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Plant Physiology 146:1020-1021 (2008) © 2008 American Society of Plant Biologists On the InsideA Key Gene Involved in Prenylated Flavonoid BiosynthesisPrenylated flavonoids protect higher plants against bacteria and fungi, and are active components in many medicinal plants that are effective against human diseases, including cancer and leishmaniasis. Prenylation reactions couple two major metabolic pathways, the shikimate-acetate and isoprenoid pathways. Prenylated flavonoids are hybrid products composed of a flavonoid core mainly attached to either 5-carbon (dimethylallyl) or 10-carbon (geranyl) prenyl groups derived from isoprenoid (terpenoid) metabolism: The prenyl groups are crucial for their biological activity. The prenylation of aromatic compounds contributes greatly to the diversity of plant secondary metabolites. This molecular diversity arises from differences in prenylation position on the aromatic ring, various lengths of prenyl chain, and further modifications of the prenyl moiety, resulting in the occurrence of more than 1,000 prenylated compounds in plants. Despite the importance and complexity of prenylated flavonoids, none of the genes responsible for the prenylation reactions has been identified despite more than 30 years of research in this field. Cell cultures of Sophora flavescens produce the prenylated flavonoid sophoraflavanone G (SFG) in large quantity. The biosynthesis of SFG involves two plastid-localized prenylation reactions. In this issue, Sasaki et al. (pp. 1075–1084) describe the identification of a plant flavonoid prenyltransferase gene encoding naringenin 8-dimethylallyltransferase (SfN8DT-1) that catalyzes the primary dimethylallylation step in SFG biosynthesis. Phylogenetic analysis shows that SfN8DT-1 has the same evolutionary origin as prenyltransferases for vitamin E and plastoquinone. The expression of SfN8DT-1 is strictly limited to the root bark where prenylated flavonoids are solely accumulated in planta. The ectopic expression of SfN8DT-1 in Arabidopsis (Arabidopsis thaliana) resulted in the formation of prenylated apigenin, quercetin, and kaempferol, as well as 8-prenylnaringenin. Parasitic Plants Affect Plant Defenses against Insects To date, research concerning induced plant defenses and cross talk during defense signaling has focused almost exclusively on herbivorous arthropods and pathogens. But plants also must defend themselves from attack by other plants. Approximately 4,500 species of flowering plants (about 1%) are parasitic and attach to other plants to obtain water and nutrients. Dodders (Cuscuta spp.) are one of the most ecologically and economically significant groups of parasitic plants. Despite their economic importance and the profound effects they have on host plants and community dynamics, relatively little is known about the defenses induced by parasitic plant attack or how these defenses affect host plant interactions with other organisms. In this issue, Runyon et al. (pp. 987–995) explore how an attack by the parasitic plant Cuscuta pentagona impacts tomato (Solanum lycopersicum) defenses against the chewing insect beet armyworm (Spodoptera exigua). In response to insect feeding, C. pentagona-parasitized tomato plants produced only one-third of the antiherbivore phytohormone jasmonic acid (JA) produced by unparasitized plants. This is not due to a siphoning off of JA from the host plant to dodder: C. pentagona vines did not translocate JA from caterpillar-infested plants. Parasitized tomato plants also failed to emit herbivore-induced volatiles after 3 d of caterpillar attack. Despite the impaired antiherbivore defenses induced by dodder, the growth of beet armyworms was slower on parasitized tomato leaves. Parasitized plants were capable of producing induced volatiles when experimentally treated with JA, indicating that resource depletion by the parasite does not fully explain the observed attenuation of volatile response to herbivore feeding. Collectively, these findings show that parasitic plants can have important consequences for host plant defense against herbivores. Plant Myosins and Cytoplasmic Streaming Myosins are involved in a wide range of actin-mediated processes. Comparative genomics has revealed that myosins are conserved throughout the eukaryotes. Land plants possess two myosin classes, XI and VIII, each of which is evolutionary related to animal and fungal class V myosins. Phylogenetic analyses by Avisar et al. (pp. 1098–1108) indicate that these two classes of myosins diverged prior to the radiation of green algae and land plants from a common ancestor, and that the common ancestor of land plants likely possessed at least seven myosins. These authors also show that the movement of Golgi stacks, mitochondria, and peroxisomes in the leaf cells of Nicotiana benthamiana is mediated mainly by myosin XI-K. Suppression of myosin XI-K function by dominant negative inhibition or RNA interference was found to dramatically reduce the movement of each of these organelle types. When similar approaches were used to inhibit functions of myosin XI-2 or XI-F, only moderate to marginal effects were observed. Organelle trafficking was virtually unaffected in response to inhibition of each of the three class VIII myosins. Interestingly, none of the tested six myosins appears to be involved in light-induced movements of chloroplasts. In addition, the authors' analysis of thousands of individual organelles revealed independent movement patterns for Golgi stacks, mitochondria, and peroxisomes, indicating that the notion of coordinated cytoplasmic streaming is not generally applicable to higher plants. In an accompanying article, Peremyslov et al. (pp. 1109–1116) report on their successful isolation of gene knockout mutants for all 13 class XI myosins present in the Arabidopsis genome. Inactivation of 11 myosin genes resulted in no discernible phenotypes under the normal growth conditions. In contrast, the knockouts of the remaining two myosin genes, XI-2 and XI-K, exhibited defects in root hair elongation suggesting that the myosin-driven motility plays a significant role in a polar tip growth. Each of these two highly expressed myosins was shown to function in the rapid movement of Golgi stacks, peroxisomes, and mitochondria in roots and leaves. These results indicate that the evolution of myosins in plants involved the opposing tendencies of functional specialization and functional redundancy. A New "Tie-Dyed" Mutant Regulation of carbon partitioning is essential for plant growth and development. To gain insight into genes controlling carbon allocation in leaves, Baker and Braun (pp. 1085–1097) have identified mutants that hyperaccumulate carbohydrates. tie-dyed2 (tdy2) is a new, recessive mutant of maize (Zea mays) that exhibits chlorotic leaf sectors containing excess starch and soluble sugars. Similar to tdy1, tdy2 mutants also have reduced stature, delayed flowering, and decreased seed yield, presumably due to the retention of carbohydrates in leaves. To determine if Tdy2 acts in the same genetic pathway as Tdy1, the authors employed double mutant constructs. F1 plants that were doubly heterozygous for tdy1 and tdy2 exhibited a moderately sectored phenotype. This type of genetic interaction whereby doubly heterozygous loci produce a mutant phenotype in the F1 generation is called second site noncomplementation. In many cases, second site noncomplementation is indicative of a physical association between the proteins encoded by the two loci, and suggests that TDY1 and TDY2 may function in the same genetic pathway and physically interact. Double mutant plants show a synergistic interaction of severely chlorotic leaves consistent with this hypothesis. The authors also report that that the leaves of tdy mutants have increased cellulose levels, suggesting that these stocks may serve as enhanced feedstocks for the production of biofuels. The Elicitor Cryptogein Stimulates Receptor-Mediated Endocytosis Cryptogein, produced by the oomycete Phytophthora cryptogea, belongs to a class of proteinaceous elicitors called elicitins, able to induce a hypersensitive response and systemic acquired resistance in plants. The mode of action of cryptogein begins with the recognition of this elicitor by an unidentified plasma membrane receptor at a high-affinity binding site. This ligand-receptor binding triggers a cascade of events that include phosphorylation processes, rapid calcium influx, ion effluxes, nitric oxide production, extracellular alkalinization, and plasma membrane depolarization both in tobacco (Nicotiana tabacum) plants and cell suspensions. In addition, the activation of a membrane-bound NADPH oxidase (NtrbohD), responsible for reactive oxygen species (ROS) production, has been shown in cryptogein-elicited cells. Leborgne-Castel et al. (pp. 1255–1266) report that cryptogein also stimulates the internalization of the lipophilic dye FM4-64, which is a marker of endocytosis, within minutes in tobacco Bright Yellow-2 (BY-2) cells. This stimulation is specific to the signal transduction pathway elicited by cryptogein because a lipid transfer protein, which binds to the same receptor as cryptogein but without triggering signaling, does not increase endocytosis. The authors also note that there is a transient increase in clathrin-coated pits at the plasma membrane coinciding with a burst of ROS production that occurs within the first 15 min after elicitation. In the presence of cryptogein, increases in both FM4-64 internalization and clathrin-mediated endocytosis are specifically blocked upon treatment with tyrphostin A23, an inhibitor of receptor-mediated endocytosis in other systems. In BY-2 cells expressing NtrbohD antisense cDNA, which are unable to produce ROS when treated with cryptogein, the stimulation of clathrin-coated pit formation is inhibited. These results indicate that the very early endocytic process induced by cryptogein in tobacco is due, at least partly, to clathrin-mediated endocytosis and is dependent on ROS production by the NADPH oxidase NtrbohD. Aquaporin Function in a Moss Aquaporins (AQPs) are transmembrane proteins that increase the water permeability of cellular membranes. Under most physiological conditions, however, water appears to simply diffuse across the lipid bilayer, and it has been difficult to show that water movement requires an increase in the water permeability of the membrane bilayer by the action of AQPs. It has been known for some time that AQPs are present in the moss Physcomitrella patens, a well-established model system for various biological and evolutionary processes in higher plants. Elucidation of the function of AQPs in bryophytes may hint at their function during the early stages of land plant evolution. Liénard et al. (pp. 1207–1218) provide evidence that AQPs may aid in the replenishment of cell water that is lost by transpiration from the "leafy" gametophore of Physcomitrella. The authors report on their successful cloning of three putative PIPs (plasma membrane intrinsic proteins) in Physcomitrella, PIP2;1, PIP2;2, and PIP2;3. Unlike the case with PIP2;3, the knocking out of PIP2;1 or PIP2;2 resulted in a significant decrease in the water permeability of protoplasts isolated from mutant gametophores. No unusual phenotypes were observed when knockout plants were grown in closed petri dishes with ample water supply. However, the suppression of PIP2;1 and PIP2;2 reduced the drought tolerance of gametophores. Based on these results, it appears that AQPs were already used by ancestral land plants some 400 million years ago to facilitate the absorption of water, and that both PIP2;1 and PIP2;2 encode functional AQPs.
Division of Health Professions and Natural Sciences FOOTNOTES www.plantphysiol.org/cgi/doi/10.1104/pp.104.900251 Related articles in Plant Physiol.:
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