Comparing photosynthetic and photovoltaic efficiencies is not a simple issue. Although both processes harvest the energy in sunlight, they operate in distinctly different ways and produce different types of products: biomass or chemical fuels in the case of natural photosynthesis and nonstored electrical current in the case of photovoltaics. In order to find common ground for evaluating energy-conversion efficiency, we compare natural photosynthesis with present technologies for photovoltaic-driven electrolysis of water to produce hydrogen. Photovoltaic-driven electrolysis is the more efficient process when measured on an annual basis, yet short-term yields for photosynthetic conversion under optimal conditions come within a factor of 2 or 3 of the photovoltaic benchmark. We consider opportunities in which the frontiers of synthetic biology might be used to enhance natural photosynthesis for improved solar energy conversion efficiency.

The world's crop productivity is stagnating whereas population growth, rising affluence, and mandates for biofuels put increasing demands on agriculture. Meanwhile, demand for increasing cropland competes with equally crucial global sustainability and environmental protection needs. Addressing this looming agricultural crisis will be one of our greatest scientific challenges in the coming decades, and success will require substantial improvements at many levels. We assert that increasing the efficiency and productivity of photosynthesis in crop plants will be essential if this grand challenge is to be met. Here, we explore an array of prospective redesigns of plant systems at various scales, all aimed at increasing crop yields through improved photosynthetic efficiency and performance. Prospects range from straightforward alterations, already supported by preliminary evidence of feasibility, to substantial redesigns that are currently only conceptual, but that may be enabled by new developments in synthetic biology. Although some proposed redesigns are certain to face obstacles that will require alternate routes, the efforts should lead to new discoveries and technical advances with important impacts on the global problem of crop productivity and bioenergy production.light capture/conversion | carbon capture/conversion | smart canopy | enabling plant biotechnology tools | sustainable crop production Increasing demands for global food production over the next several decades portend a huge burden on the world's shrinking farmlands. Increasing global affluence, population growth, and demands for a bioeconomy (including livestock feed, bioenergy, chemical feedstocks, and biopharmaceuticals) will all require increased agricultural productivity, perhaps by as much as 60-120% over 2005 levels (e.g., refs. 1 and 2), putting increased productivity on a collision course with environmental and sustainability goals (3). The 45 y from 1960 to 2005 saw global food production grow ∼160%, mostly (135%) by improved production on

Cd is a potentially toxic metal that can accumulate in the human body with a half-life exceeding 10 years. There is evidence that low-leve1 exposure to Cd, derived from the diet, is associated with renal dysfunction (Buchet et al., 1990). Cd exposure has also been linked with pulmonary emphysema (Ryan et al., 1982) and possibly bone demineralization (Bhattacharyya et al., 1988). Pollution of the biosphere with this toxic metal has accelerated dramatically since the beginning of the industrial revolution (Nriago, 19791, and Cd accumulation in soil and water now poses a major environmental and human health problem, which is in need of an effective and affordable solution. The use of metal-accumulating plants to remove toxic metals, including Cd, from soil and aqueous streams has been proposed as a possible solution to this problem (reviewed by . This process of using plants for environmental restoration is termed "phytoremediation." Cd is a particularly favorable target metal for this new technology because it is readily transported and accumulated in the shoots of severa1 plant species (Wagner, 1994 The primary point of entry for Cd into plants is through the roots; however, for its efficient remova1 from the soil it must first be translocated to the harvestable parts of the shoot. To understand this translocation process, we undertook a study of Cd transport and accumulation in E. juncea, a high biomass crop plant within the Brassicaceae family, which has been identified as a potentially useful plant for phytoremediation (Banuelos and Meek, 1990). For comparison we have investigated a low biomass wild species within the same family, Thlaspi caerulescens, known to accumulate high levels of Cd in its shoots under certain conditions (Baker et al., 1994; Brown et al., 1994Brown et al., , 1995. MATERIALS AND METHODS Plant MaterialSeeds of Indian mustard (Brassica juncea L., cv 4263081, identified as a metal accumulator , and 24-28°C). The solutions were continuously aerated with an aquarium air pump and changed every 3 d. On the day of treatment, Cd was added to the hydroponic medium as CdSO, and the solutions were changed daily thereafter. Cd Accumulation in PlantsTotal shoot and root accumulation of Cd, Mn, and Cu in B. juncea and T. caerulescens were determined after 7 d of exposure to Cd, using a direct current plasma spectrometer (model SS-7, Fisons, Beverly, MA). Roots and shoots were harvested, washed in deionized water for 2 min, air dried at 60°C for 2 d, and then ground into a fine powder using a pestle and mortar.

The bioaccumulation of arsenic by plants may provide a means of removing this element from contaminated soils and waters. However, to optimize this process it is important to understand the biological mechanisms involved. Using a combination of techniques, including x-ray absorption spectroscopy, we have established the biochemical fate of arsenic taken up by Indian mustard (Brassica juncea). After arsenate uptake by the roots, possibly via the phosphate transport mechanism, a small fraction is exported to the shoot via the xylem as the oxyanions arsenate and arsenite. Once in the shoot, the arsenic is stored as an As III -tris-thiolate complex. The majority of the arsenic remains in the roots as an As III -tris-thiolate complex, which is indistinguishable from that found in the shoots and from As III -tris-glutathione. The thiolate donors are thus probably either glutathione or phytochelatins. The addition of the dithiol arsenic chelator dimercaptosuccinate to the hydroponic culture medium caused a 5-fold-increased arsenic level in the leaves, although the total arsenic accumulation was only marginally increased. This suggests that the addition of dimercaptosuccinate to arsenic-contaminated soils may provide a way to promote arsenic bioaccumulation in plant shoots, a process that will be essential for the development of an efficient phytoremediation strategy for this element.Arsenic may play an essential role in animal nutrition (Uthus, 1992(Uthus, , 1994, perhaps in Met metabolism, but there is no doubt that the element is principally renowned for its toxicity (National Research Council, 1977). Indeed, arsenic toxicity in humans has recently become evident on a very large scale in Bangladesh (Dhar et al., 1997), and the National Research Council has recently recommended that the maximum contaminant level standard for drinking water in the U.S. be lowered from the current value of 50 g L Ϫ1 (National Research Council, 1999). Arsenic is also toxic to plants and microorganisms and has been used in pesticides, herbicides, preservatives, and pharmaceuticals (National Research Council, 1977). Many of these uses continue today, and therefore it is important to remediate past contamination (Dutre et al., 1998). In this paper we address arsenate uptake by Indian mustard (Brassica juncea) plants growing hydroponically. Our data suggest that arsenate (As V ) enters the roots as a phosphate analog and is promptly reduced to As III . Little arsenic is transported to the aboveground tissues. The addition of dimercaptosuccinate to the hydroponic growth solution caused significant amounts of arsenic to move into the shoot, perhaps offering a way of removing arsenate from contaminated soils. MATERIALS AND METHODS Plant GrowthIndian mustard (Brassica juncea [L.] Czern. variety 426308) (Kumar et al., 1995) plants were grown under microbiologically controlled conditions such that their roots were maintained axenically. Seeds were surfacesterilized in 2.6% (w/v) sodium hypochlorite for 30 min, rinsed four times in autoclaved de-i...

The ability of Thlaspi goesingense Hálácsy to hyperaccumulate Ni appears to be governed by its extraordinary degree of Ni tolerance. However, the physiological basis of this tolerance mechanism is unknown. We have investigated the role of vacuolar compartmentalization and chelation in this Ni tolerance. A direct comparison of Ni contents of vacuoles from leaves of T. goesingense and from the non-tolerant non-accumulator Thlaspi arvense L. showed that the hyperaccumulator accumulates approximately 2-fold more Ni in the vacuole than the non-accumulator under Ni exposure conditions that were non-toxic to both species. Using x-ray absorption spectroscopy we have been able to determine the likely identity of the compounds involved in chelating Ni within the leaf tissues of the hyperaccumulator and non-accumulator. This revealed that the majority of leaf Ni in the hyperaccumulator was associated with the cell wall, with the remaining Ni being associated with citrate and His, which we interpret as being localized primarily in the vacuolar and cytoplasm, respectively. This distribution of Ni was remarkably similar to that obtained by cell fractionation, supporting the hypothesis that in the hyperaccumulator, intracellular Ni is predominantly localized in the vacuole as a Ni-organic acid complex.