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Plant Physiology 136:2577-2578 (2004) © 2004 American Society of Plant Biologists On the InsideTranscriptomics of Gravi- and Mechano-Stimulated RootsGravitropic and mechanical stimulation both induce differential growth in plant roots. Kimbrough et al. (pp. 2790–2805) have performed a whole genome microarray analysis of Arabidopsis root apices after gravitropic and mechanical stimulation. They monitored the time course of transcript abundance of 22,744 genes after stimulation, and report on the earliest detected changes in transcript levels in the Arabidopsis root apices due to gravity and mechanical stimulation. Some of these transcriptional changes in response to gravity occur within 2 min in Arabidopsis apical root tissues while statolith sedimentation is still occurring. In response to gravistimulation, they observed the up-regulation of certain transcription factors, cell wall modifying enzymes, transporters, and stress-related genes. No genes were down-regulated in a gravity specific manner in any of the experiments. Mechanical stimulation was sensed by the root apex and led to significant changes in transcript abundance of 1,696 genes, of which 26 were specifically regulated by mechanical stimulation and not by gravity. In addition to 9 transcripts with unknown function, several hitherto unexpected players (a mitochondrial H+-ATPase, a putative sulfate transporter, a putative cytochrome P450, and a wound-induced protein) were found to be up-regulated within 2 min after mechanical stimulation, whereas rapid decreases in the abundance of mRNA were observed for genes encoding a cyclic nucleotide-regulated K+-channel, an expansin, a RING-finger transcription factor, a sugar transporter, and a pathogen response protein. Proteomics of Salt Tolerance in a Halophytic Alga The halophytic green alga Dunaliella salina responds to salt stress by a massive accumulation of glycerol (its internal osmotic element), enhanced elimination of Na+ ions, and accumulation of specific proteins. However, a comprehensive analysis of salt up-regulated genes in Dunaliella has been hindered by the paucity of accurate protein sequences. Liska et al. (pp. 2806–2817) have adopted a proteomic approach to identify salt-induced proteins in Dunaliella in an effort to clarify the molecular basis for the alga‘s extreme salt tolerance. Seventy-six salt-induced proteins were selected from 2D gel separations of different subcellular fractions and analyzed by mass spectrometry. A variety of techniques, particularly MS BLAST searches, enabled the identification of 80% of the salt-induced proteins. Salinity stress up-regulated key enzymes in the Calvin cycle, in starch mobilization, in redox energy production, in protein biosynthesis and degradation, and in a homolog of a marine bacterial Na+-Redox transporter. The results indicate that Dunaliella responds to high salinity by an enhancement of photosynthetic CO2 assimilation. Amazingly, the authors report that carbon uptake activity is over 2-fold faster in cells grown in 3 M NaCl than in 0.5 M NaCl. The resulting increases in photosynthates and metabolites are presumably needed for the synthesis of massive amounts of glycerol. Engineering Salt Tolerance through the Plastid Genome Plants utilize a number of protective mechanisms to maintain normal cellular metabolism and prevent damage to cellular components in response to salt stress. One metabolic adaptation to salt stress is the accumulation of osmoprotectants such as glycine betaine, a quaternary compound that helps to maintain an osmotic balance with the environment and stabilize the quaternary structure of complex proteins. The metabolic pathway for glycine betaine synthesis in higher plants is regulated by the chloroplast enzyme betaine aldehyde dehydrogenase (BADH). In this issue, Kumar et al. (pp. 2843–2854) examine whether the overexpression of BADH via chloroplast genetic engineering confers heightened salt tolerance in carrot (Daucus carota). There are many advantages to expressing transgenes in chloroplasts. Since the chloroplast genome of carrot shows strict maternal inheritance, the risk of chloroplast transgene escape by crop-to-weed hybridization is virtually nil. The authors report that they have successfully engineered the carrot chloroplast genome to overexpress the badh gene and produced a chloroplast transgenic line that is able to survive in 400 mM NaCl, the levels at which halophytes survive salt stress. In vitro transgenic carrot cells transformed with the badh transgene were visually green in color when compared to untransformed carrot cells and this offered a visual selection for transgenic lines. BADH enzyme activity was enhanced 8-fold in transgenic carrot cell cultures, grew 7-fold more, and accumulated 50-fold more betaine than untransformed cells grown in liquid medium containing 100 mM NaCl. The transformation of the carrot plastid genome reported here is the first example of a successful stable plastid transformation using non-green explants via somatic embryogenesis. Osmotic Adjustment Mutants
Drought decreases the water potential ( Different Modes of Maternal Inheritance in Syringa Species The majority of angiosperm species display maternal inheritance of the plastid genome. Epifluorescence microscopy, combined with fluorescent staining of organellar DNA, has been used to reveal the fate of paternal plastids prior to fertilization. Such studies have revealed that the amount of organellar DNA increases in male reproductive cells of species that display biparental cytoplasmic inheritance, but decreases in cells of species that display maternal cytoplasmic inheritance. Liu et al. (pp. 2762–2770) have used this epifluorescent method to examine 19 species in the genus Syringa. Plastid DNA was detectable in the generative cells of only 12 of the 19 species, indicating that species within the genus Syringa display differences in plastid inheritance. Some Syringa species exhibit maternal mitochondrial inheritance, whereas others exhibit biparental plastid inheritance. Epifluorescence microscopy detected very little mitochondrial DNA in the mature generative cells of any Syringa species, despite the preservation of mitochondria in the cell. This suggests that most of the mitochondrial DNA in the generative cell may be degraded during pollen development, and any remainder, in the zygote. This is similar to the situation in animal cells, where any residual mitochondrial DNA in the mature sperm is degraded in the zygote. The difference between the modes of potential plastid inheritance among the species suggests different phylogenies for the species; it also supports recent conclusions of molecular, systematic studies of the Syringa. The authors suggest that the mechanisms that control plastid inheritance in angiosperms arose independently, and may have developed later than those for mitochondrial inheritance. Transcriptomics of Nitrate Addition Nitrogen is the most important inorganic nutrient in plants, and a major constituent of proteins and nucleic acids, as well as many cofactors and secondary metabolites. Nitrogen affects all levels of plant function. Nitrate (NO3–) addition induces genes involved in NO3– uptake and reduction, and the production of organic acids to act as acceptors and counter anions. Genes are induced in the oxidative pentose phosphate pathway to provide reducing equivalents for NO3– assimilation. NO3– addition also modifies resource allocation, growth, and development by modulating shoot-root allocation and lateral root growth, accelerating senescence, and promoting flowering and tuber initiation. The breadth of response to NO3– makes it a rich but challenging area for post-genomic study. In this issue, Scheible et al. (pp. 2483–2499) have analyzed the transcriptome of Arabidopsis to identify processes affected by long-term nitrogen deprivation or short-term NO3– nutrition in Arabidopsis. Two days of nitrogen deprivation led to coordinate repression of the majority of the genes assigned to photosynthesis, chlorophyll synthesis, plastid protein synthesis, induction of many genes for secondary metabolism, and reprogramming of mitochondrial electron transport. NO3– readdition led to rapid, widespread, and coordinated changes. As expected, there were marked and rapid changes for many genes directly involved in NO3– transport and assimilation and in the generation of reducing equivalents and organic acid skeletons. By 3 h, most genes assigned to amino acid and nucleotide biosynthesis and scavenging were induced, while most genes assigned to amino acid and nucleotide breakdown were repressed. There was coordinate induction of many genes assigned to RNA synthesis and processing and most of the genes assigned to amino acid activation and protein synthesis. Specific genes encoding expansin and tonoplast intrinsic proteins as well as cell wall modifying enzymes were induced, indicating activation of cell expansion and growth in response to NO3– nutrition. There were also rapid responses in the expression of many genes involved in regulation, including genes for trehalose and hormone metabolism as well as transcription factors and signal transduction proteins.
Department of Natural Sciences Mercy College Dobbs Ferry, New York 10522 FOOTNOTES www.plantphysiol.org/cgi/doi/10.1104/pp.104.900121. Related articles in Plant Physiol.:
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w) of soil, thereby hindering the ability of plant roots to absorb water. Osmotic adjustment avoids excessive dehydration through the accumulation of intercellular solutes, which lowers cellular