An Indirect Organogenesis-Based Citrus Transformation System Monitored Using Visible Reporter Markers

Abstract

Genetic transformation of citrus is essential for functional genomics and trait improvement, yet remains limited by prolonged regeneration cycles, genotype-dependent responses, and inefficient screening of transformed tissues. Here, we established an Agrobacterium-mediated indirect organogenesis-based transformation system for Carrizo citrange and evaluated two visible reporter systems, the anthocyanin regulator ROSEA1 and the betalain biosynthetic cassette RUBY, for monitoring transformed tissues throughout regeneration. The regeneration protocol enabled visible callus initiation within 7 days following a 5-day pre-culture treatment and recovery of PCR-confirmed transgenic plantlets within 5 months. Both visible reporters were compatible with callus development and shoot regeneration throughout the indirect organogenesis workflow. During callus proliferation, visible pigmentation closely corresponded with GFP fluorescence, enabling rapid, non-destructive identification of transformed tissues. However, reporter performance diverged during shoot regeneration: RUBY maintained stable pigmentation throughout regeneration, whereas ROSEA1-associated pigmentation progressively declined despite continued GFP fluorescence. Consequently, RUBY showed an approximately 4.7-fold higher pigment-positive shoot fraction than ROSEA1 at week 17, and the RUBY-positive fraction was similar to the GFP-positive fraction in the same treatment. Putative transgenic events were confirmed by PCR in independent lines of both constructs, together with GFP fluorescence in leaves and root tips. Together, these findings describe a practical indirect organogenesis-based transformation platform for citrus and demonstrate that both ROSEA1 and RUBY are effective visual reporters during callus proliferation, whereas RUBY provides more reliable visual identification during shoot regeneration. RUBY is therefore recommended as the visible reporter of choice for citrus transformation extending beyond the callus stage. The workflow was established in Carrizo citrange, among the most readily transformed citrus genotypes, and extension to other varieties remains to be demonstrated.