2\',3\'-cAMP is a positional isomer of the well-established second messenger 3\',5\'-cAMP, but little is known about the biology of this noncanonical cyclic nucleotide monophosphate (cNMP). Toll/interleukin-1 receptor (TIR) domains of nucleotide-binding leucine-rich repeat (NLR) immune receptors have the NADase function necessary but insufficient to activate plant immune responses. Here, we show that plant TIR proteins, besides being NADases, act as 2\',3\'-cAMP/cGMP synthetases by hydrolyzing RNA/DNA. Structural data show that a TIR domain adopts distinct oligomers with mutually exclusive NADase and synthetase activity. Mutations specifically disrupting the synthetase activity abrogate TIR-mediated cell death in Nicotiana benthamiana (Nb), supporting an important role for these cNMPs in TIR signaling. Furthermore, the Arabidopsis negative regulator of TIR-NLR signaling, NUDT7, displays 2\',3\'-cAMP/cGMP but not 3\',5\'-cAMP/cGMP phosphodiesterase activity and suppresses cell death activity of TIRs in Nb. Our study identifies a family of 2\',3\'-cAMP/cGMP synthetases and establishes a critical role for them in plant immune responses.

Helitrons are DNA transposable elements that are widely present in the genomes of diverse eukaryotic taxa. Helitrons are distinct from other transposons in their ability to capture gene fragments and their rolling-replication mechanism. Brassica rapa is a mesopolyploid species and one of the most important vegetable and oil crops globally. A total of 787 helitrons were identified in the B. rapa genome and were assigned to 662 families and 700 subfamilies. More than 21,806 repetitive sequences were found within the helitrons, whose G+C content correlated negatively to that of the host helitron. Each helitron contained an average of 2.9 gene fragments and 1.9 intact genes, of which the majority were annotated with binding functions in metabolic processes. In addition, a set of 114 nonredundant microRNAs were detected within 174 helitrons and predicted to regulate a set of 787 nonredundant target genes. These results suggest that helitrons contribute to genomic structural and transcriptional variation by capturing gene fragments and generating microRNAs.