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Plant Physiology 140:1-2 (2006) © 2006 American Society of Plant Biologists On the InsideBrassinosteroids and Nonclimacteric Fruit RipeningIn contrast to climacteric fruit, where ethylene is pivotal, the hormonal control of ripening in grape (Vitis vinifera) and other nonclimacteric fruits is poorly understood. Brassinosteroids (BRs) are steroidal hormones, essential for normal plant growth and development but not previously implicated in the ripening of nonclimacteric fruit. Symons et al. (pp. 150–158) present evidence that increases in endogenous BR levels, but not indole-3-acetic acid or GA levels, are associated with ripening in grapes. The application of BRs to grape berries significantly promoted ripening, while brassinazole, an inhibitor of BR biosynthesis, significantly delayed fruit ripening. Putative grape homologs of genes encoding BR biosynthesis enzymes and the BR receptor were isolated. Expression analysis of these genes during berry development revealed transcript accumulation patterns that were consistent with a dramatic increase in endogenous BR levels at the onset of fruit ripening. These results provide evidence that changes in endogenous BR levels influence ripening in this nonclimacteric fruit. This may provide significant insights into the mechanisms controlling ripening in grapes, which has direct implications for the logistics of grape production and processing. On the Scent of a Rose Genomic approaches have led to the identification of several genes potentially involved in scent production by rose (Rosa sp.) petals. Two species of orcinol O-methyltransferases (OOMTs), encoded by two closely related genes (OOMT1 and OOMT2), catalyze the last two steps of the biosynthetic pathway leading to the production of the phenolic methyl ether 3,5-dimethoxytoluene (DMT), the major scent compound of many rose (Rosa hybrida) varieties (Fig. 1 ). Modern roses are descended from both European and Chinese species, the latter being producers of phenolic methyl ethers, but not the former. Scalliet et al. (pp. 18–29) have investigated why phenolic methyl ether production occurs in some but not all rose varieties. In DMT-producing varieties, OOMTs are localized specifically in the petal, predominantly in the adaxial epidermal cells. In these cells, OOMTs become increasingly associated with membranes during petal development, suggesting that the scent biosynthesis pathway catalyzed by these enzymes may be directly linked to the cells' secretory machinery. OOMT gene sequences were detected in two non-DMT-producing rose species of European origin, but no mRNA transcripts were detected and these varieties lacked both OOMT protein and enzyme activity. These data indicate that upregulation of OOMT gene expression may have been a critical step in the evolution of scent production in roses.
Onset of Gravicompetency It is widely held that root gravisensing involves interactions between various cellular structures, such as statoliths, endoplasmic reticulum, actin microfilaments, and vacuoles. Many researchers favor the hypothesis that sedimenting amyloplasts exert a pressure on actin filaments that are connected to mechanosensitive ion channels in the plasma membrane, thereby leading to channel activation and a subsequent chain of events. However, much less is known about the onset of gravicompetency in seeds and whether those cellular structures that supposedly play a role in gravisensing are even present or functional at this stage of development. Ma and Hasenstein (pp. 159–166) have determined the precise inception of gravisensitivity in flax (Linum usitatissimum) roots by clinorotating germinating seeds after various periods of static orientation (gravistimulation). The onset of gravisensing was defined as the time needed to induce at least 50% of all the newly emerged roots to grow in the direction of the gravity vector. The time was measured from the onset of imbibition until the beginning of subsequent clinorotation. Gravitropic competency was established about 8 h after imbibition, 11 h prior to germination. Amyloplasts began to appear 10 to 12 h after imbibition, when the majority of the primary roots were found to be graviperceptive. After 12 h of imbibition, only the external layer of the root cap exhibited actin filaments and these were quite wispy. The actin cytoskeleton did not fully develop in the columella cells within the first 48 h after imbibition. Moreover, the microfilament inhibitor Latrunculin B did not affect the onset of gravisensitivity or germination. These results indicate that gravisensing is accompanied by the development of amyloplasts, and that the actin cytoskeleton is not required for gravisensing during seed germination. Thermotolerance Mechanism of Plant Mitochondria To gain insight into the temperature tolerance of seeds and seedlings, Stupnikova et al. (pp. 326–335) have characterized mitochondria from desiccation-tolerant seeds and from desiccation-sensitive pea (Pisum sativum) seedling epicotyls under a wide range of temperatures. The authors report that exogenous NADH is able to fuel respiration at temperatures where other substrates fail. Thus, NADH can power oxidative phosphorylation at surprisingly low temperatures around 0°C and even below in the case of seeds. Seed mitochondria exhibited remarkable temperature tolerance in response to both cold and heat stress, maintaining a well-coupled respiration between –3.5°C and 40°C (the proper functioning of seed mitochondria at –3.5°C may be the lowest recorded for any organism). In contrast, epicotyl mitochondria were inefficient below 0°C and uncoupled above 30°C. Both seed and epicotyl mitochondria exhibited an Arrhenius break temperature at 7°C, although they differed in phospholipid composition. Seed mitochondria had a lower phosphatidylethanolamine to phosphatidylcholine ratio and appeared less susceptible to lipid peroxidation. Contrary to the paradigm of homeoviscous adaptation, the extremely cold-resistant seed mitochondria had less unsaturated fatty acids. Seed mitochondria also accumulated large amounts of heat shock protein HSP22 and late embryogenesis abundant protein PsLEAm. The authors attribute the wide temperature tolerance of seed mitochondria to differences in membrane composition and to the accumulation of stress proteins involved in desiccation tolerance. Finally, the authors propose that a major physiological role for exogenous NADH oxidation is to enable oxidative phosphorylation to continue at low temperatures to maintain ATP levels. A Novel Mechanism of Lys Synthesis As in bacteria, Lys biosynthesis in plants occurs by way of a pathway that utilizes the intermediate diaminopimelic acid (DAP). However, three variants of the DAP pathway have been described in prokaryotes and it is not clear which, if any of them, occurs in plants. Hudson et al. (pp. 292–301) report the discovery of a new Lys biosynthesis pathway in Arabidopsis (Arabidopsis thaliana) that utilizes a novel transaminase that specifically catalyzes the interconversion of tetrahydrodipicolinate and LL-diaminopimelate. Because this single transaminase catalyzes the direct formation of LL-DAP from tetrahydrodipicolinate, it circumvents the DapD, DapC, and DapE steps of the acyl pathways found in prokaryotes. Indeed, the LL-DAP aminotransferase encoded by locus At4g33680 was able to complement the dapD and dapE mutants of Escherichia coli. This result in conjunction with the kinetic properties and substrate specificity of the enzyme, indicated that LL-DAP aminotransferase functions in the Lys biosynthetic direction under in vivo conditions. Orthologs of At4g33680 were identified in all the cyanobacterial species whose genomes have been sequenced. The Synechocystis sp. ortholog encoded by locus sll0480 showed the same functional properties as At4g33680. These results demonstrate that the Lys biosynthesis pathway in plants and cyanobacteria is distinct from the pathways that have so far been defined in microorganisms. This discovery adds a fourth variation to the list of DAP pathways found in nature. A Photorespiratory Mutant
Mutants of the photorespiratory C2 cycle display a conditional-lethal phenotype in which they are unable to thrive at ambient conditions but grow normally under conditions, such as high CO2, that suppress photorespiration. Mitochondrial Ser hydroxymethyltransferase (SHMT) is one of the key enzymes of the photorespiratory C2 cycle. The Arabidopsis mutant shm (now designated shm1-1) is defective in mitochondrial SHMT activity and displays a lethal photorespiratory phenotype when grown at ambient CO2, but is virtually unaffected at elevated CO2. The Arabidopsis genome harbors seven putative SHM genes, two of which (SHM1 and SHM2) feature predicted mitochondrial targeting signals. Voll et al. (pp. 59–66) mapped shm1-1 to the position of the SHM1 gene (At4g37930) and determined that the mutation is due to a G
Department of Natural Sciences, Mercy College, Dobbs Ferry, New York 10522 FOOTNOTES www.plantphysiol.org/cgi/doi/10.1104/pp.104.900182. Related articles in Plant Physiol.:
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A transition at the 5' splice site of intron 6 of SHM1, causing aberrant splicing and a premature termination of translation. Promoter 
-glucuronidase analyses revealed that SHM1 is predominantly expressed in leaves while SHM2 is mainly transcribed in the shoot apical meristem and roots. The expression of wild-type SHM1 under the control of either the cauliflower mosaic virus 35S or the SHM1 promoter in shm1-1 abolished the photorespiratory phenotype of the shm mutant. Surprisingly, however, the expression of SHM2 in shm1-1 under control of either the cauliflower mosaic virus 35S or the SHM1 promoter failed to complement the photorespiratory shm phenotype, indicating that SHM2 either does not encode a fully functional SHMT protein or that the protein is not targeted to mitochondria. These findings unequivocally demonstrate that At4g37930 (AtSHM1) is crucial for plant growth in ambient air and for proper functioning of the C2 cycle. 
