Performing Department
Crop Sciences
Non Technical Summary
Current global challenges highlight the need for more sustainable agricultural systems and increases in crop yields. These needs are fundamentally dependent on future crops having the ability to efficiently photosynthesize under changing environmental conditions. Furthermore, leaf photosynthesis is related to the plant's water availability, which also directly impacts yield. Thus, it is necessary to understand how crops balance CO2 uptake for photosynthesis with transpirational water loss to enable the development of crops to meet global food, feed, and fuel demands. This is evident when considering the scale of modern agriculture. In 2020, nearly 92 million acres of corn were planted in the United States, and it has been estimated that an acre of corn transpires 3,000 to 4,000 gallons of water a day. Even modest gains in reducing transpiration, while maintaining rates of photosynthesis, would constitute large water savings for maize cultivated under both irrigated and unirrigated conditions. These water savings would become even more important under water stress conditions, which are predicted to become frequent and severe in the future.In addition to the economic importance of maize, the vast genetic resources make it an ideal system for studying complex traits. Surprisingly, maize has long been overlooked as a model for studying photosynthesis, and only recently has water-use efficiency (WUE) been actively targeted by breeding programs. Traditionally private companies, non-profit research institutes, government agencies, and academics have focused on developing drought tolerant lines of maize. Traits such as reduced leaf senescence, a variety of reproductive traits (most notably reduced anthesis silking interval), and yield have been the targets of selection in breeding programs that focus on drought tolerance. Root architecture is less tractable than above ground traits, but can have significant effects on drought tolerance. Likewise, although the hormone abscisic acid (ABA) certainly plays a role in drought tolerance, manipulation of a hormone signaling pathway can be difficult due to pleiotropic effects. Indeed, drought itself is difficult because the timing and intensity of the water stress event can determine whether a trait contributes to drought stress or susceptibility.Only recently have breeding efforts included selection for aspects of transpiration, which combines breeding methodology with a body of physiological work relating photosynthesis, temperature, vapor pressure deficit (VPD), transpiration, and WUE. This approach is corroborated by the findings of elevated CO2 studies and the corresponding reduction in transpiration. Transpiration efficiency can be affected by a variety of morphological, anatomical, and physiological traits. Leaf area has to be factored into whole plant WUE, noting the required balance between reducing leaf area for improved transpiration efficiency and having sufficient leaf area for light interception for photosynthesis. Variation has also been observed in response to VPD. Some lines show sensitivity to VPD, and have reduced transpiration at high VPD, creating a breakpoint in the linear response. Although stomatal response kinetics have not been investigated across lines of maize, interspecies comparisons have noted differences in the response that would impact transpiration. Finally, other factors such as mesophyll conductance also can affect transpiration and photosynthesis. Taken together, these studies show a variety of mechanisms, that when optimized through breeding or biotechnology approaches, could result in substantial water savings. Traits related to transpiration would be complementary to many of the traits for stress tolerance under drought. The goal of this project is to characterize maize germplasm that has been modified for enhanced WUE across several traits using biotechnology and breeding approaches.
Animal Health Component
50%
Research Effort Categories
Basic
50%
Applied
50%
Developmental
(N/A)
Goals / Objectives
Goal One: Assess the impacts of transgene constructs and gene mutation on plant growth and water-use efficiency.Goal Two: Develop and yield test hybrids with enhanced water-use efficiency.
Project Methods
Goal One: To modify the CO2 signaling pathway with the goal of improving transpiration efficiency, we chose to overexpress SLAC1 in Z. mays using a guard cell promoter. A guard cell promoter enables the engineering of the CO2 signaling in a specific cell type, which could reduce deleterious side effects. Based on the pathway elucidated in Arabidopsis and the mutant analysis in maize, overexpression of SLAC1 will enhance stomatal closure. More S-type anion channels should increase the speed of stomatal closure and likely make the plant hypersensitive to elevated CO2, ABA signaling, and reduced light intensity. Interestingly it has been shown in Arabidopsis that drought stress induces an up-regulation of SLAC1, indicating that there is a natural mechanism that can be exploited. Plants expressing this construct will likely be sensitive to all environmental stimuli because SLAC1 lies downstream of the convergence point of the independent signaling pathways.A propensity for stomatal closure would facilitate water savings due to reduced transpirational water loss. Importantly, the CO2 concentrating mechanism used in C4 photosynthesis maintains maximum photosynthetic rates at low intercellular CO2 concentrations, which may allow plants overexpressing SLAC1 in guard cells to maintain normal rates of photosynthesis. Because this carbon pump only operates in C4 species, we would expect overexpression of SLAC1 in C3 species to result in reduced growth. The overexpression of SLAC1 in Arabidopsis resulted in slowed stomatal opening in response to light due to an interaction with KAT1 (potassium channel in arabidopsis 1); however, the implications for plant growth and CO2 signaling were not reported. In addition to the CO2 concentrating mechanism, stomatal opening in response to light is much faster in Z. mays when compared to Arabidopsisand therefore, an attenuation of stomatal opening by overexpressing SLAC1 may not have a negative effect in Z. mays. Several transgenic events have been generated and are currently undergoing backcrossing into the reference inbred line B73 and several elite inbred lines from different heterotic pools that will be used to make hybrids. Once the transgenic lines have been verified and backcrossing is completed, we will perform physiological characterization under controlled conditions as well as agronomic trials in the field to investigate the effects of the SLAC1 overexpression construct on WUE.We have also built an RNAi construct to knock down HT1. Based on the pathway elucidated in Arabidopsis, a knockdown of HT1 will render plants insensitive to changes in CO2 levels (Hashimoto et al., 2006). The RNAi construct was designed to target both of the highly homologous HT1 gene copies in maize. Because HT1 is upstream of the convergence point of the independent signaling pathways, knockdown mutants should only affect stomatal response to CO2 and not other stimuli. However, the most severe ht1-2 mutants in Arabidopsis showed slowed stomatal opening specifically in response to red light (Matrosova et al., 2015). Fortunately, oftentimes independent RNAi events have different levels of silencing, which can create an allelic series of expression. Such a series would be ideal for fine-tuning the expression of HT1 for increased WUE. Events carrying this construct are undergoing a similar backcrossing scheme and characterization to that of SLAC1 overexpressing lines.Goal Two: The grow out of 25 hybrid lines and their inbred parents at the controlled drought locations in Mexico were used to quantify drought tolerance and WUE in the selected lines. The tissue samples that were taken from the trials will allow us to estimate the heritability of δ13C and the effect water limitation has on δ13C in hybrid maize. We have previously shown how water limitation can affect δ13C in inbred lines of maize, but it is unknown if this interaction is similar in hybrid maize. This is an important and potentially limiting factor that needs to be more deeply characterized if this method is to be implemented by plant breeding programs. Through our collaboration with CIMMYT, the subset of lines grown in three locations in Mexico using the same watering regime they employ for drought trials for their breeding program. We now need to process the isotope samples and combine them with the agronomic phenotypes that were collected. We will also grow the 25 hybrids and their parents in a well-watered location at the University of Illinois South Farm to record δ13C and agronomic phenotypes in a temperate environment.In addition to the hybrids that were grown in Mexico, my lab is constantly making and testing new hybrids based on information from our more basic experiments. Our goal is to better understand traits that make hybrids more WUE, including efficiency in well-watered conditions. Lines that are wasteful water users under well-watered conditions exhaust the soil moisture supply rather than conserving it for dry periods. We will continue to use soil moisture probes, drone imaging, leaf measurements, and emerging methods and technologies to capture line differences related to WUE in a production environment.Together these objectives fully leverage the resources available in maize to address the link between photosynthesis and transpiration. The results of these experiments will enable the breeding and engineering of more efficient crops.