Conserved and divergent roles of DELLA proteins in Physcomitrium (Physcomitrella) patens

DELLA proteins are master growth regulators that repress responses to the phytohormones gibberellins (GAs). Manipulation of DELLA signalling was instrumental in the development of high‐yielding crop varieties that saved millions from starvation during the “Green Revolution”. In order to infer how DELLA signalling has evolved within the land plant lineage, the functions of DELLA proteins in the bryophyte Physcomitrium (Physcomitrella) patens were examined in the present study. It was found that PpDELLAs are regulated differently from flowering DELLA proteins, as they are not degraded by diterpenes, they do not interact with GID1 homologues, they are not directly regulated by the 26S proteasome and they do not regulate abiotic stress responses. However, similarly to flowering plant DELLAs, they are involved in the regulation of reproductive development and they act as transcriptional ‘hubs’, possibly interacting with transcription factors belonging to similar families as flowering plant DELLA interactors. Using IP-MS and yeast two-hybrid, three photoreceptors were identified as PpDELLA protein-protein interactors, however no involvement of PpDELLAs in light responses was observed. In addition, while the Arabidopsis DELLA AtRGA1 did not interact with the seed dormancy regulator AtDOG1, PpDELLAs interacted with PpDOG1s, and the two proteins displayed similar functions in spore germination. This work shows that DELLA proteins in P. patens have both conserved and divergent functions. Unravelling how PpDELLAs function at the molecular level may provide novel ways of genetic modification that can be used to engineer better crops and contribute towards mitigating the effects of global warming and achieving global food security