Complex sentence: In "Arabidopsis", the signal is initiated by the production of messenger RNAs coding a transcription factor called CONSTANS (CO), which promotes transcription of another gene called Flowering Locus T (FT).
Split into simpler sentences: In "Arabidopsis", the signal is initiated by the production of messenger RNA (mRNA) coding a transcription factor called CONSTANS (CO). CO protein promotes transcription of another gene called Flowering Locus T (FT).