue cultures, recognizes an antigen in Arabidopsis that is associated specifically with the plasma membrane of sieve elements (Fig. 1 ). This protein, which has structural and sequence characteristics typical of the Arabidopsis family of early nodulin (ENOD)-like proteins, accumulates at the earliest stages of sieve element differentiation. The sieve element-specific ENOD-like protein (SE-ENOD) appears to be expressed and processed through the endomembrane system in nucleate sieve elements prior to selective autophagy of the organelles and is ultimately attached to the sieve element plasma membrane by a glycosylphosphatidylinositol anchor. T-DNA insertion mutants show a minimally altered growth phenotype under normal growth conditions with a significant reduction in the reproductive potential of the plant. The localization and structural studies of the SE-ENOD are consistent with the hypothesis that SE-ENOD is involved in cell surface interactions within the extracellular region of the sieve element plasma membrane.
Actin and Auxin-Dependent Patterning The physiological process underlying auxin-dependent patterning is the cell-to-cell polar transport of auxin. The polarity of auxin transport is linked to the cycling of pin-formed (PIN) proteins. When plants are treated with brefeldin A (BFA), plasma membrane-localized PIN1 proteins are trapped in intracellular compartments. This effect of BFA on PIN1 internalization and cycling is interrupted by the actin depolymerization drug cytochalasin D, indicating that the endosomal exocytosis of PIN1 is actin dependent. BFA also causes the bundling of cortical actin strands into dense bundles. These observations suggest a possible connection between auxin signaling, vesicle flow, and the organization of actin filaments. Maisch and Nick (pp. 1695–1704) have employed tobacco (Nicotiana tabacum) cv Bright-Yellow 2 (BY-2) as model system to study intracellular communication in patterning. They show that cell division within cell files is partially synchronized, leading to higher frequencies of files with even cell numbers as compared to files with uneven cell numbers. By inhibiting the polar auxin flux using low concentrations of 1-N-naphthylphthalamic acid, they demonstrate that the pattern of cell division within BY-2 cell files depends on auxin. To address the role of actin in this synchrony, they induced a bundled configuration of actin by overexpressing mouse talin. They report that the constitutive bundling of actin impairs the synchrony of cell division and increases the sensitivity to 1-N-naphthylphthalamic acid. Moreover, when they added polar transportable auxins (but not auxin per se), both the normal organization of actin and the synchrony of cell division were restored. They conclude that actin is not only responsive to changes in the cellular content of auxin, but also that the organization of actin filaments is crucial for auxin transport. These results are consistent with the idea of a regulatory mechanism in which auxin controls its own transport through changing the organization of actin filaments. Carbon Availability and Mycorrhizae
The mutualistic interaction in arbuscular mycorrhiza (AM) is characterized by an exchange of mineral nutrients and carbon. The major benefit of AM, which is the supply of phosphate to the plant, has been well studied. However, less is known about the regulatory function of carbon availability on AM formation. Schaarschmidt et al. (pp. 1827–1840) have analyzed the effect of enhanced levels of hexoses in the root, the main form of carbohydrate used by the fungus, on AM formation. Their strategy for enhancing the hexose content of roots involved expressing a yeast invertase gene in different subcellular locations in the roots of tobacco. Despite increased hexose levels in these roots, no effect was seen on the colonization density of the mycorrhizal fungus Glomus intraradices as measured by the assessment of fungal structures, or the level of fungus-specific palmitvaccenic acid (C16:1 Effects of Overproducing Abscisic Acid The overexpression of genes that respond to drought stress is a seemingly attractive approach for improving drought resistance in crops. Such an approach, however, could potentially affect not only water-use efficiency but also crop productivity. With this in mind, Thompson et al. (pp. 1905–1917) have characterized two tomato (Solanum lycopersicum) lines that overexpress a gene encoding 9-cis-epoxycarotenoid dioxygenase, the enzyme that catalyzes a key rate-limiting step in abscisic acid (ABA) biosynthesis. As expected, both lines contained more ABA than the wild type, and had higher transpiration efficiencies because of their lower stomatal conductances and greater root hydraulic conductivities. In well-watered plants, the negative effects of ABA overaccumulation were reduced assimilation rate, leaf interveinal flooding and chlorosis (particularly under high-humidity environments in young plants), and delayed germination and establishment. However, under standard glasshouse conditions, these effects were insufficient to reduce biomass production, presumably because of counteracting positive effects on leaf expansion through improvements in water status, turgor, and antagonism of epinastic growth. Given the pleiotropic effects of elevated ABA levels, the authors propose that the optimization of 9-cis-epoxycarotenoid dioxygenase transgene activity for level of expression, developmental timing, and tissue specificity may be necessary to produce agronomically useful genotypes in which the potential negative physiological effects of high ABA, such as delayed establishment or flowering, are avoided, while the positive effects on water economy and leaf growth are maintained. Histone Acetylation and Flowering Histone acetylation is an important posttranslational modification that is correlated with gene activation. In eukaryotes, the p300/CBP family plays a major role in transcriptional regulation by promoting acetylation of both histones and non-histone proteins. In Arabidopsis, there are five p300/CBP histone acetyltransferase homologs (AtHACs). Although the structural conservation and diversification of AtHACs have been characterized, the functions of AtHACs in gene regulation and developmental control in Arabidopsis remain unknown. Deng et al. (pp. 1660–1668) have isolated Arabidopsis mutants with T-DNA insertions in the HAC1 gene and investigated their effects on plant development. They demonstrate that HAC1 played an important role in vegetative and reproductive development. Lesions in AtHAC1 caused pleiotropic developmental defects, including delayed flowering, a shortened primary root, and partially reduced fertility. Analysis of the molecular basis of late flowering in hac1 mutants showed that the hac1 plants respond normally to daylength, GA treatment, and vernalization. Furthermore, the expression level of the flowering repressor FLOWERING LOCUS C (FLC) is increased in hac1 mutants, indicating that the late-flowering phenotype of hac1 mutants is mediated by FLC. The authors propose that HAC1 affects flowering time by epigenetic modification of factors upstream of FLC, and that HAC1 is critical for the normal regulation of flowering time in Arabidopsis.
Department of Natural Sciences, Mercy College, Dobbs Ferry, New York 10522 FOOTNOTES www.plantphysiol.org/cgi/doi/10.1104/pp.104.900222 Related articles in Plant Physiol.:
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11), or the plant phosphate content. Roots of Medicago truncatula, transformed to express genes encoding an apoplast-, cytosol-, or vacuolar-located yeast-derived invertase, had increased hexose-to-Suc ratios compared to 
-glucuronidase-transformed roots. However, transformations with the invertase genes did not affect mycorrhizal formation. These data suggest the carbohydrate supply in AM cannot be improved by increased hexose levels in plant roots, implying that under normal conditions sufficient carbon is available in mycorrhizal roots. Reduced hexose levels in roots, however, can reduce AM colonization and proliferation. In tobacco plants with defective phloem loading or with decreased acid invertase activity in roots, there was diminished mycorrhizal formation.