Expression analysis of MADS box genes in pollen, egg cells and zygotes
--Heuer, S, Lorz, H, Dresselhaus, T

MADS box genes represent a highly conserved gene family of transcription factors in plants. During recent years an increasing number has been isolated from different plant species. They were first identified due to their functions in flower organ development, but now it becomes obvious that MADS box genes are also expressed during embryogenesis and in vegetative organs.

To address the question of whether MADS box genes are also expressed during the fertilization process, we have screened cDNA libraries of maize egg cells, in vitro zygotes and mature pollen with different maize MADS box probes under medium stringent conditions. Homologous as well as novel MADS box genes should be detected under the conditions applied. Figure 1 shows the control experiment of the screening. Hybridization signals were obtained in the cDNA libraries of egg cells and pollen, as well as in the seedlings library, which served as a positive control. Under the conditions used no signal could be detected in the zygote library.

The subsequent plaque screening of the egg cell, zygote and pollen libraries led to the isolation of two novel MADS box genes from the pollen library and one novel MADS box gene from the egg cell library. No MADS box gene was isolated from the zygote library. Gene expression studies have shown that two MADS box genes are expressed in egg cells and are completely downregulated in the zygote library. All three genes are expressed in organs other than egg cells and pollen, respectively, with an expression pattern which indicates that they are highly regulated. RNA in situ hybridizations and transformation experiments are in progress and will gain more information about these novel MADS box genes.

Figure 1. MADS box genes detected in cDNA libraries of egg cells (EC), mature pollen (P) and seedlings (S), but not in cDNA libraries of in vitro zygotes (Z). The libraries were screened with radiolabelled maize MADS box probes. A representative hybridization with the ZAG3/5 probe is shown. Cross-hybridization with all other MADS box probes has occurred due to the conservation of the MADS box (lane 5-10). (* For references see EMBL data bases: ZAG1: L18924; ZAG 3: L46397; ZAG5: L46398; ZEM2: X85335; ZEM3: X85336; ZMM1: X81199; ZAP1: L46400; ZAG2: X80206; ZAG4: L46399)


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