Robert T. Furbank

Suppression of Sucrose Synthase Gene Expression Represses Cotton Fiber Cell Initiation, Elongation, and Seed Development

Cotton is the most important textile crop as a result of its long cellulose-enriched mature fibers. These single-celled hairs initiate at anthesis from the ovule epidermis. To date, genes proven to be critical for fiber development have not been identified. Here, we examined the role of the sucrose synthase gene (Sus) in cotton fiber and seed by transforming cotton with Sus suppression constructs. We focused our analysis on 0 to 3 days after anthesis (DAA) for early fiber development and 25 DAA, when the fiber and seed are maximal in size. Suppression of Sus activity by 70% or more in the ovule epidermis led to a fiberless phenotype. The fiber initials in those ovules were fewer and shrunken or collapsed. The level of Sus suppression correlated strongly with the degree of inhibition of fiber initiation and elongation, probably as a result of the reduction of hexoses. By 25 DAA, a portion of the seeds in the fruit showed Sus suppression only in the seed coat fibers and transfer cells but not in the endosperm and embryo. These transgenic seeds were identical to wild-type seeds except for much reduced fiber growth. However, the remaining seeds in the fruit showed Sus suppression both in the seed coat and in the endosperm and embryo. These seeds were shrunken with loss of the transfer cells and were <5% of wild-type seed weight. These results demonstrate that Sus plays a rate-limiting role in the initiation and elongation of the single-celled fibers. These analyses also show that suppression of Sus only in the maternal seed tissue represses fiber development without affecting embryo development and seed size. Additional suppression in the endosperm and embryo inhibits their own development, which blocks the formation of adjacent seed coat transfer cells and arrests seed development entirely.

The Control of Single-Celled Cotton Fiber Elongation by Developmentally Reversible Gating of Plasmodesmata and Coordinated Expression of Sucrose and K+ Transporters and Expansin

Each cotton fiber is a single cell that elongates to 2.5 to 3.0 cm from the seed coat epidermis within ∼16 days after anthesis (DAA). To elucidate the mechanisms controlling this rapid elongation, we studied the gating of fiber plasmodesmata and the expression of the cell wall–loosening gene expansin and plasma membrane transporters for sucrose and K+, the major osmotic solutes imported into fibers. Confocal imaging of the membrane-impermeant fluorescent solute carboxyfluorescein (CF) revealed that the fiber plasmodesmata were initially permeable to CF (0 to 9 DAA), but closed at ∼10 DAA and re-opened at 16 DAA. A developmental switch from simple to branched plasmodesmata was also observed in fibers at 10 DAA. Coincident with the transient closure of the plasmodesmata, the sucrose and K+ transporter genes were expressed maximally in fibers at 10 DAA with sucrose transporter proteins predominately localized at the fiber base. Consequently, fiber osmotic and turgor potentials were elevated, driving the rapid phase of elongation. The level of expansin mRNA, however, was high at the early phase of elongation (6 to 8 DAA) and decreased rapidly afterwards. The fiber turgor was similar to the underlying seed coat cells at 6 to 10 DAA and after 16 DAA. These results suggest that fiber elongation is initially achieved largely by cell wall loosening and finally terminated by increased wall rigidity and loss of higher turgor. To our knowledge, this study provides an unprecedented demonstration that the gating of plasmodesmata in a given cell is developmentally reversible and is coordinated with the expression of solute transporters and the cell wall–loosening gene. This integration of plasmodesmatal gating and gene expression appears to control fiber cell elongation.

Raising yield potential of wheat. II. Increasing photosynthetic capacity and efficiency

Past increases in yield potential of wheat have largely resulted from improvements in harvest index rather than increased biomass. Further large increases in harvest index are unlikely, but an opportunity exists for increasing productive biomass and harvestable grain. Photosynthetic capacity and efficiency are bottlenecks to raising productivity and there is strong evidence that increasing photosynthesis will increase crop yields provided that other constraints do not become limiting. Even small increases in the rate of net photosynthesis can translate into large increases in biomass and hence yield, since carbon assimilation is integrated over the entire growing season and crop canopy. This review discusses the strategies to increase photosynthesis that are being proposed by the wheat yield consortium in order to increase wheat yields. These include: selection for photosynthetic capacity and efficiency, increasing ear photosynthesis, optimizing canopy photosynthesis, introducing chloroplast CO(2) pumps, increasing RuBP regeneration, improving the thermal stability of Rubisco activase, and replacing wheat Rubisco with that from other species with different kinetic properties.

The C4 pathway: An efficient CO2 pump

The C4 pathway is a complex combination of both biochemical and morphological specialisation, which provides an elevation of the CO2 concentration at the site of Rubisco. We review the key parameters necessary to make the C4 pathway function efficiently, focussing on the diffusion of CO2 out of the bundle sheath compartment. Measurements of cell wall thickness show that the thickness of bundle sheath cell walls in C4 species is similar to cell wall thickness of C3 mesophyll cells. Furthermore, NAD-ME type C4 species, which do not have suberin in their bundle sheath cell walls, do not appear to compensate for this with thicker bundle sheath cell walls. Uncertainties in the CO2 diffusion properties of membranes, such as the plasmalemma, choroplast and mitochondrial membranes make it difficult to estimate bundle sheath diffusion resistance from anatomical measurements, but the cytosol itself may account for more than half of the final calculated resistance value for CO2 leakage. We conclude that the location of the site of decarboxylation, its distance from the mesophyll interface and the physical arrangement of chloroplasts and mitochondria in the bundle sheath cell are as important to the efficiency of the process as the properties of the bundle sheath cell wall. Using a mathemathical model of C4 photosynthesis, we also examine the relationship between bundle sheath resistance to CO2 diffusion and the biochemical capacity of the C4 photosynthetic pathway and conclude that bundle sheath resistance to CO2 diffusion must vary with biochemical capacity if the efficiency of the C4 pump is to be maintained. Finally, we construct a mathematical model of single cell C4 photosynthesis in a C3 mesophyll cell and examine the theoretical efficiency of such a C4 photosynthetic CO2 pump.

The mechanisms contributing to photosynthetic control of electron transport by carbon assimilation in leaves.

Abstract‘Photosynthetic control’ describes the processes that serve to modify chloroplast membrane reactions in order to co-ordinate the synthesis of ATP and NADPH with the rate at which these metabolites can be used in carbon metabolism. At low irradiance, optimisation of the use of excitation energy is required, while at high irradiance photosynthetic control serves to dissipate excess excitation energy when the potential rate of ATP and NADPH synthesis exceed demand. The balance between ΔpH, ATP synthesis and redox state adjusts supply to demand such that the [ATP]/[ADP] and [NADPH]/[NADP+] ratios are remarkably constant in steady-state conditions and modulation of electron transport occurs without extreme fluctuations in these pools.

New phenotyping methods for screening wheat and barley for beneficial responses to water deficit

This review considers stomatal conductance as an indicator of genotypic differences in the growth response to water stress. The benefits of using stomatal conductance are compared with photosynthetic rate and other indicators of genetic variation in water stress tolerance, along with the use of modern phenomics technologies. Various treatments for screening for genetic diversity in response to water deficit in controlled environments are considered. There is no perfect medium: there are pitfalls in using soil in pots, and in using hydroponics with ionic and non-ionic osmotica. Use of mixed salts or NaCl is recommended over non-ionic osmotica. Developments in infrared thermography provide new and feasible screening methods for detecting genetic variation in the stomatal response to water deficit in controlled environments and in the field.

Achieving yield gains in wheat

Wheat provides 20% of calories and protein consumed by humans. Recent genetic gains are <1% per annum (p.a.), insufficient to meet future demand. The Wheat Yield Consortium brings expertise in photosynthesis, crop adaptation and genetics to a common breeding platform. Theory suggest radiation use efficiency (RUE) of wheat could be increased ~50%; strategies include modifying specificity, catalytic rate and regulation of Rubisco, up-regulating Calvin cycle enzymes, introducing chloroplast CO(2) concentrating mechanisms, optimizing light and N distribution of canopies while minimizing photoinhibition, and increasing spike photosynthesis. Maximum yield expression will also require dynamic optimization of source: sink so that dry matter partitioning to reproductive structures is not at the cost of the roots, stems and leaves needed to maintain physiological and structural integrity. Crop development should favour spike fertility to maximize harvest index so phenology must be tailored to different photoperiods, and sensitivity to unpredictable weather must be modulated to reduce conservative responses that reduce harvest index. Strategic crossing of complementary physiological traits will be augmented with wide crossing, while genome-wide selection and high throughput phenotyping and genotyping will increase efficiency of progeny screening. To ensure investment in breeding achieves agronomic impact, sustainable crop management must also be promoted through crop improvement networks.

The Development of C4 Rice: Current Progress and Future Challenges

Another “green revolution” is needed for crop yields to meet demands for food. The international C4 Rice Consortium is working toward introducing a higher-capacity photosynthetic mechanism—the C4 pathway—into rice to increase yield. The goal is to identify the genes necessary to install C4 photosynthesis in rice through different approaches, including genomic and transcriptional sequence comparisons and mutant screening.

Regulation of Photosynthesis in C3 and C4 Plants: A Molecular Approach.

Most plants use the C3 pathway of photosynthesis, also called the photosynthetic carbon reduction cycle (PCR), shown in Figure 1A. C3 plants have a single chloroplast type that performs all of the reactions that convert light energy into the chemical energy that is used to fix COp and to synthesize the reduced carbon compounds upon which all life depends. Ribulose-i ,5-bisphosphate carboxylaseloxygenase (Rubisco) catalyzes primary carbon fixation, in which a fivecarbon sugar phosphate, ribulose-l,5-bisphosphate (RuBP), and COp are converted to two molecules of the threecarbon compound 3-phosphoglycerate (hence the name C3). Phosphoglycerate is then phosphorylated and reduced by the products of the light reactions of photosynthesis (ATP and NADPH) to produce triose phosphate (TP). TP can be exported from the chloroplast via the chloroplast envelope phosphate (Pi) transporter to the cytosol and used in the synthesis of sucrose, which is then translocated throughout the plant (see Sonnewald et al., 1994), or it can be retained within the chloroplast for starch synthesis or recycling to RuBP. Rubisco also catalyzes the fixation of Op in a process known as photorespiration, which competes directly with fixation of COp. At air levels of Coa, for every three COp molecules fixed by Rubisco to form 3-phosphoglycerate, approximately one O2 molecule is fixed, producing Sphosphoglycerate and 3-phosphoglycolate (Figure 1A). Because 3-phosphoglycolate cannot be used in the PCR cycle, it must be recycled to phosphoglycerate via the photorespiratory pathway, expending ATP and NADPH. This competition between 0 2 and COp and the energy costs associated with recycling phosphoglycolate largely determine the efficiency of C3 photosynthesis in air (Hatch, 1988; Woodrow and Berry, 1988). The C4 pathway is a complexadaptation of the C3 pathway that overcomes the limitation of photorespiration and is found in a diverse collection of species, many of which grow in hot climates with sporadic rainfall. The C4 pathway effectively suppresses photorespiration by elevating the C02 concentration at the site of Rubisco using a biochemical C02 pump. C4 plants have two chloroplast types, each found in a specialized cell type. Leaves of C4 plants show extensive vascularization,

Carbohydrate Content and Enzyme Metabolism in Developing Canola Siliques

Little biochemical information is available on carbohydrate metabolism in developing canola (Brassica napus L.) silique (pod) wall and seed tissues. This research examines the carbohydrate contents and sucrose (Suc) metabolic enzyme activities in different aged silique wall and seed tissues during oil filling. The silique wall partitioned photosynthate into Suc over starch and predominantly accumulated hexose. The silique wall hexose content and soluble acid invertase activity rapidly fell as embryos progressed from the early- to late-cotyledon developmental stages. A similar trend was not evident for alkaline invertase, Suc synthase (SuSy), and Suc-phosphate synthase. Silique wall SuSy activities were much higher than source leaves at all times and may serve to supply the substrate for secondary cell wall thickening. In young seeds starch was the predominant accumulated carbohydrate over the sampled developmental range. Seed hexose levels dropped as embryos developed from the early- to midcotyledon stage. Hexose and starch were localized to the testa or liquid endosperm, whereas Suc was evenly distributed among seed components. With the switch to oil accumulation, seed SuSy activity increased by 3.6-fold and soluble acid invertase activity decreased by 76%. These data provide valuable baseline knowledge for the genetic manipulation of canola seed carbon partitioning.