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Laccase Confers Biotic Stress Tolerance in Cotton
Cotton (Gossypium spp.) is a globally cultivated crop of vast economic importance. Pathogens and pests are major limitations to cotton yield and quality. Verticillium wilt, caused by the fungus Verticillium dahliae, is the disease most detrimental to cotton production. Cotton bollworm (Helicoverpa armigera), once the most serious insect pests of cotton, has been well controlled by the widely planted Bt cotton. Monocultures of Bt cotton, however, can lead to pest resistance, and in fields planted with Bt cotton, the decreased spraying of pesticides has resulted in non-Bt-targeted pests becoming new key pests, particularly piercing-sucking insects such as cotton aphid (Aphis gosypii). Phenylpropanoid metabolism is the most important secondary metabolic pathway involved in plant defense against biotic and abiotic stresses. Lignin synthesis is one of the branches of the phenylpropanoid pathway, and the polymerization of monolignol to form lignin provides mechanical strength and reinforces cell walls to provide a physical barrier to limit pathogen colonization. Previous work has shown that laccases function as lignin polymerization enzymes. Using this information, Hu et al. (pp. 1808–1823) have developed a new strategy for the engineering of cotton resistance to both fungal pathogens and insect pests. They report that the overexpression of GhLac1 leads to an increase in lignification that is associated with increased tolerance to V. dahliae and to the insect pests cotton bollworm and cotton aphid. Suppression of GhLac1 expression leads to a redirection of the metabolic flux through the phenylpropanoid pathway, leading to the accumulation of jasmonic acid and secondary metabolites that confer resistance to V. dahliae and cotton bollworm but to an increased susceptibility to cotton aphid. Plant laccases therefore provide a new molecular tool to engineer pest and pathogen resistance in crops.
Anthocyanins on Demand
Anthocyanins are vacuolar pigments derived from the phenylpropanoid pathway that are produced in many different plant species. The role of anthocyanin accumulation under stress in vegetative tissues is probably linked to the scavenging of reactive oxygen species. Anthocyanins are powerful antioxidants, and as part of human diet in seeds, fruits and leaves are proposed to have health-promoting properties. It has been shown that the consumption of anthocyanins can lower the risk of cancer, diabetes, and cardiovascular diseases. To be able to breed for fruits and vegetables that are rich in anthocyanins, it is important to understand both their biosynthesis and functions in plants. To this end, Outchkourov et al. (pp. 1862–1878) have developed an inducible dexamethasone-regulated switch that can deliver, on demand, anthocyanin accumulation in different tissues of the tomato (Solanum lycopersicum) cultivar MicroTom. Following the application of dexamethasone, anthocyanin formation was induced within 24 h in vegetative tissues and in undifferentiated cells. This inducible system reveals a range of phenomena that accompany anthocyanin biosynthesis in tomato, including profound physiological and architectural changes, depending on the tissue, such as root branching, root epithelial cell morphology, seed germination, and leaf conductance. A number of pathways that are not known to be involved in anthocyanin biosynthesis were also observed to be regulated.
Mineral Deposits in Ficus Leaves
Mineral deposits occur in many but not all plant leaves. In those leaves that do have mineral deposits, the mineral type, morphology, and the distributions within the leaves are under strict control. In fact, mineralization in certain leaves is a well-preserved trait throughout evolution, indicating that such controlled mineralization is advantageous. The most widespread leaf minerals are silica and calcium oxalate, and, to a lesser extent, amorphous calcium carbonate. Many roles have been ascribed to calcium oxalates in leaves, including calcium regulation, detoxification of heavy metals, leaf defense, and an internal CO2 source. Silica deposition (phytoliths and small particles) can occur in any part of the leaf epidermis, leaf mesophyll, and in the vascular tissue. Cystoliths are calcified bodies composed of hydrated amorphous calcium carbonate deposited in specialized epidermal cells. In many plant species, cystoliths or calcium oxalates function as light scatterers, increasing the leaf photosynthetic yield. Ficus leaves can deposit one or more of these major mineral types found in leaves. To better understand the functions of these minerals and the control that the leaf exerts over mineral deposition, Pierantoni et al. (pp. 1751–1763) investigated leaves from 10 Ficus species from vastly different environments. They report that each Ficus species is characterized by a unique 3D mineral distribution, which is preserved in different environments. The mineral distribution patterns are generally different on the adaxial and abaxial sides of the leaf. All species examined have abundant calcium oxalate deposits around the veins. The authors used micromodulated fluorimetry to examine the effect of cystoliths on photosynthetic efficiency in two species having cystoliths abaxially and adaxially (Ficus microcarpa) or only abaxially (Ficus carica). In F. microcarpa both adaxial and abaxial cystoliths efficiently contribute to light redistribution inside the leaf and hence increase photosynthetic efficiency, whereas in F. carica the abaxial cystoliths do not increase photosynthetic efficiency.
Glc-Induced Trophic Shift in an Endosymbiotic Dinoflagellate
Dinoflagellates in the genus Symbiodinium have the ability to enter into endosymbiotic associations with corals, providing the metabolic basis for the highly productive and biologically diverse coral-reef ecosystems. The Symbiodinium-coral association is highly susceptible to environmental perturbations such as high temperature that can result in the loss of the algae from coral tissue (“coral bleaching”) and ultimately lead to the death of the host and destruction of the reef. This phenomenon is plaguing reef communities worldwide. Metabolite exchange is essential to the success of coral-dinoflagellate symbioses. The algal symbionts are photosynthetically competent, and much of the CO2 that they fix can be accessed by the host via translocation of photosynthetic products (e.g. sugars, amino acids, lipids, carbohydrates, and small peptides) from the algae to the host. Although carbon flow between the partners is a hallmark of this mutualism, the mechanisms governing this flow and its impact on symbiosis remain poorly understood. Symbiodinium strain SSB01 can grow photoautotrophically, or it can grow mixotrophically or heterotrophically when supplied with Glc, a metabolite normally transferred from the alga to its host. Xiang et al. (pp. 1793–1807) now show that Glc supplementation of SSB01 cultures causes a loss of pigmentation and photosynthetic activity, disorganization of thylakoid membranes, accumulation of lipid bodies, and alterations of cell-surface morphology. Glc-supplemented cells exhibited a marked reduction in levels of plastid transcripts encoding photosynthetic proteins, although most nuclear-encoded transcripts (including those for proteins involved in lipid synthesis and formation of the extracellular matrix) exhibited little change in their abundances (except for a doubling of some nuclear-encoded transcripts for sugar transporters). Thus, Symbiodinium undergoes dramatic physiological changes in response to Glc that are not reflected by major changes in the abundances of nuclear-encoded transcripts and thus presumably reflect posttranscriptional regulatory processes.
Epigenetic Divergence Associated with Heterosis
Heterosis refers to the tendency of a crossbred individual to show qualities superior to those of both parents. The phenomenon has been exploited extensively in agricultural breeding for decades and has improved crop performance enormously. Despite its commercial impact, knowledge of the molecular basis underlying heterosis remains incomplete. Most studies have focused on finding genetic explanations, resulting in the classical dominance and overdominance models of heterosis. Genetic explanations, however, do not sufficiently explain or predict heterosis. There is growing evidence that epigenetic factors also play a role in heterosis. Recent studies have implicated changes in DNA methylation and small RNAs in hybrid performance; however, it remains unclear whether epigenetic changes are a cause or a consequence of heterosis. Lauss et al. (pp. 1627–1645) have analyzed a large panel of more than 500 Arabidopsis (Arabidopsis thaliana) epigenetic hybrid plants (epiHybrids), which they derived from near-isogenic but epigenetically divergent parents. This experimental system allowed them to quantify the contribution of parental methylation differences to heterosis. They measured traits such as leaf area, growth rate, flowering time, main stem branching, rosette branching, and final plant height and observed several strong positive and negative heterotic phenotypes among the epiHybrids. Using an epigenetic quantitative trait locus mapping approach, they succeeded in identifying specific differentially methylated regions in the parental genomes that are associated with hybrid performance. Sequencing of methylomes, transcriptomes, and genomes of selected parent-epiHybrid combinations further showed that these parental differentially methylated regions most likely mediate the remodeling of methylation and transcriptional states at specific loci in the hybrids. Taken together, their data suggest that locus-specific epigenetic divergence between the parental lines can directly or indirectly trigger heterosis in Arabidopsis hybrids independent of genetic changes. These results add to a growing body of evidence that points to epigenetic factors as one of the key determinants of hybrid performance.
Boron Transport in Rice
Boron (B) is an essential micronutrient for plant growth and development. Its major physiological function is to maintain the structure of the cell wall by crosslinking pectic polysaccharides through borate-diol bonding of two rhamnogalacturonan II molecules. B is immobile in most plant species. Therefore, a continuous supply of B is required to maintain growth of newly developing tissues, and deficiency of B results in the cessation of root elongation, reduced leaf expansion, and loss of fertility. On the other hand, B also shows toxicity to plants when present in excess. Both B deficiency and toxicity cause crop losses in many areas of the world. B is preferentially delivered to developing tissues in many plant species under conditions of low or no transpiration, but the molecular mechanisms underlying the preferential distribution of B to these developing tissues are poorly understood. Here, Shao et al. (pp. 1739–1750) present evidence that a member of nodulin 26-like intrinsic protein (NIP), OsNIP3;1, is involved in this preferential distribution in rice (Oryza sativa). OsNIP3;1 was highly expressed in the nodes, and its expression was up-regulated by B deficiency but down-regulated by high B. OsNIP3;1 was localized at the xylem parenchyma cells of enlarged vascular bundles of nodes facing toward the xylem vessels. Furthermore, this protein was rapidly degraded within a few hours in response to high B. Knockout of this gene scarcely affected the uptake and root-to-shoot translocation of B, but altered B distribution in different organs in the shoot. These results indicate that OsNIP3;1 located in the nodes is involved in the preferential distribution of B to the developing tissues by unloading B from the xylem in rice and that it is regulated at both the transcriptional and protein level in response to external B level.
