Insertion of YFP at the Arabidopsis AFL1 and P5CS1 loci shows the potential, and potential limitations, of gene targeting for functional analysis of stress-related genes

Crispr/CAS9-enabled homologous recombination to insert a tag in frame with an endogenous gene can circumvent difficulties such as context-dependent promoter activity that complicate analysis of gene expression and protein accumulation patterns. However, there have been few reports examining whether such Gene Targeting/Gene Tagging (GT) can alter expression of the target gene. The enzyme encoded by {Delta}1-pyrroline-5-carboxylate synthetase 1 (P5CS1) is key for stress-induced proline synthesis and drought resistance, yet its expression pattern and protein localization have been difficult to assay. We used GT to insert YFP in frame with the 5 or 3 ends of the endogenous P5CS1 and At14a-Like 1 (AFL1) coding regions. Insertion at the 3 end of either gene generated homozygous lines with expression of the gene-YFP fusion indistinguishable from the wild type allele. However, for P5CS1 this occurred only after selfing and advancement to the T5 generation allowed initial homozygous lethality of the insertion to be overcome. Once this was done, the GT-generated P5CS1-YFP plants revealed new information about P5CS1 localization and tissue-specific expression. In contrast, insertion of YFP at the 5 end of either gene blocked expression. The results demonstrate that GT can be useful for functional analyses of genes that are problematic to properly express by other means but also show that, in some cases, GT can disrupt expression of the target gene. Summary statementGene tagging of Arabidopsis thaliana P5CS1 and AFL1 shows the potential of GT for functional analysis of stress-related genes, but also provides examples of how GT can dramatically disrupt expression of the target gene.