- © 2010 American Society of Plant Biologists
Improving the Oil Yield of Maize Grain
The grain of maize (Zea mays) serves as the most important feedstock for meat, egg, milk, and fuel production in the world. Approximately 65% of maize grain is used for feeding animals. High oil maize shows a greater feed efficiency than normal oil maize in animal feed trials because the caloric content of oil is 225% greater than that of starch on a weight basis. Previous attempts to increase oil synthesis in plants have focused mainly on manipulation of oil pathway genes. As an alternative to single-enzyme approaches, the targeting of transcription factors provides an attractive solution for altering complex traits, with the caveat that the alteration of transcription factors may lead to undesirable pleiotropic effects. LEAFY COTYLEDON1 (LEC1) and WRINKLED1 (WRI1) have been identified as two key transcription factors involved in the regulation of oil accumulation in Arabidopsis (Arabidopsis thaliana). Shen et al. (pp. 980–987) report that the overexpression of ZmLEC1 increases seed oil in maize by as much as 48% but reduces seed germination and leaf growth. Since ZmWRI1 is a transcription factor that acts downstream, its overexpression might be expected to have less pleiotropic effects. Indeed, the authors report that the overexpression of ZmWRI1 results in an oil increase similar to overexpression of ZmLEC1 without affecting germination, seedling growth, or grain yield. These results highlight the potential application of transcription factors for increasing oil production in major crops and identify ZmWRI1 as a promising target for increasing oil production in crops.
A GA Precursor Regulates Moss Development
GAs play crucial roles in many aspects of plant growth and development, including seed germination, stem elongation, leaf expansion, trichome development, and flower and fruit development. But when during the course of plant evolution did GAs come to be so important? The moss Physcomitrella patens lacks both bioactive GAs and functional versions of many of the key signal pathway components associated with GA signal transduction in higher plants. P. patens does, however, produce ent-kaurene, a common precursor for GAs, and it does possess a functional ent-kaurene synthase (PpCPS/KS). A study by Hayashi et al. (pp. 1085–1097) suggests that ent-kaurene plays an important role in the early protonemal stage of moss development. To assess the biological role of ent-kaurene in P. patens, the authors generated a PpCPS/KS disruption mutant, which does not accumulate ent-kaurene. The loss of ent-kaurene production was confirmed by gas chromatography-mass spectrometry analysis. Phenotypically, the mutant had a defect in the protonemal differentiation of the chloronemata to caulonemata. The phenotypic defect was recovered by the application of ent-kaurene or ent-kaurenoic acid. Treatment with uniconazole, an inhibitor of ent-kaurene oxidase in GA biosynthesis, mimics the protonemal phenotypes of the PpCPS/KS mutant, which were also restored by ent-kaurenoic acid treatment. GA9 methyl ester, an antheridiogen of schizaeaceous ferns, also rescued the protonemal defect of the mutants, but GA3 and GA4, which are active GAs in angiosperms, did not. These results are consistent with the idea that P. patens utilizes GA-type diterpenes synthesized from ent-kaurene as endogenous growth regulators in protonemal development.
Inhibiting Flower Organ Abscission
Following flower pollination, unwanted floral organs are abscised and shed from the plant. The cells of the abscission zone recognize abscission signals that activate cell wall-loosening proteins, such as endoglucanases, polygalacturonases, and expansins. These enzymes weaken the middle lamella that binds adjacent plant cells to each other, thereby facilitating organ detachment from the main body of the plant. Floral organ abscission in Arabidopsis has proven to be a useful model system for studying the genetics underlying the abscission process. Wei et al. (pp. 1031–1045) report that a member of the DOF (for DNA binding with one finger) transcription factor family, AtDOF4.7, was expressed robustly in the abscission zone of Arabidopsis. The constitutive expression of AtDOF4.7 produced a phenotype that exhibited a defect in abscission. Anatomical analyses showed that the formation of the abscission zone was normal but that the middle lamella failed to dissolve between the cell walls. AtDOF4.7, a nuclear-localized transcription factor, demonstrated binding activity to the promoter of an abscission-related polygalacturonase gene, PGAZAT. The overexpression of AtDOF4.7 resulted in down-regulation of PGAZAT. These results suggest that AtDOF4.7 participates in the control of abscission as part of the transcription complex that directly regulates the expression of cell wall hydrolysis enzymes.
The Genetic Control of Leaf Size
The final size of plant organs, such as leaves, is under tight regulation by environmental and genetic factors that spatially and temporally coordinate cell expansion and cell cycle activity. Gonzalez et al. (pp. 1261–1279) have sought to gain more insight into the genetic control of leaf size in Arabidopsis by performing a comparative analysis of transgenic lines that produce enlarged leaves under standardized environmental conditions. To this end, they selected five genes, belonging to very different functional classes, that all positively affect leaf size when overexpressed: AVP1, GRF5, JAW, BRI1, and GA20OX1. An analysis of these lines, when grown under two experimental conditions, showed that the increase in leaf area depends on leaf position and environment and that all five lines affect leaf development in a distinct way. Nevertheless, in all five cases, an increase in cell number was, entirely or predominantly, responsible for the leaf size enlargement. Analyses of the hormone levels, transcriptome, and metabolome of individual lines were consistent with the above cellular analysis and showed that enhanced organ growth in these lines is governed by different, seemingly independent, molecular pathways. An analysis of transgenic lines simultaneously overexpressing several combinations of two growth-enhancing genes in Arabidopsis lent further support to the notion of independent converging pathways controlling leaf size in Arabidopsis.
Delayed Leaf Senescence Increases Yield in Rice
One approach toward increasing yield in rice (Oryza sativa) is to delay leaf senescence, thereby extending the time available for photosynthesis. One method for delaying senescence is directly spraying leaves with cytokinins (CKs). CKs can act to inhibit the transcription of Cys proteases, delaying degradation of nucleic acids and proteins. SAG39 is a rice gene that encodes a Cys protease. Liu et al. (pp. 1239–1249) have isolated the promoter for SAG39 (PSAG39) and determined that SAG39 expression is relatively low in mature leaves, but that SAG39 mRNA levels increase over the course of senescence, with the maximum accumulation of transcripts occurring at the latest stages of senescence. To test if PSAG39 could be useful for increasing rice yields by increasing CK content and delaying senescence, homozygous transgenic plants were obtained by linking PSAG39 to a gene (ipt) encoding isopentenyltransferase, an enzyme found in Agrobacterium tumefaciens that catalyzes a critical rate-limiting step in CK synthesis, and introducing this construct into a rice cultivar. The chlorophyll level of the flag leaf in these transgenic lines was used to monitor senescence. The “stay-green” phenotype of PSAG39:ipt transgenic rice was confirmed. Changes in the CK content in the transgenic rice led to early flowering and a greater number of emerged panicles 70 d after germination.
Flagellin Receptor Mediates Stomatal Response to Pathogen Invasion
Stomatal pores are major portals for pathogen entry into plants. Accordingly, guard cells have developed mechanisms to regulate stomatal aperture in response to pathogens. Previously, it has been found that a strain of the bacterial pathogen Pseudomonas syringae induces stomatal closure in Arabidopsis within 1 h post inoculation. However, after 3 to 4 h, the closed stomata reopen. The ability of Pseudomonas to reopen stomata is dependent on the polyketide toxin coronatine (COR), a virulence factor that had previously been shown to be important for bacterial multiplication within the mesophyll space, disease symptom development, and induction of systemic susceptibility of infected plants. The FLAGELLIN-SENSING2 (FLS2) receptor kinase is an important antipathogenic protein produced by Arabidopsis. FLS2 recognizes bacterial flagellin and initiates a battery of downstream defense responses to reduce bacterial invasion through stomata in the epidermis and bacterial multiplication in the apoplast of infected plants. Zeng and He (pp. 1188–1198) have conducted experiments to further characterize stomatal regulation during Pseudomonas infection of Arabidopsis plants. COR-deficient Pseudomonas mutants were severely reduced in virulence when inoculated onto the leaf surface of wild-type plants, but this defect was rescued almost fully in fls2 mutant plants. The responses of fls2 plants to COR were similar to those of the Arabidopsis G-protein alpha subunit1-3 mutant, which is defective in abscisic acid-regulated stomatal closure, but were distinct from those of the Arabidopsis non-expressor of PR genes1 mutant, which is defective in salicylic acid-dependent stomatal closure and apoplast defense.
Tomato Trichome Proteomics
Trichomes are specialized cells present on the surfaces of many plants and are capable of synthesizing and either storing or secreting large amounts of specialized metabolites, including many chemicals of medicinal interest. Because trichomes are located on plant surfaces and produce biologically active metabolites, they can protect against a number of environmental stresses, including herbivores and pathogens. Unlike Arabidopsis, which only makes nonglandular trichomes, cultivated tomato (Solanum lycopersicum) also contains glandular trichomes. Proteomic techniques provide a useful set of tools for discovery of enzymes and pathways. These techniques are especially powerful in studies of enriched cell types. The epidermal location of trichomes simplifies the analysis of their RNAs, proteins, and metabolites. The lack of genome sequence information limits the ability to discover proteins representing new genes in nonmodel species. Proteomics studies of such species rely on EST sequences. Because ESTs are enriched in protein-coding sequences, they can be especially useful for analyzing proteomics data. Schilmiller et al. (pp. 1212–1223) have generated a large trichome-specific EST collection and used it in combination with shotgun proteomics data from isolated tomato trichomes. These datasets were mined to identify genes and proteins expressed in trichomes. The utility of this approach was demonstrated by the authors’ characterization of a novel sesquiterpene synthase that produces β-caryophyllene and α-humulene in tomato trichomes.
