Distinct phytochrome actions in nonvascular plants revealed by targeted inactivation of phytobilin biosynthesis

Abstract

The red/far-red light photoreceptor phytochrome mediates photomorphological responses in plants. For light sensing and signaling, phytochromes need to associate with open-chain tetrapyrrole molecules as the chromophore. Biosynthesis of tetrapyrrole chromophores requires members of ferredoxin-dependent bilin reductases (FDBRs). It was shown that LONG HYPOCOTYL 2 (HY2) is the only FDBR in flowering plants producing the phytochromobilin (PΦB) for phytochromes. However, in the moss Physcomitrella patens, we found a second FDBR that catalyzes the formation of phycourobilin (PUB), a tetrapyrrole pigment usually found as the protein-bound form in cyanobacteria and red algae. Thus, we named the enzyme PUB synthase (PUBS). Severe photomorphogenic phenotypes, including the defect of phytochrome-mediated phototropism, were observed in Physcomitrella patens when both HY2 and PUBS were disrupted by gene targeting. This indicates HY2 and PUBS function redundantly in phytochrome-mediated responses of nonvascular plants. Our studies also show that functional PUBS orthologs are found in selected lycopod and chlorophyte genomes. Using mRNA sequencing for transcriptome profiling, we demonstrate that expression of the majority of red-light-responsive genes are misregulated in the pubs hy2 double mutant. These studies showed that moss phytochromes rapidly repress expression of genes involved in cell wall organization, transcription, hormone responses, and protein phosphorylation but activate genes involved in photosynthesis and stress signaling during deetiolation. We propose that, in nonvascular plants, HY2 and PUBS produce structurally different but functionally similar chromophore precursors for phytochromes. Holophytochromes regulate biological processes through light signaling to efficiently reprogram gene expression for vegetative growth in the light.

Marine algae and land plants share conserved phytochrome signaling systems

“…S6 and Table S4). Notable exceptions were genes for both plastid-targeted ferredoxindependent bilin reductases (FDBRs) phycourobilin synthase (PUBS) and phycocyanobilin:ferredoxin oxidoreductase (PCYA), bilin chromophore biosynthetic enzymes that catalyze conversion of biliverdin to phycourobilin (PUB) and phycocyanobilin (PCB), respectively (26). Expression patterns of MpPUBS and MpPCYA differed, suggesting distinct roles in bilin-dependent signaling pathways.…”

Retrograde bilin signaling enables Chlamydomonas greening and phototrophic survival

“…HMOX2 encodes an enzyme with a predicted C-terminal transmembrane endoplasmic reticulumanchoring domain like those of mammalian heme oxygenases (32), suggesting a gene of eukaryotic origin but lost in the streptophyte lineage. Recent studies indicate that some chlorophyte species even possess a second plastid-targeted bilin reductase (33).…”

Insights into the Biosynthesis and Assembly of Cryptophycean Phycobiliproteins

“…Generally, phycobilin biosynthesis starts with the oxygenolytic cleavage of heme by ferredoxin-dependent HO yielding the first open-chain tetrapyrrole biliverdin IX␣ (BV IX␣) (16 -18). Further reduction of BV obtaining PCB, PEB, and phytochromobilin (P⌽B) is catalyzed by ferredoxin-dependent bilin reductases (FDBR) (16,19,20). In cyanobacteria, PEB biosynthesis is performed by 15,16-DHBV:ferredoxin oxidoreductase (PebA) converting BV IX␣ to the intermediate DHBV, which is subsequently reduced to PEB by the second FDBR PEB:ferredoxin oxidoreductase (PebB) (16,21).…”