3dCAP-Wheat: An Open-Source Comprehensive Computational Framework Precisely Quantifies Wheat Foliar, Nonfoliar, and Canopy Photosynthesis.

Canopy photosynthesis is the sum of photosynthesis of all above-ground photosynthetic tissues. Quantitative roles of nonfoliar tissues in canopy photosynthesis remain elusive due to methodology limitations. Here, we develop the first complete canopy photosynthesis model incorporating all above-ground photosynthetic tissues and validate this model on wheat with state-of-the-art gas exchange measurement facilities. The new model precisely predicts wheat canopy gas exchange rates at different growth stages, weather conditions, and canopy architectural perturbations. Using the model, we systematically study (1) the contribution of both foliar and nonfoliar tissues to wheat canopy photosynthesis and (2) the responses of wheat canopy photosynthesis to plant physiological and architectural changes. We found that (1) at tillering, heading, and milking stages, nonfoliar tissues can contribute ~4, ~32, and ~50% of daily gross canopy photosynthesis (A cgross; ~2, ~15, and ~-13% of daily net canopy photosynthesis, A cnet) and absorb ~6, ~42, and ~60% of total light, respectively; (2) under favorable condition, increasing spike photosynthetic activity, rather than enlarging spike size or awn size, can enhance canopy photosynthesis; (3) covariation in tissue respiratory rate and photosynthetic rate may be a major factor responsible for less than expected increase in daily A cnet; and (4) in general, erect leaves, lower spike position, shorter plant height, and proper plant densities can benefit daily A cnet. Overall, the model, together with the facilities for quantifying plant architecture and tissue gas exchange, provides an integrated platform to study canopy photosynthesis and support rational design of photosynthetically efficient wheat crops.

Diurnal and Seasonal Variations of Photosynthetic Energy Conversion Efficiency of Field Grown Wheat.

Improving canopy photosynthetic light use efficiency and energy conversion efficiency (ε c ) is a major option to increase crop yield potential. However, so far, the diurnal and seasonal variations of canopy light use efficiency (LUE) and ε c are largely unknown due to the lack of an efficient method to estimate ε c in a high temporal resolution. Here we quantified the dynamic changes of crop canopy LUE and ε c during a day and a growing season with the canopy gas exchange method. A response curve of whole-plant carbon dioxide (CO2) flux to incident photosynthetically active radiation (PAR) was further used to calculate ε c and LUE at a high temporal resolution. Results show that the LUE of two wheat cultivars with different canopy architectures at five stages varies between 0.01 to about 0.05 mol CO2 mol-1 photon, with the LUE being higher under medium PAR. Throughout the growing season, the ε c varies from 0.5 to 3.7% (11-80% of the maximal ε c for C3 plants) with incident PAR identified as a major factor controlling variation of ε c . The estimated average ε c from tillering to grain filling stages was about 2.17%, i.e., 47.2% of the theoretical maximal. The estimated season-averaged radiation use efficiency (RUE) was 1.5-1.7 g MJ-1, which was similar to the estimated RUE based on biomass harvesting. The large variations of LUE and ε c imply a great opportunity to improve canopy photosynthesis for greater wheat biomass and yield potential.

A single point mutation in the C-terminal extension of wheat Rubisco activase dramatically reduces ADP inhibition via enhanced ATP binding affinity.

Ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) activase (Rca) is a AAA+ enzyme that uses ATP to remove inhibitors from the active site of Rubisco, the central carboxylation enzyme of photosynthesis. Rca α and ß isoforms exist in most higher plant species, with the α isoform being identical to the ß form but having an additional 25-45 amino acids at the Rca C terminus, known as the C-terminal extension (CTE). Rca is inhibited by ADP, and the extent of ADP sensitivity of the Rca complex can be modulated by the CTE of the α isoform, particularly in relation to a disulfide bond structure that is specifically reduced by the redox-regulatory enzyme thioredoxin-f. Here, we introduced single point mutations of Lys-428 in the CTE of Rca-α from wheat (Triticum aestivum) (TaRca2-α). Substitution of Lys-428 with Arg dramatically altered ADP inhibition, independently of thioredoxin-f regulation. We determined that the reduction in ADP inhibition in the K428R variant is not due to a change in ADP affinity, as the apparent constant for ADP binding was not altered by the K428R substitution. Rather, we observed that the K428R substitution strongly increased ATP substrate affinity and ATP-dependent catalytic velocity. These results suggest that the Lys-428 residue is involved in interacting with the γ-phosphate of ATP. Considering that nucleotide-dependent Rca activity regulates Rubisco and thus photosynthesis during fluctuating irradiance, the K428R substitution could potentially provide a mechanism for boosting the performance of wheat grown in the dynamic light environments of the field.

A Conserved Sequence from Heat-Adapted Species Improves Rubisco Activase Thermostability in Wheat.

The central enzyme of photosynthesis, Rubisco, is regulated by Rubisco activase (Rca). Photosynthesis is impaired during heat stress, and this limitation is often attributed to the heat-labile nature of Rca. We characterized gene expression and protein thermostability for the three Rca isoforms present in wheat (Triticum aestivum), namely TaRca1-ß, TaRca2-α, and TaRca2-ß. Furthermore, we compared wheat Rca with one of the two Rca isoforms from rice (Oryza sativa; OsRca-ß) and Rca from other species adapted to warm environments. The TaRca1 gene was induced, whereas TaRca2 was suppressed by heat stress. The TaRca2 isoforms were sensitive to heat degradation, with thermal midpoints of 35°C ± 0.3°C, the temperature at which Rubisco activation velocity by Rca was halved. By contrast, TaRca1-ß was more thermotolerant, with a thermal midpoint of 42°C, matching that of rice OsRca-ß. Mutations of the TaRca2-ß isoform based on sequence alignment of the thermostable TaRca1-ß from wheat, OsRca-ß from rice, and a consensus sequence representing Rca from warm-adapted species enabled the identification of 11 amino acid substitutions that improved its thermostability by greater than 7°C without a reduction in catalytic velocity at a standard 25°C. Protein structure modeling and mutational analysis suggested that the thermostability of these mutational variants arises from monomeric and not oligomeric thermal stabilization. These results provide a mechanism for improving the heat stress tolerance of photosynthesis in wheat and potentially other species, which is a desirable outcome considering the likelihood that crops will face more frequent heat stress conditions over the coming decades.

Dissecting regulatory pathways of G1/S control in Arabidopsis: common and distinct targets of CYCD3;1, E2Fa and E2Fc.

Activation of E2F transcription factors at the G1-to-S phase boundary, with the resultant expression of genes needed for DNA synthesis and S-phase, is due to phosphorylation of the retinoblastoma-related (RBR) protein by cyclin D-dependent kinase (CYCD-CDK), particularly CYCD3-CDKA. Arabidopsis has three canonical E2F genes, of which E2Fa and E2Fb are proposed to encode transcriptional activators and E2Fc a repressor. Previous studies have identified genes regulated in response to high-level constitutive expression of E2Fa and of CYCD3;1, but such plants display significant phenotypic abnormalities. We have sought to identify targets that show responses to lower level induced changes in abundance of these cell cycle regulators. Expression of E2Fa, E2Fc or CYCD3;1 was induced using dexamethasone and the effects analysed using microarrays in a time course allowing short and longer term effects to be observed. Overlap between CYCD3;1 and E2Fa modulated genes substantiates their action in a common pathway with a key role in controlling the G1/S transition, with additional targets for CYCD3;1 in chromatin modification and for E2Fa in cell wall biogenesis and development. E2Fc induction led primarily to gene downregulation, but did not antagonise E2Fa action and hence E2Fc appears to function outside the CYCD3-RBR pathway, does not have a direct effect on cell cycle genes, and promoter analysis suggests a distinct binding site preference.

BABY BOOM target genes provide diverse entry points into cell proliferation and cell growth pathways.

Ectopic expression of the Brassica napus BABY BOOM (BBM) AP2/ERF transcription factor is sufficient to induce spontaneous cell proliferation leading primarily to somatic embryogenesis, but also to organogenesis and callus formation. We used DNA microarray analysis in combination with a post-translationally regulated BBM:GR protein and cycloheximide to identify target genes that are directly activated by BBM expression in Arabidopsis seedlings. We show that BBM activated the expression of a largely uncharacterized set of genes encoding proteins with potential roles in transcription, cellular signaling, cell wall biosynthesis and targeted protein turnover. A number of the target genes have been shown to be expressed in meristems or to be involved in cell wall modifications associated with dividing/growing cells. One of the BBM target genes encodes an ADF/cofilin protein, ACTIN DEPOLYMERIZING FACTOR9 (ADF9). The consequences of BBM:GR activation on the actin cytoskeleton were followed using the GFP:FIMBRIN ACTIN BINDING DOMAIN2 (GFP:FABD) actin marker. Dexamethasone-mediated BBM:GR activation induced dramatic changes in actin organization resulting in the formation of dense actin networks with high turnover rates, a phenotype that is consistent with cells that are rapidly undergoing cytoplasmic reorganization. Together the data suggest that the BBM transcription factor activates a complex network of developmental pathways associated with cell proliferation and growth.