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Plant Physiology 139:1-2 (2005) © 2005 American Society of Plant Biologists On the InsidePeroxisome BiogenesisPlant peroxisomes are involved in a wide range of functions including lipid metabolism, photorespiration, nitrogen metabolism, stress response, and synthesis of some plant hormones. In yeast (Saccharomyces cerevisiae) and mammals, various steps in peroxisome biogenesis require the function of PEX proteins (peroxins). PEX12 is a RING finger peroxisomal membrane protein involved in the import of matrix proteins. In humans, mutations in PEX12 lead to failure of matrix protein import and result in Zellweger syndrome, a lethal neurological disorder. To determine the role of PEX12 in plant cellular functions and to address its role in plant development, Fan et al. (pp. 231–239) characterized Arabidopsis (Arabidopsis thaliana) PEX12 and identified mutants deficient in this gene. RNAi plants with partial reduction of the PEX12 transcript exhibit impaired peroxisome biogenesis and function, inhibition of plant growth, and reduced fertility. These findings suggest that the Arabidopsis PEX12 protein is required for peroxisome biogenesis and plays an essential role throughout plant development. Nitrogen Deprivation and Symbiotic Gland Development Gunnera is the only flowering plant genus known to host cyanobacteria, specifically Nostoc. N2 fixation by the symbiotic cyanobacteria is able to fulfill the entire nitrogen needs of the plant. Unlike other multicellular plant hosts, Gunnera harbors Nostoc intracellularly, surrounding the Nostoc filaments with a membrane most likely of plasma membrane origin. The specialized glands on the Gunnera stem serve as portals for the entry of motile Nostoc filaments (hormogonia) into the plant (Fig. 1). Almost all molecular research to date on plant-cyanobacterial associations has focused on the cyanobacterial partner. A major reason for this bias is that symbiotic cyanobacteria (mainly Nostoc species) have proven easy to grow and manipulate genetically in the laboratory. Recently, the genome of Nostoc punctiforme, a symbiotic strain, has been completely sequenced. In order to facilitate the molecular characterization of cellular interactions between N. punctiforme and Gunnera plants, Chiu et al. (pp. 224–230) have developed a simple protocol for the routine establishment of the symbiosis between Nostoc and Gunnera manicata in aseptic culture. During this process, they discovered that Gunnera stem glands could not develop unless the plant was deprived of exogenous combined nitrogen. Moreover, treatment of plants with the auxin transport inhibitor 1-naphthylphthalamic acid prevented gland development on N-limited medium, most likely by preventing resource reallocation from leaves to the stem. Understanding the nature of Gunnera plant's unusual adaptation to an N-limited environment may shed light on the evolution of plant-cyanobacterium symbioses and may suggest a route to establish productive associations between N2-fixing cyanobacteria and crop species.
Pronounced Expansion of Plant Transcription Factor Families Transcription factors (TFs) usually form gene families that vary considerably in size among organisms. In Arabidopsis, at least 1,500 genes are TFs, and 45% of these TFs belong to families common to Caenorhabditis elegans, Drosophila melanogaster, and Saccharomyces cerevisiae. Some of these TF families are much larger in Arabidopsis, suggesting differential expansion. For example, the Myb family has 190 members in Arabidopsis but only six in Drosophila, three in Caenorhabditis, and 10 in Saccharomyces. The finding that genes involved in transcription have been preferentially retained after the most recent whole genome duplication event in the Arabidopsis lineage suggest important roles of TF duplicates in plant evolution. To investigate if differences exist in the expansion patterns of TF gene families between plants and other eukaryotes, Shiu et al. (pp. 18–26) used Arabidopsis TFs to identify TF DNA-binding domains. These DNA-binding domains were then used to identify related sequences in 25 other eukaryotic genomes. Interestingly, among 19 families that are shared between animals and plants, more than 14 are larger in plants than in animals. They also found that among TF families shared by plants, animals, and fungi, the expansion of TFs has been much more dramatic in plants than in other eukaryotes. Moreover, this elevated expansion rate of plant TF does not stem from the generally higher polyploidy of plant genomes, but is due to a higher degree of expansion compared to other plant genes. The high rate of expansion among plant TF genes and their propensity for parallel expansion suggest frequent adaptive responses to selection pressure common among higher plants. Bacterial-Like Glycerol Channel in Mosses Major intrinsic proteins (MIPs) are a large and ancient protein family found in all three domains of life. MIPs form channels that facilitate the passive transport of water and other small polar molecules such as glycerol across membranes. Plants have more genes encoding different MIPs than any other type of organism; Arabidopsis, for example, has at least 35 genes coding for MIPs. Based on phylogenetic comparisons, plant MIPs have been classified into four subfamilies, plasma membrane intrinsic proteins (PIPs), tonoplast intrinsic proteins (TIPs), Nod26-like intrinsic proteins (NIPs), and small-basic intrinsic proteins (SIPs). PIPs and TIPs serve as water-conducting aquaporins, whereas NIPs have been reported to transport both glycerol and water. The specificity of SIPs is not yet known. All four subfamilies have also been identified in the primitive nonvascular moss Physcomitrella patens, suggesting that they may be present in all terrestrial plants. Gustavsson et al. (pp. 287–295) have identified a gene encoding a novel fifth type of MIP in the moss P. patens. Phylogenetic analyses show that this protein, GlpF-like intrinsic protein (GIP1;1), is closely related to a subclass of glycerol transporters in bacteria that in addition to glycerol are highly permeable to water. Unlike its bacterial counterparts, however, sequence-based predictions of substrate specificity suggest that GIP1;1 is permeable for glycerol but not for water. This is confirmed by functional analyses of heterologously expressed GIP1;1 in Xenopus laevis oocytes. A likely explanation of the occurrence of this bacterial-like MIP in P. patens is horizontal gene transfer. Interestingly, NIPs also facilitate the transport of glycerol in plants and have also previously been suggested to have evolved from a horizontally transferred bacterial gene. Fibers and Cavitation Resistance Xylem must withstand both the mechanical stresses associated with negative pressure as well as the risk of air entering the hydraulic pathway. Failure to do so may lead to cavitation of water columns and blockage of water transport. Natural selection favors increased cavitation resistance among woody evergreen plants occurring in drought-prone environments; however, not all plants adapted to arid environments have high cavitation resistance. One explanation for this is that there may be tradeoffs associated with increased cavitation resistance such as increased xylem construction costs or decreased conductive efficiency. Jacobsen et al. (pp. 546–556) examined the possible mechanical and hydraulic costs to increased cavitation resistance among six co-occurring species of chaparral shrubs in southern California. The correlations they uncovered between wood anatomical traits and cavitation resistance suggest a greater role for fibers in imparting conductive safety than previously considered. While it is thought that resistance to cavitation is primarily a function of pore size in pit membranes, it may be that the mechanical properties of xylem fibers, as well as of other cell types associated with vessels, are important in determining resistance to cavitation. Fibers may influence the risk of vessel implosion by reducing stresses that exacerbate pit membrane deflection or stresses that lead to vessel collapse and microfracture of the cell wall. A Regulator of Photosystem Stoichiometry under High-Light Conditions
Because PSII and PSI function in tandem during photosynthetic electron flow, photosynthetic organisms must maintain a balance of the two photosystems. Light quantity, as well as light quality, induces the changes in PSI/PSII ratio in cyanobacteria. PSI/PSII ratio decreases in cyanobacteria upon shifting to high light through the selective suppression in the amount of PSI, in addition to the general decrease in the amount of both photosystems. The role of the regulation of photosystem stoichiometry during acclimation to high light is apparently not to maintain optimal photosynthesis but to protect the cells from oxidative damage through the suppression of photosynthetic electron transfer. The disruption of genes involved in the transcription, translation, assembly, or biogenesis of PSI or PSII may be responsible for the change in photosystem stoichiometry. However, only a few cases have been reported for "regulatory mutants" of photosystem stoichiometry under high-light conditions. Fujimori et al. (pp. 408–416) report on a putative transcriptional regulator, Sll1961, which is involved in the modulation of photosystem stoichiometry during acclimation to high-light conditions. Characterization of the disruption mutant of this gene indicates that the mutants fail to properly suppress the amount of PSI under high-light conditions. DNA microarray analysis revealed that the expression of sll1773 was drastically induced in the sll1961 mutant upon exposure to high light for 3 h. The sll1773 gene encodes a pirin-like protein called PirA. Pirin-like proteins are found in many different organisms ranging from Archaebacteria to mammals. Previous studies in higher plants have indicated that the gene expression of pirin was induced during apoptosis, and that pirin interacts with the
Department of Natural Sciences, Mercy College, Dobbs Ferry, New York 10522 FOOTNOTES www.plantphysiol.org/cgi/doi/10.1104/pp.104.900173. Related articles in Plant Physiol.:
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-subunit of G proteins. In the cyanobacterium Synechocystis sp. PCC 6803, the pirA (sll1773) gene is the only gene that encodes a pirin homolog. These results demonstrate that a transcriptional regulator, Sll1961, and its possible target proteins including Sll1773, may be responsible for the regulation of photosystem stoichiometry in response to high light. 
