Characterizaion and Chemical Rescure of the Phenotype of ors-3, a Knockout mutant line of PpORS in Physcomitrella patens
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2ʹ-Oxoalkylresorcinol synthase from Physcomitrella patens (PpORS) is basal to all plant type III polyketide synthases in phylogenetic trees, and may resemble closely their most recent common ancestor. PpORS knockouts were previously generated and partially characterized (Li et al., Planta, 2018, 247: 527–541). This study aimed to investigate further the in planta functions and evolutionary roles of PpORS. Ors-3 (an ors knockout line) was first subjected to dehydration stress. The ability of ors-3 to recover after dehydration is significantly compromised in contrast to the control strain. Ors-3 also loses water faster than the control strain. These results together with the previous data suggested that ors mutants possess a defective cuticle (Li et al., 2018). In ors-3, the expression of two putative paralogs of PpORS was also examined to reveal that their expression levels did not appear to be up-regulated in ors-3 as compared to those in the control. Chemical rescue of ors-3 has been achieved. Thus, the ability of ors-3 to survive dehydration is restored in a dose dependent manner by in vitro PpORS products, purified oxoalkylresorcinol, and also by long chain alkylresorcinol analogs. Exogenous 14Clabelled in vitro PpORS products are incorporated as an insoluble biopolymer, and most of the radoioactivity was recovered after acid hydrolysis. Taken together, these data indicate that PpORS-produced 2ʹ-oxoalkylresorcinols are constituents of the moss cuticular biopolymer that confer resistance to dehydration, and imply that an ancestral ORS in early land plants may have contributed to their successful colonization of the land.