Structure of two maize phytase genes and their spatio-temporal expression during seedling development

Up to 80% of Zea mays L. grain phosphorus is stored in the form of phytin i n the embryo. Our objective is to determine the control of phytin mobilizat ion during germination and seedling growth. A maize phytase cDNA, phy S11, has been previously characterized (Maugenest et al., Biochem J 322: 511-517 , 1997). In the present work, phy S11 was used to screen a maize genomic li brary and two distinct genes, PHYT I and PHYT II, were isolated and sequenc ed. The transcribed sequences of these two genes presented a strong homolog y whereas the untranscribed upstream and downstream sequences appeared very different. Northern blot analysis and in situ hybridization showed a high accumulation of phytase mRNA at the early steps of germination in the coleo rhiza, radicle cortex and coleoptile parenchyma. Phytase expression was als o detected at a lower extent in the scutellum. In adult plants, northern bl ot analyses revealed low but significant levels of phytase mRNA in the root s. In situ hybridizations on root cross-sections localized phytase mRNA in rhizodermis, endodermis and pericycle layers. Immunolocalization analysis s howed phytase accumulation at the same sites as its mRNA. A RT-PCR approach was used in an attempt to discriminate between the transcripts from each g ene in the different situations. These experiments indicate that both genes are expressed during germination, whereas only PHYT I is expressed in adul t roots. This suggests that signals responsible for phytase gene expression in roots are different from those responsible for gene expression during g ermination.