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Plant Physiology 149:352-353 (2009) © 2009 American Society of Plant Biologists On the InsideTwo Distinct Functions for Citrate Transporters in Grasses The greatest limitation to plant productivity on acid soils is aluminum (Al) toxicity. The efflux of organic anions is an important mechanism for Al resistance in cereal and noncereal species. The nature of organic anions (malate, citrate, or oxalate) released from roots differs between species. Members of the multidrug and toxin compound extrusion (MATE) family of proteins control Al-activated citrate efflux from barley (Hordeum vulgare) and sorghum (Sorghum bicolor). MATE proteins are widely present in bacteria, fungi, plants, and mammals. These proteins are characterized by having 400 to 700 amino acids with 12 transmembrane helices. Plants have many more types of MATE transporters than do bacteria or animals. Arabidopsis (Arabidopsis thaliana), for example, has 58 MATE orthologs, whereas rice (Oryza sativa) appears to have more than 40. Two contributions in this issue concern the functions of citrate-transporting MATE proteins in grasses. Yokosho et al. (pp. 297–305) performed a functional analysis of a MATE-encoding gene (OsFRDL1) of rice, the closest homolog of the gene encoding the barley citrate efflux protein. They demonstrate that there was no difference in the Al-induced secretion of citrate from the roots between a knockout line and wild-type rice. It appears that OsFRD1L, which is expressed chiefly in the pericycle of the root, is not involved in the Al-induced secretion of citrate like its homolog in barley, but in the efficient translocation of Fe into the xylem. A second contribution, by Ryan et al. (pp. 340–351), also concerns MATE proteins that excrete citrate. This study provides a detailed examination of the efflux of citrate from wheat (Triticum aestivum) roots, a species that shows a large intraspecific variation in Al resistance. A major mechanism of Al resistance in wheat is related to the Al-dependent release of malate anions from roots. The current authors present findings consistent with the idea that citrate efflux is a second major Al resistance mechanism in wheat. Thus, contrary to some previous proposals, Al resistance in wheat is a multigenic trait with distinct mechanisms.
Lignin is a heterogeneous cell wall polymer in plants. The deposition of lignin reinforces plant cell walls, facilitates water transport, provides compressive strength to conducting tissues, and acts as a mechanical barrier to pathogens. Typically, lignin makes up 20% to 30% of the cell wall material in woody species and therefore represents a significant proportion of plant biomass. In recent years, poplar (Populus spp.) has emerged as a model angiosperm tree species for the investigation of wood-related topics, including lignification. Coniferous gymnosperms, however, differ significantly from arborescent angiosperms such as poplar in terms of their anatomical, physiological, and biochemical attributes. The impact of lignin perturbations on gymnosperm performance is still largely unexplored, despite the significant ecological and economic importance of gymnosperm species such as pine (Pinus spp.) trees. To better understand the physiological role of lignin in coniferous gymnosperms, Wagner et al. (pp. 370–383) investigated how the silencing of 4-coumarate-CoA ligase (4CL), a key enzyme in the biosynthesis of lignin, affects plant phenotype, wood anatomy, and chemical wood composition in the conifer species Pinus radiata. They report that the severe suppression of 4CL substantially impacted plant phenotype in P. radiata and resulted in dwarfed plants with a "bonsai tree-like" appearance. Microscopic analyses of stem sections revealed substantial morphological changes in both wood and bark tissues. This included the formation of weakly lignified tracheids that displayed signs of collapse and the development of circumferential bands of axial parenchyma. The suppression of 4CL also affected carbohydrate metabolism. The most dramatic change was an approximately 2-fold increase in galactose content in wood due to increased compression wood formation. The molecular, anatomical, and analytical data presented verify that 4CL is associated with lignin biosynthesis and illustrate that 4CL silencing leads to complex physiological and morphological changes in P. radiata.
Many terrestrial plants form mutually beneficial root symbioses with soil microbes. The most widespread symbioses are arbuscular mycorrhizae (AM), the "fungus roots" formed between the vast majority of vascular flowering plants and biotrophic fungi belonging to the phylum Glomeromycota. Additionally, certain angiosperms, particularly the legumes, enter into root symbioses with N2-fixing rhizobial soil bacteria. Although the AM and rhizobial symbioses are morphologically distinct, the two are mechanistically related in legumes. A number of legume genes required for nodulation are also essential for the AM interaction. Moreover, some the host genes that are induced during legume-rhizobia symbioses are also up-regulated during AM symbioses. The overlap of the two symbiotic pathways has led to the hypothesis that the evolutionarily younger legume root nodule symbiosis may have evolved from the more ancient AM symbiosis. In recent years, the development of genetic and genomic tools for the two model legumes Medicago truncatula and Lotus japonicus has greatly facilitated the cloning of genes required for root symbioses. Intriguingly, all cloned legume symbiosis genes, including both the common symbiosis genes and genes only required for rhizobial symbiosis, have orthologs in nonlegumes. This finding offers an opportunity to address the evolution of root symbioses in plants by characterizing ortholog functionality across the legume and nonlegume boundary. Chen et al. (pp. 306–317) show that orthologs of CASTOR and POLLUX, the twin homologous genes in L. japonicus that encode putative ion channel proteins, are ubiquitously present and highly conserved in both legumes and nonlegumes. Using rice as a study system, they employed reverse genetic tools (knockout mutants and RNA interference) to demonstrate that the rice orthologs of CASTOR and POLLUX, namely Os-CASTOR and Os-POLLUX, are indispensible for mycorrhizal symbiosis in rice, and that Os-POLLUX can restore nodulation, but not rhizobial infection, to the dmi1 (doesn't make infections1) mutant of M. truncatula that putatively has a defective ion channel in its nuclear envelope.
Parthenocarpy refers to the production of fruit in the absence of pollination or fertilization. Seedless tomatoes (Solanum lycopersicum) are currently obtained by treating tomato flowers with exogenous auxin or inhibitors of auxin transport or other phytohormones. Parthenocarpic fruit development has also been conferred to tomato either by increasing auxin synthesis within the placenta/ovules or by increasing the auxin sensitivity in the fruit or by manipulating genes of the auxin or GA signal transduction pathway. Molesini et al. (pp. 534–548) show that a novel gene family, consisting of two genes encoding small peptides (only 53 amino acids long), regulates fruit initiation in tomato. The genes that encode these AUCSIA (for auxin cum silencing action) proteins are preferentially expressed in flower buds before anthesis. The authors show that RNA interference-mediated suppression of Aucsia genes causes parthenocarpic fruit development and a 100-fold increase in total IAA content of pre-anthesis flower buds. Furthermore, Aucsia-silenced plants display other auxin-related phenotypes such as alterations of leaf development, reduced auxin-induced rhizogenesis, reduced polar auxin transport in roots, and increased sensitivity to 1-naphthylphthalamic acid, an inhibitor of polar auxin transport. Collectively, these data indicate that AUCSIA peptides play a role in auxin-regulated processes in tomato and most likely, due to the high degree of sequence conservation, also in other Angiosperms. Indeed, Aucsia genes were probably present in green plants before the evolution of multicellularity and the colonization of land.
Model systems can be chosen for their ability to address particular questions or to represent particular phylogenetic groups. This latter approach is especially promising in grasses, in which the presence of several completed genomes (and the likelihood of more) provides an evolutionary genomic context for each new species sequenced. Multiple sequenced genomes, coupled with the ease of comparative analysis amongst the highly colinear chromosomes of grasses, allow new genomic sequences to be successfully annotated and related to genetic information from other grass species. Foxtail millet (Setaria italica) is an ancient grain crop species whose genome is currently being sequenced. According to Doust et al. (pp. 137–141), the rationale for sequencing foxtail millet is that it is closely related to the bioenergy grasses switchgrass (Panicum virgatum), napiergrass (Pennisetum purpureum), and pearl millet (Pennisetum glaucum). Foxtail millet provides a valuable tool for investigating the C4 grasses, particularly those that are being developed as biomass sources for biofuel production. It is expected that the draft foxtail millet genome will provide an assembly guide for any future switchgrass sequencing projects. Indeed, on-going work in switchgrass indicates that there is strong colinearity between switchgrass and foxtail millet. These two species last shared a common ancestor only about 13 million years ago. Relative divergence time is an important consideration when choosing model species for the genetic dissection of traits such as vegetative branching or C4 photosynthetic traits in bioenergy grasses, as closer phylogenetic relationships may coincide with more genetic, genomic, and physiological similarity. Some genetic resources such as genetic maps and a small collection of ESTs are already available for foxtail millet, but most of the tool development and genetic research in foxtail millet will be sequence driven. Foxtail millet does, however, currently lack some essential tools needed to become an ideal model system. Mutagenized populations, for example, are few, as are reverse genetics tools for characterizations of gene functions.
Plants take up sulfur (S) primarily as the sulfate anion, which is often found in low concentrations in the soil. Sulfate is mobile and commonly lost through soil leaching. In the past, the effects of leaching have likely been masked by the high input of S from atmospheric pollution. However, the drastic reductions in S deposition in North America and Europe over the last decade have led to an equally dramatic rise in reported cases of S deficiency in crop species. Despite the importance of S for plant nutrition, the role of the AM symbioses in S uptake has received little attention. Using an AM symbiosis of monoxenic cultured roots, Allen and Shachar-Hill (pp. 549–560) demonstrate that the transfer of S is nutritionally significant and regulated both by S availability to the host and by S metabolite availability to the extraradical mycelia. The authors have also identified some putative fungal homologs of S uptake and metabolism genes, and provide evidence indicating that S uptake is transcriptionally regulated at the level of sulfate permease.
Division of Health Professions and Natural Sciences FOOTNOTES www.plantphysiol.org/cgi/doi/10.1104/pp.104.900282 Related articles in Plant Physiol.:
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