Application of Emerging Technologies for Precision Breeding in Maize.

New breeding technologies have the potential to focus crop improvement much more tightly on target traits. This will allow precise modification of specific features of commercial hybrids, and provide an avenue to introgress valuable novel variation from under-utilized genetic resources.  An example of modifying a commercial hybrid is the development of waxy corn. The high-amylopectin waxy phenotype is naturally caused by a wx deletion, and phenotypically equivalent deletions can be created de-novo in high-quality hybrids using CRISPR-Cas enabled advanced breeding technology. Because waxy corn is typically produced using an identity-preserved system, it allows the establishment of a solid foundation for the success of future products generated via CRISPR-Cas. Such products could include hybrids with greater resilience to drought.  An example is the use of CRISPR-Cas to substitute the native maize GOS2 promoter, which confers a moderate level of constitutive expression, into the promoter region of the native ARGOS8 gene. The ARGOS8 variant has elevated ARGOS8 transcripts relative to the native allele and these transcripts are detectable in all the tissues tested, which is the expected result from using the GOS2 promoter. A field study showed that compared to the wild type, the ARGOS8 variant increased grain yield under flowering stress conditions and had no yield loss under well-watered conditions. These results demonstrate the utility of the CRISPR-Cas system in generating novel allelic variation for breeding drought tolerant crops.