A maize enzyme from the 2-oxoglutarate-dependent oxygenase family with
unique kinetic properties, mediates resistance against pathogens and
regulates senescence
Abstract
In plants, salicylic acid (SA) hydroxylation regulates SA homoeostasis,
playing an essential role during plant development and response to
pathogens. This reaction is catalyzed by SA hydroxylase enzymes, which
hydroxylate SA producing 2,3- dihydroxybenzoic acid (2,3-DHBA) and/or
2,5-dihydroxybenzoic acid (2,5-DHBA). Several SA hydroxylases have been
recently identified and characterized from different plant species;
however, no such activity has been previously reported in maize. In this
work, we describe the identification and characterization of a new SA
hydroxylase in maize plants. This enzyme, with high sequence similarity
to previously described SA hydroxylases from Arabidopsis and rice,
converts SA into 2,5-DHBA; however, it shows different kinetics
properties to those from previously characterized enzymes, and it also
catalyzes the conversion of the flavonoid dihydroquercetin into
quercetin in in vitro activity assays, suggesting that the maize
enzyme may have different roles in vivo as those previously
reported from other species. Despite this, ZmS5H can complement the
resistance to pathogen and early senescence phenotypes of Arabidopsis
s3h mutant plants. Finally, we characterized a maize mutant in
the S5H gene ( s5hMu) that has altered
growth, senescence and increased resistance against Colletotrichum
graminicola infection, showing not only changes in SA and 2,5-DHBA but
also variations in flavonol levels. Together, the results presented here
provide evidence that SA hydroxylases in different plant species have
evolved to show differences in catalytic properties that may be
important to fine tune SA levels and other phenolic compounds such as
flavonols to regulate different aspects of plant development and defense
against pathogens.