The origin, evolution and functional divergence of HOOKLESS1 in plants

Apical hooks are functional innovations only observed in angiosperms, which effectively protect the apical meristems out of damage during plant seedlings penetrating soil covers. Acetyltransferase like protein HOOKLESS1 (HLS1) in Arabidopsis thaliana is required for hook formation. However, the origin and evolution of HLS1 in plants are still not solved. Here, we traced the evolution of HLS1 and found that HLS1 originated in embryophytes. Moreover, we found that Arabidopsis HLS1 delayed plant flowering time, in addition to their well-known functions in apical hook development and newly reported roles in thermomorphogenesis. We further revealed that HLS1 interacted with transcription factor CO and repressed the expression of FT to delay flowering. Lastly, we compared the functional divergence of HLS1 among eudicot ( A. thaliana ), bryophytes ( Physcomitrium patens and Marchantia polymorpha ) and lycophyte ( Selaginella moellendorffii ). Although HLS1 from these bryophytes and lycophyte partially rescued the thermomorphogenesis defects in hls1-1 mutants, the apical hook defects and early flowering phenotypes could not be reversed by either P. patens, M. polymorpha or S. moellendorffii orthologs. These results illustrate that HLS1 proteins from bryophytes or lycophyte are able to modulate thermomorphogenesis phenotypes in A. thaliana likely through a conserved gene regulatory network. Our findings shed new light on the understanding of the functional diversity and origin of HLS1, which controls the most attractive innovations in angiosperms. HOOKLESS1, a positive regulator of seedling hook establishment, originated in embryophytes and evolved to gain hook development function in angiosperms.