Characterization of the expression of two maize root-preferential genes at the protein level
The zrp2 gene encodes an mRNA that accumulates preferentially in roots of maize. To study zrp2 gene expression at the protein level, an antibody was raised against an E. coli-produced ZRP2 protein. Western analyses using this antibody revealed an approximately 100 kDa protein in seedling roots that was not detected in seedling leaves. This size is about 30 kDa larger than the predicted molecular weight. Within seedling roots, low levels of ZRP2 protein were detected in the root tip and high levels were detected throughout the rest of the root. In mature maize plants, the ZRP2 protein was present at high levels in roots and stems. Other organs of mature plants lacked detectable amounts of this protein. In vitro translation of ZRP2 mRNA resulted in the production of a 100 kDa polypeptide indicating that the observed size of ZRP2 protein is unlikely to be due to a post-translational modification. To gain an understanding of the function of ZRP2 protein, maize roots were subjected to various external stimuli. Results of these studies showed that zrp2 expression is transiently down-regulated by abscisic acid both at the mRNA and protein levels. zrp2 expression was also observed to be down-regulated under high salt concentrations;The maize zrp4 gene encodes an O-methyltransferase whose mRNA has been shown to accumulate preferentially in the endodermis of maize roots. In this study, distribution of ZRP4 protein was investigated using antibodies generated against an E. coli expressed ZRP4 protein. In maize seedlings, ZRP4 protein was detected at high levels in roots but was not detected in leaves. Within seedling roots, this protein was detected at low levels in root tips and at high levels in the remainder of the root. In mature plants, high levels of ZRP4 protein was detected in young prop roots whereas low levels were found in the mature roots. The ZRP4 protein was not detected in other organs of the mature plant. These results suggest that ZRP4 protein distribution closely follows the mRNA accumulation pattern and that zrp4 gene expression is developmentally regulated in the maize root.