PpORS-produced 2'-oxo-5-pentacosylresorcinol (2'-oxo-C25-RL) restored
dehydration tolerance in ors-3, a knockout mutant of PpORS. Feeding experiments with [14C]-2'-oxo-C25-RL suggested the role of PpORS products in cuticular
polymer that confer
dehydration resistance. 2'-Oxoalkylresorcinol synthase from the moss Physcomitrium (Physcomitrella) patens (PpORS) is the earliest diverged member of plant type III
polyketide synthases, and produces very-long-chain 2'-oxoalkylresorcinols in vitro. Targeted knockouts of PpORS (
ors) exhibited an abnormal phenotype (increased susceptibility to
dehydration), and a defective cuticle in
ors was suggested (Li et al., Planta 247:527-541, 2018). In the present study, we investigated chemical rescue of the
ors phenotype and also metabolic fates of the PpORS products in the moss. Using C24-CoA as substrate, 2'-oxo-5-pentacosylresorcinol (2'-oxo-C25-RL) and two minor
pyrones were first enzymatically prepared as total in vitro products. When a knockout mutant (ors-3) and control strains were grown in the presence of the total in vitro products or purified 2'-oxo-C25-RL, the ability of ors-3 and the control to survive
dehydration stress increased in a dose-dependent manner. Structurally analogous long-chain alkylresorcinols also rescued the
ors phenotype, although less efficiently. When the moss was grown in the presence of 14C-radiolabeled 2'-oxo-C25-RL, 96% of the radioactivity was recovered only after
acid hydrolysis. These findings led us to propose that 2'-oxoalkylresorcinols are the functional in planta products of PpORS and are incorporated into cuticular
biopolymers that confer resistance to
dehydration. In addition, the earliest diverging
ORS clade in phylogenetic trees of plant type III PKSs exclusively comprises bryophyte
enzymes that share similar active site substitutions with PpORS. Further studies on these bryophyte
enzymes may shed light on their roles in early plant evolution and offer a novel strategy for improving
dehydration tolerance in plants.