Absence of SICKLE triggers programmed cell death by disturbing alternative splicing and decay of mRNAs.

Programmed cell death (PCD) plays fundamental roles in plant development and responses to environmental stresses. Here, we report a protein, SICKLE (SIC), that represses PCD. In Arabidopsis (Arabidopsis thaliana), the loss-of-function mutant of SIC, sic-4, hyper-accumulated lariat intronic RNAs (lariRNAs) and exhibited PCD. The gene encoding a RNA debranching enzyme 1 (DBR1), a rate-limiting enzyme for lariRNAs decay, was overexpressed to reduce the level of lariRNAs in the sic-4 mutant, which led to suppression of PCD. Meanwhile, another lariRNAs hyper-accumulating mutant, dbr1-2, also exhibited PCD, further indicating that sic-4 PCD is caused by hyper-accumulation of lariRNAs. Transcriptional profiling analyses revealed that the sic-4 mutation disturbed alternative splicing and decay of mRNAs associated with salicylic acid (SA) homeostasis, a well-known molecule functioning in PCD regulation. Moreover, SA is dramatically increased in sic-4 and the disruption of SA biosynthesis and signaling suppressed PCD in the mutant, demonstrating that SA functions downstream of sic-4. Taken together, our results demonstrate that SIC is involved in regulating SA-triggered PCD.