David M. Rosenthal

Summary: Leaf shape, a spectacularly diverse plant trait, varies across taxonomic levels, geography and in response to environmental differences. However, comprehensive intraspecific analyses of leaf shape variation across variable environments is surprisingly absent. Here, we performed a multilevel analysis of leaf shape using diverse accessions of sweet potato (Ipomoea batatas), and uncovered the role of genetics, environment, and G×E on this important trait. We examined leaf shape using a variety of morphometric analyses, and complement this with a transcriptomic survey to identify gene expression changes associated with shape variation. Additionally, we examined the role of genetics and environment on leaf shape by performing field studies in two geographically separate common gardens. We showed that extensive leaf shape variation exists within I. batatas, and identified promising candidate genes associated with this variation. Interestingly, when considering traditional measures, we found that genetic factors are largely responsible for most of leaf shape variation, but that the environment is highly influential when using more quantitative measures via leaf outlines. This extensive and multilevel examination of leaf shape shows an important role of genetics underlying a potentially important agronomic trait, and highlights that the environment can be a strong influence when using more quantitative measures of leaf shape.

Leaf shape, a spectacularly diverse plant trait, varies across taxonomic levels, geography, and in response to environmental differences. However, comprehensive intraspecific analyses of leaf shape variation across variable environments is surprisingly absent. Here, we perform a multi-level analysis of leaf shape using diverse accessions of sweetpotato ( Ipomoea batatas ), and uncover the role of genetics, environment, and GxE on this important trait. We examine leaf shape using a variety of morphometric analyses, and complement this with a transcriptomic survey to identify gene expression changes associated with shape variation. Additionally, we examine the role of genetics and environment on leaf shape by performing field studies in two geographically separate common gardens. We show that extensive leaf shape variation exists within I. batatas , and identify promising candidate genes underlying this variation. Interestingly, when considering traditional measures, we find that genetic factors are largely responsible for most of leaf shape variation, but that the environment is highly influential when using more quantitative measures via leaf outlines. This extensive and multi-level examination of leaf shape shows an important role of genetics underlying a potentially important agronomic trait, and highlights that the environment can be a strong influence when using more quantitative measures of leaf shape.

A greater understanding of natural variation in photosynthesis will inform strategies for crop improvement by revealing overlooked opportunities. We use Arabidopsis thaliana ecotypes as a model system to assess (i) how photosynthesis and photorespiration covary and (ii) how mesophyll conductance influences water use efficiency (WUE). Phenotypic variation in photorespiratory CO₂ efflux was correlated with assimilation rates and two metrics of photosynthetic capacity (i.e. V_Cmax and J_max); however, genetic correlations were not detected between photosynthesis and photorespiration. We found standing genetic variation—as broad-sense heritability—for most photosynthetic traits, including photorespiration. Genetic correlation between photosynthetic electron transport and carboxylation capacities indicates that these traits are genetically constrained. Winter ecotypes had greater mesophyll conductance, maximum carboxylation capacity, maximum electron transport capacity, and leaf structural robustness when compared with spring ecotypes. Stomatal conductance varied little in winter ecotypes, leading to a positive correlation between integrated WUE and mesophyll conductance. Thus, variation in mesophyll conductance can modulate WUE among A. thaliana ecotypes without a significant loss in assimilation. Genetic correlations between traits supplying energy and carbon to the Calvin–Benson cycle are consistent with biochemical models, suggesting that selection on either of these traits would improve all of them. Similarly, the lack of a genetic correlation between photosynthesis and photorespiration suggests that the positive scaling of these two traits can be broken.

Photosynthetic efficiency is a critical determinant of crop yield potential, although it remains below the theoretical optimum in modern crop varieties. Enhancing mesophyll conductance (i.e. the rate of carbon dioxide diffusion from substomatal cavities to the sites of carboxylation) may increase photosynthetic and water use efficiencies. To improve water use efficiency, mesophyll conductance should be increased without concomitantly increasing stomatal conductance. Here, we partition the variance in mesophyll conductance to within- and among-cultivar components across soybean (Glycine max) grown under both controlled and field conditions and examine the covariation of mesophyll conductance with photosynthetic rate, stomatal conductance, water use efficiency, and leaf mass per area. We demonstrate that mesophyll conductance varies more than 2-fold and that 38% of this variation is due to cultivar identity. As expected, mesophyll conductance is positively correlated with photosynthetic rates. However, a strong positive correlation between mesophyll and stomatal conductance among cultivars apparently impedes positive scaling between mesophyll conductance and water use efficiency in soybean. Contrary to expectations, photosynthetic rates and mesophyll conductance both increased with increasing leaf mass per area. The presence of genetic variation for mesophyll conductance suggests that there is potential to increase photosynthesis and mesophyll conductance by selecting for greater leaf mass per area. Increasing water use efficiency, though, is unlikely unless there is simultaneous stabilizing selection on stomatal conductance.

The recent study by McGrath and Lobell (2013 Environ. Res. Lett. 8 014054) assesses the interaction of a changing climate and the carbon dioxide fertilization effect (CFE) on crop productivity. By accounting for the differential response of individual crops and using a finer geographic scale to assess climate effects on crops they have found that previous estimates of the CFE have likely overestimated future yields in some regions while underestimating yields in others. While this work improves our estimates of potential crop yields in an elevated CO₂ atmosphere, it also highlights knowledge gaps regarding the response of major crops to the effects of elevated CO₂ under the sub-optimal growing conditions predicted for many regions in the future.

Microarray analysis was performed on RNA isolated from guard cells that were manually dissected from leaves of Arabidopsis. By pooling our data with those of two earlier studies on Arabidopsis guard cell protoplasts, we provide a robust view of the guard-cell transcriptome, which is rich in transcripts for transcription factors, signaling proteins, transporters, and carbohydrate-modifying enzymes. To test the hypothesis that photosynthesis-derived sugar signals guard cells to adjust stomatal opening, we determined the profile of genes expressed in guard cells from leaves that had been treated with sucrose. The results revealed that expression of 440 genes changed in guard cells in response to sucrose. Consistent with this hypothesis, these genes encoded cellular functions for photosynthesis and transport of sugars, water, amino acids, and ions. Plants of T-DNA insertion lines for 50 genes highly responsive to sucrose were examined for defects in guard cell function. Twelve genes not previously known to function in guard cells were shown to be important in leaf conductance, water-use efficiency, and/or stomate development. Of these, three are of particular interest, having shown effects in nearly every test of stomatal function without a change in stomatal density: TPS5 (At4g17770), a TRAF domain-containing protein (At1g65370), and a WD repeat–containing protein (At1g15440).

The heterotrimeric G-protein complex provides signal amplification and target specificity. The Arabidopsis (Arabidopsis thaliana) Gβ-subunit of this complex (AGB1) interacts with and modulates the activity of target cytoplasmic proteins. This specificity resides in the structure of the interface between AGB1 and its targets. Important surface residues of AGB1, which were deduced from a comparative evolutionary approach, were mutated to dissect AGB1-dependent physiological functions. Analysis of the capacity of these mutants to complement well-established phenotypes of Gβ-null mutants revealed AGB1 residues critical for specific AGB1-mediated biological processes, including growth architecture, pathogen resistance, stomata-mediated leaf-air gas exchange, and possibly photosynthesis. These findings provide promising new avenues to direct the finely tuned engineering of crop yield and traits.

Globally, cassava is the second most important root crop after potatoes and the fifth most important crop overall in terms of human caloric intake. In addition to its growing global importance for feed, fuel, and starch, cassava has long been vital to food security in Sub‐Saharan Africa. Climate change is expected to have its most severe impact on crops in food insecure regions, yet little is known about how cassava productivity will respond to climate change. The most important driver of climate change is globally increasing atmospheric CO₂ concentration ([CO₂]). However, the potential for cassava to enhance food security in an elevated [CO₂] world is uncertain as greenhouse and open top chamber (OTC) study reports are ambiguous. Studies have yielded misleading results in the past regarding the effect of elevated [CO₂] on crop productivity, particularly in cases where pots restricted sink growth. To resolve these conflicting results, we compare the response of cassava to growth at ambient (ca. 385 ppm) and elevated [CO₂] (585 ppm) under field conditions and fully open air [CO₂] elevation. After three and half months of growth at elevated [CO₂], above ground biomass was 30% greater and cassava root tuber dry mass increased over 100% (fresh weight increased 89%). High photosynthetic rates and photosynthetic stimulation by elevated [CO₂], larger canopies, and a large sink capacity all contributed to cassava's growth and yield stimulation. Cassava exhibited photosynthetic acclimation via decreased Rubisco capacity early in the season prior to root tuber initiation when sink capacity was smaller. Importantly, and in contrast to a greenhouse study, we found no evidence of increased leaf N or total cyanide concentration in elevated [CO₂]. All of our results are consistent with theoretical expectations; however, the magnitude of the yield increase reported here surpasses all other C₃ crops and thus exceeds expectations.