Systematic optimization of Cas12a base editors in wheat and maize using the ITER platform

Plant biotechnology needs new methods that accelerate design-build-test-learn cycles to develop new gene editing reagents. We have established ITER (Iterative Testing of Editing Reagents) based on arrayed protoplast transfections and high-content imaging, allowing one optimization cycle –from design to results– within three weeks. We validated ITER in wheat and maize protoplasts using Cas9 cytosine and adenine base editors and used it to develop an optimized LbCas12a-ABE system. Sequential improvement of five components –NLS, crRNA, LbCas12a nuclease, adenine deaminase and linker– led to a systematic, stepwise increase in ABE activity at extrachromosomal GFP reporter (from 0.5% to 40%) and endogenous target sites. We confirmed the activity of LbCas12a-ABE in stable wheat transformants and leveraged these improvements to develop a highly mutagenic LbCas12a nuclease and a LbCas12a-CBE. Our data show that ITER is a sensitive, versatile, and high-throughput platform that can be harnessed to accelerate the development of genome editing technologies in plants. Measurement of editing rates at endogenous target sites in wheat or maize