Enhanced thylakoid photoprotection can increase yield and canopy radiation use efficiency in rice.

High sunlight can raise plant growth rates but can potentially cause cellular damage. The likelihood of deleterious effects is lowered by a sophisticated set of photoprotective mechanisms, one of the most important being the controlled dissipation of energy from chlorophyll within photosystem II (PSII) measured as non-photochemical quenching (NPQ). Although ubiquitous, the role of NPQ in plant productivity remains uncertain because it momentarily reduces the quantum efficiency of photosynthesis. Here we used plants overexpressing the gene encoding a central regulator of NPQ, the protein PsbS, within a major crop species (rice)  to assess the effect of photoprotection at the whole canopy scale. We accounted for canopy light interception, to our knowledge for the first time in this context. We show that in comparison to wild-type plants, psbS overexpressors increased canopy radiation use efficiency and grain yield in fluctuating light, demonstrating that photoprotective mechanisms should be altered to improve rice crop productivity.

A step change in the transfer of interspecific variation into wheat from Amblyopyrum muticum.

Despite some notable successes, only a fraction of the genetic variation available in wild relatives has been utilized to produce superior wheat varieties. This is as a direct result of the lack of availability of suitable high-throughput technologies to detect wheat/wild relative introgressions when they occur. Here, we report on the use of a new SNP array to detect wheat/wild relative introgressions in backcross progenies derived from interspecific hexaploid wheat/Ambylopyrum muticum F1 hybrids. The array enabled the detection and characterization of 218 genomewide wheat/Am. muticum introgressions, that is a significant step change in the generation and detection of introgressions compared to previous work in the field. Furthermore, the frequency of introgressions detected was sufficiently high to enable the construction of seven linkage groups of the Am. muticum genome, thus enabling the syntenic relationship between the wild relative and hexaploid wheat to be determined. The importance of the genetic variation from Am. muticum introduced into wheat for the development of superior varieties is discussed.

Generation of amphidiploids from hybrids of wheat and related species from the genera Aegilops, Secale, Thinopyrum, and Triticum as a source of genetic variation for wheat improvement.

We aim to improve diversity of domesticated wheat by transferring genetic variation for important target traits from related wild and cultivated grass species. The present study describes the development of F1 hybrids between wheat and related species from the genera Aegilops, Secale, Thinopyrum, and Triticum and production of new amphidiploids. Amphidiploid lines were produced from 20 different distant relatives. Both colchicine and caffeine were successfully used to double the chromosome numbers. The genomic constitution of the newly formed amphidiploids derived from seven distant relatives was determined using genomic in situ hybridization (GISH). Altogether, 42 different plants were analysed, 19 using multicolour GISH separating the chromosomes from the A, B, and D genomes of wheat, as well as the distant relative, and 23 using single colour GISH. Restructuring of the allopolyploid genome, both chromosome losses and aneuploidy, was detected in all the genomes contained by the amphidiploids. From the observed chromosome numbers there is an indication that in amphidiploids the B genome of wheat suffers chromosome losses less frequently than the other wheat genomes. Phenotyping to realize the full potential of the wheat-related grass germplasm is underway, linking the analyzed genotypes to agronomically important target traits.

Does growth under elevated CO2 moderate photoacclimation in rice?

Acclimation of plant photosynthesis to light irradiance (photoacclimation) involves adjustments in levels of pigments and proteins and larger scale changes in leaf morphology. To investigate the impact of rising atmospheric CO2 on crop physiology, we hypothesize that elevated CO2 interacts with photoacclimation in rice (Oryza sativa). Rice was grown under high light (HL: 700 µmol m⁻² s⁻¹), low light (LL: 200 µmol m⁻² s⁻¹), ambient CO2 (400 µl l⁻¹) and elevated CO2 (1000 µl l⁻¹). Leaf six was measured throughout. Obscuring meristem tissue during development did not alter leaf thickness indicating that mature leaves are responsible for sensing light during photoacclimation. Elevated CO2 raised growth chamber photosynthesis and increased tiller formation at both light levels, while it increased leaf length under LL but not under HL. Elevated CO2 always resulted in increased leaf growth rate and tiller production. Changes in leaf thickness, leaf area, Rubisco content, stem and leaf starch, sucrose and fructose content were all dominated by irradiance and unaffected by CO2. However, stomata responded differently; they were significantly smaller in LL grown plants compared to HL but this effect was significantly suppressed under elevated CO2. Stomatal density was lower under LL, but this required elevated CO2 and the magnitude was adaxial or abaxial surface-dependent. We conclude that photoacclimation in rice involves a systemic signal. Furthermore, extra carbohydrate produced under elevated CO2 is utilized in enhancing leaf and tiller growth and does not enhance or inhibit any feature of photoacclimation with the exception of stomatal morphology.

The photoprotective protein PsbS exerts control over CO(2) assimilation rate in fluctuating light in rice.

A direct impact of chloroplastic protective energy dissipation (qE) on photosynthetic CO(2) assimilation has not been shown directly in plants in the absence of photoinhibition. To test this empirically we transformed rice to possess higher (overexpressors, OE) and lower (RNA interference, RNAi) levels of expression of the regulatory psbS gene and analysed CO(2) assimilation in transformants in a fluctuating measurement light regime. Western blots showed a several-fold difference in levels of PsbS protein between RNAi and OE plants with the wild type (WT) being intermediate. At a growth light intensity of 600 µmol m(-2) sec(-1) , the carboxylation capacity, electron transport capacity and dark adapted F(v)/F(m) (ratio of variable to maximum fluorescence) were inhibited in RNAi plants compared with WT and OE. The PsbS content had a significant impact on qE (measured here as non-photochemical quenching, NPQ) but the strongest effect was observed transiently, immediately following the application of light. This capacity for qE was several-fold lower in RNAi plants and significantly higher in OE plants during the first 10 min of illumination. At steady state the differences were reduced: notably at 500 µmol m(-2) sec(-1) all plants had the same NPQ values regardless of PsbS content. During a series of light-dark transitions the induction of CO(2) assimilation was inhibited in OE plants, reducing integrated photosynthesis during the light period. We conclude that the accumulation of PsbS and the resultant qE exerts control over photosynthesis in fluctuating light, showing that optimization of photoprotective processes is necessary for maximum photosynthetic productivity even in the absence of photoinhibitory stress.

Effects of season-dependent irradiance levels and nitrogen-deficiency on photosynthesis and photoinhibition in field-grown rice (Oryza sativa).

Photoinhibition and acclimation of photosynthesis in rice plants grown under N-sufficient (NS) and N-deficient (ND) field conditions were investigated during the tropical wet (WS) and dry (DS) seasons in the Philippines. Diurnal patterns of CO2 assimilation were examined. There was a transient peak in CO2 assimilation in the leaves of the NS plants in the early morning during the DS and the WS, which was not seen in the ND plants in either season. ND leaves had lower Ribulose bisphosphate carboxylase/oxygenase (Rubisco) contents and lower chlorophyll contents. A lowered quantum yield of photosystem II (phiPSII) was observed in the ND plants at an intermediate irradiance though no differences between N treatments were seen at high irradiance. Analysis of carotenoids indicated a small increase in the de-epoxidation state of the xanthophyll cycle (DES) at mid-day in the ND leaves compared to NS. Photoinhibition was greater in ND leaves when incident mid-day irradiance was increased by altering the leaf angle. Although Rubisco contents were lower in ND plants, photosynthesis in situ did not decline proportionally. For NS plants, Chlorophyll content, but not Rubisco content, was season-dependent and results are discussed in terms of the interaction between irradiance use and N content of rice leaves.

Acclimation of rice photosynthesis to irradiance under field conditions.

Acclimation to irradiance was measured in terms of light-saturated photosynthetic carbon assimilation rates (P(max)), Rubisco, and pigment content in mature field-grown rice (Oryza sativa) plants in tropical conditions. Measurements were made at different positions within the canopy alongside irradiance and daylight spectra. These data were compared with a second experiment in which acclimation to irradiance was assessed in uppermost leaves within whole-plant shading regimes (10% low light [LL], 40% medium light [ML], and 100% high light [HL] of full natural sunlight). Two varieties, japonica (tropical; new plant type [NPT]) and indica (IR72) were compared. Values for Rubisco amount, chlorophyll a/b, and P(max) all declined from the top to the base of the canopy. In the artificial shading experiment, acclimation of P(max) (measured at 350 microL L(-1) CO(2)) occurred between LL and ML for IR72 with no difference observed between ML and HL. The Rubisco amount increased between ML and HL in IR72. A different pattern was seen for NPT with higher P(max) (measured at 350 microL L(-1) CO(2)) at LL than IR72 and some acclimation of this parameter between ML and HL. Rubisco levels were higher in NPT than IR72 contrasting with P(max). Comparison of data from both experiments suggests a leaf aging effect between the uppermost two leaf positions, which was not a result of irradiance acclimation. Results are discussed in terms of: (a) acclimation of photosynthesis and radiation use efficiency at high irradiance in rice, and (b) factors controlling photosynthetic rates of leaves within the canopy.

Are there associations between grain-filling rate and photosynthesis in the flag leaves of field-grown rice?

Rate of grain filling in terms of dry mass accumulated per panicle per day was measured in field-grown rice in the dry season in the Philippines and compared to rates of light-saturated photosynthesis per unit leaf area (P(max)) measured at 350 micro l l(-1) CO(2) for 21 d after flowering. Five new plant type (tropical japonica) varieties (NPT) and one indica variety (IR72) were used and these gave some variation in rates and patterns of grain filling. A rapid grain-filling phase (RGFP) occurred approximately 10 d after flowering in most varieties. There was no consistent relationship in any variety between the rate of grain-filling and P(max) and chlorophyll content, both of which remained mostly unchanged throughout grain filling. Significant declines in the amount of total leaf protein and ribulose bisphosphate carboxylase-oxygenase (Rubisco) occurred, but these did not occur at the same time as the RGFP in all varieties. A decrease in the ratio of chlorophyll a/b preceded these changes and a transient rise in chlorophyll content was also observed in four varieties at this time. There was no significant change in leaf non-structural carbohydrate content during or following the RGFP. It is concluded that the decline in Rubisco and protein content in NPT was not reflected in photosynthetic activity. Hence in these field experiments Rubisco accumulated to a level in excess of photosynthetic requirements, serving as a store of nitrogen for grain filling.