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Plant Physiology 149:599-600 (2009) © 2009 American Society of Plant Biologists On the InsideAn Unusual Role for GNOM in Hydrotropism Hydrotropism refers to the bending of roots in response to gradients of water. Elucidating the molecular mechanisms underlying hydrotropism is important not only for understanding how terrestrial plants adapt to changes in moisture, but also for improving crop yields and biomass production. Previously, Hideyuki Takahashi's research group isolated an ahydrotropic mutant of Arabidopsis (Arabidopsis thaliana), mizu-kussei1 (miz1), and showed that MIZ1 encodes a protein of unknown function. In this issue, Miyazawa et al. (pp. 835–840) have identified a second ahydrotropic mutant, miz2, that turns out to be a unique allele of gnom. GNOM encodes a guanine-nucleotide exchange factor for ADP-ribosylation factor-type G proteins. Nearly all of the defects associated with gnom alleles involve disruptions in polar auxin transport and the proper localization of auxin efflux carriers, leading to severe defects in apical-basal pattern formation during embryogenesis. However, previous studies have revealed that the hydrotropic responsiveness of Arabidopsis roots are unaffected by auxin efflux inhibitors. Moreover, unlike other gnom alleles, miz2 mutants exhibit neither reduced gravitropism nor morphological defects nor alterations in auxin responsiveness nor abnormal localization of auxin efflux proteins. Since it is known that GNOM-mediated vesicular trafficking is inhibited by brefeldin A (BFA), the authors treated wild-type seedlings with BFA and monitored the effects of the drug on hydrotropism. They report that BFA inhibited root hydrotropism in a dose-dependent manner. Thus, it appears that miz2 is a GNOM mutant that affects hydrotropism by a mechanism distinct from the role that GNOM plays in regulating polar auxin transport and gravitropism.
The passage of a fire over an area is soon followed by a flush of new growth involving the recruitment of new seedlings from the seed bank of the charred soil. Smoke has been identified as a broadly effective stimulant that enhances the germination of approximately 1,200 species in more than 80 genera. Attempts to study the effects of smoke on plant function have been confounded by smoke's complex chemical composition. Recently, however, a family of butenolide molecules called karrikins has been identified as active agents in smoke. The parent molecule, 3-methyl-2H-furo[2,3-c]pyran-2-one (KAR1), is a potent stimulant that enhances the germination of some species at subnanomolar concentrations. To gain a better understanding of the mechanism by which karrikins trigger seed germination and explore their interaction with abscisic acid (ABA) and GA, Nelson et al. (pp. 863–873) have examined KAR1 responses in Arabidopsis mutants altered in phytohormone signaling. They have found that Arabidopsis rapidly and sensitively perceives karrikins, and that karrikins trigger the germination of dormant Arabidopsis seeds far more effectively than known phytohormones or a structurally related strigolactone. Studies of phytohormone mutants indicate that ABA suppresses KAR1-induced germination, and that GA synthesis is required for KAR1 activity. Neither ABA nor GA levels in seeds are appreciably affected by KAR1 treatment prior to radicle emergence, despite marked differences in germination outcome. KAR1 stimulation of Arabidopsis germination is light dependent and reversible by far-red exposure. The requirements for light and GA biosynthesis reported here provide important insights into KAR1's mode of action.
In addition to being fundamental sources of fuel for carbon and energy metabolism, sugars function as signaling molecules in higher plants. Recent studies have revealed that the Glc sensing and signaling system in Arabidopsis is mediated by hexokinase. Thus, in addition to catalyzing hexose phosphorylation, hexokinase is also a Glc sensor. Transgenic plants expressing catalytically inactive hexokinase 1 (AtHXK1) mutant alleles in a Glc-insensitive (gin2 [for glucose insensitive2]) mutant background have revealed that the catalytic and sensory functions of AtHXK1 can be uncoupled in Arabidopsis. However, with the exception of Arabidopsis HXK1, the role of hexokinases as Glc sensors has yet to be demonstrated in other plant species. To investigate the functions of rice (Oryza sativa) hexokinase isoforms and their possible roles in Glc sensing, Cho et al. (pp. 745–759) have characterized two rice genes, OsHXK5 and OsHXK6, which are evolutionarily related to AtHXK1. The subcellular localization of OsHXK5 and OsHXK6 suggests that these two proteins retain a dual-targeting ability to mitochondria and nuclei. In transient expression assays, these two OsHXKs and their catalytically inactive alleles dramatically enhanced the Glc-dependent repression of specific proteins. Moreover, the expression of OsHXK5, OsHXK6, or their mutant alleles complemented the gin2-1 mutant, thereby resulting in wild-type characteristics in seedling development, Glc-dependent gene expression, and plant growth. Furthermore, transgenic rice plants overexpressing OsHXK5 or OsHXK6 exhibited hypersensitive plant growth retardation and enhanced repression of the photosynthetic gene RbcS in response to Glc treatment. These results provide evidence that OsHXK5 and OsHXK6 function as Glc sensors in rice.
Autophagy, the bulk degradation of intracellular proteins and organelles, plays an important role in nutrient recycling during senescence. During autophagy, a portion of the cytoplasm, including entire organelles, is engulfed in membrane-bound vesicles and delivered to hydrolytic vacuoles. A recent genome-wide search confirmed that Arabidopsis has many genes homologous to the yeast autophagy genes (ATGs). It was recently demonstrated that Rubisco and other stromal proteins are mobilized to the vacuole during ATG-dependent autophagy by means of small spherical bodies called Rubisco-containing bodies (RCBs). The pinching off of RCBs may account for the observation that during senescence, there is a significant decrease in both the number and the size of chloroplasts. Wada et al. (pp. 885–893) have examined autophagy and degradation directly in living Arabidopsis leaf cells to determine whether autophagy is responsible for chloroplast shrinkage and whether it is involved in the vacuolar degradation of whole chloroplasts during leaf senescence. They report that the number and size of chloroplasts decreased in darkened leaves of wild type, while the number remained constant and the size decrease was suppressed in atg4a4b-1. When individual leaves of transgenic plants expressing stroma-targeted fluorescent protein underwent dark senescence, a large accumulation of fluorescence was observed to occur in the vacuolar lumen. Moreover, fluorescing chloroplasts and RCBs were also observed in the vacuole. In contrast, neither the stroma-targeted fluorescent protein nor chloroplasts nor RCBs were observed to accumulate in the vacuoles of the autophagy-deficient mutant. Thus, the authors have succeeded in demonstrating chloroplast autophagy in living cells and have provided direct evidence of chloroplast transport into the vacuole.
Cauliflower mosaic virus (CaMV) is a pararetrovirus that infects plants. Pararetroviruses replicate through reverse transcription just like retroviruses, but the viral particles contain DNA instead of RNA. Replication of CaMV involves the production of a polycistronic RNA intermediate, the 35S RNA. The 35S RNA contains a highly structured 600-nucleotide-long leader sequence with six to eight short open reading frames. This leader is followed by seven tightly arranged longer open reading frames that encode all the viral proteins. P6 is the most abundant viral protein encoded by 35S RNA and has been shown to be the major constituent of amorphous, electron-dense inclusion bodies believed to be the sites of virion assembly. One of P6's many proposed functions is to control the export of the 35S RNA from the nucleus to the cytoplasm, thereby drawing the 35S RNA into the nascent P6 inclusion bodies where viral proteins are translated. The cytoskeleton has been implicated in the intracellular trafficking of a number of plant viral proteins. These studies suggest that the trafficking of viral proteins along actin filaments is a mechanism utilized by highly divergent RNA viruses. Harries et al. (pp. 1005–1016) have utilized a fusion between the C terminus of P6 and GFP to visualize P6 inclusions in living cells. They demonstrate that the fusion of P6 with GFP does not interfere with its function. Moreover, they demonstrate that P6-GFP inclusion bodies move intracellularly and are associated with microtubules, actin microfilaments, and the endoplasmic reticulum. Although P6-GFP inclusion bodies associated with microtubules appear stationary, P6-GFP bodies can move along microfilaments and this trafficking is severely reduced by treatment with the actin inhibitor latrunculin B. Latrunculin B treatment of Nicotiana edwardsonii leaves inhibits the formation of local lesions by CaMV, indicating that P6 trafficking along microfilaments is necessary for CaMV cell-to-cell movement. Additionally, the association of P6-GFP inclusion bodies with microtubules prevents the disruption of microtubules by oryzalin, suggesting a tight association between these two proteins. The motility of P6 along microfilaments represents an entirely new property for this protein, and these results suggest a role for P6 in the intracellular and cell-to-cell movement of CaMV.
Many signal transduction pathways involve the cleavage of phosphatidylinositols (PIs). During PI signal transduction, phospholipase C cleaves phosphatidylinositol (4,5)bisphosphate (PtdInsP2) to form diacylglycerol and the Ca2+-mobilizing messenger inositol trisphosphate. In the last decade, signaling via PtdInsP2 itself rather than via its cleavage products has become the focus of intense study in animal cells. PtdInsP2 has been shown to interact with membrane ion channels and other ion transporters, underscoring its importance in conveying information about stimuli and in maintaining cellular ionic homeostasis. To understand the regulation of plant ion channels by PIs, Ma et al. (pp. 1127–1140) have examined the effect of PtdInsP2 on the outward-rectifying K+ efflux channel (NtORK) of tobacco (Nicotiana tabacum). In their research, they have employed tobacco plants altered in their basal plasma membrane PtdInsP2 levels. Using the patch clamp method in a whole-cell configuration, with the cytosolic concentrations of Ca2+ and protons tightly controlled by appropriate buffers, they have established a negative correlation between the basal level of PtdInsP2 and NtORK activity. Pharmacological manipulations of the PtdInsP2 membrane levels also lend support to this conclusion.
Division of Health Professions and Natural Sciences FOOTNOTES www.plantphysiol.org/cgi/doi/10.1104/pp.104.900283 Related articles in Plant Physiol.:
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