Genome editing technologies enable precise modifications of DNA sequences in vivo and offer great promise for crop improvement. CRISPR/Cas9 (Clustered Regularly Interspaced Short Palindromic Repeats/CRISPR-associated Cas9) has revolution-

Reporters have been widely used to visualize gene expression, protein localization, and other cellular activities, but the commonly used reporters require special equipment, expensive chemicals, or invasive treatments. Here, we construct a new reporter RUBY that converts tyrosine to vividly red betalain, which is clearly visible to naked eyes without the need of using special equipment or chemical treatments. We show that RUBY can be used to noninvasively monitor gene expression in plants. Furthermore, we show that RUBY is an effective selection marker for transformation events in both rice and Arabidopsis. The new reporter will be especially useful for monitoring cellular activities in large crop plants such as a fruit tree under field conditions and for observing transformation and gene expression in tissue culture under sterile conditions.

CRISPR gene-editing technology has successfully generated targeted mutations in rice and many other plant species (Ma et al., 2015). Assessment of heritability and phenotypic stability of CRISPR-edited plants requires the elimination of the CRISPR construct. The presence of the CRISPR construct makes it difficult to distinguish the mutations transmitted from the previous generation from newly generated mutations by the CRISPR construct at the current generation. The existence of the CRISPR construct also greatly increases the risk of off-target effects, making phenotypic stability a concern. Moreover, removal of the transgenes is likely a prerequisite for gaining regulatory approval of CRISPR-edited crops for commercial applications. Development of an efficient and easy-to-perform technology for the removal of the CRISPR construct after DNA modifications have been completed will have a major impact on using gene-editing technology for crop improvement.

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