Characterization of pedicel β‐glucuronidase activity in developing maize (Zea mays) kernels

A conspicuous endogenous maize (Zea mays L.) β-glucuronidase (GUS) activity was observed in histochemical assays of non-transformed maize kernels, confounding the use of Escherichia coli gusA as a reporter gene. Appearance of the endogenous activity was developmentally dependent and highly tissue-specific, being localized to the upper pedicel (basal maternal kernel) tissues where the black layer forms in the latter stages of kernel development. Pedicel homogenates exhibited GUS activity using either p-nitrophenyl-β-D -glucuronide or 4-methylumbelliferyl-β-D -glucuronide (MUG) as substrates. Pedicel GUS was apparently not the result of endophyte contamination of enzyme isolates since no endophytes could be cultured. The MUG-based activity had a pH optimum of 4 to 5 and was separable into two isoforms by anion exchange chromatography with K_m values for MUG of 2.2 and 2.7 µM for the early- and late-eluting forms, respectively. The pedicel GUS isoforms had very similar characteristics: native M_r of approximately 32000, stimulation by assay at 60°C, inhibition at high ionic strength or in the presence of EDTA and relative insensitivity to the E. coli GUS inhibitor saccharic acid-1,4-lactone. Only the early-eluting form, however, was capable of hydrolyzing the histocbemical GUS substrate 5-bromo-4-chloro-3-indoyl-β-D -glucuronide. Neither isoform exhibited antifungal activity against Fusarium moniliforme. In contrast to the in vitro activity, pedicel endogenous GUS measured histochemically was completely inhibited by saccharic acid-1,4-lactone, unaffected by EDTA and greatly decreased by incubation at elevated assay temperature. A modification of the standard histochemical GUS assay allowed complete suppression of endogenous GUS activity while enhancing E. coli-derived GUS activity in kernels transiently expressing the gusA gene. Possible roles of these endogenous GUS activities within the black layer region of the kernel pedicel are proposed.