Maize Genetics Cooperation Newsletter vol
81 2007
Oregon State University
Complex developmental processes are often controlled by the interplay of positive and antagonistic, or modulating, signaling pathways. The switch between embryogenesis and the maturation phase of maize embryo development involves the interaction of antagonistic signaling pathways governed by abscisic acid (ABA) and gibberellic acids (GAs). Abscisic acid (ABA) is a highly conserved hormone signal required to induce maturation phase in developing plant embryos. During embryonic development in cereals, bioactive GAs accumulate, peaking prior to the ABA peak that initiates maturation phase. Although ABA�s role in maturation is highly conserved in plants, a preceding GA peak is found only in cereals, and its significance is unclear. We have previously used both genetics and manipulation of hormone levels in culture to support our idea that the pre-maturation GA peak antagonizes ABA in controlling maturation-phase processes in maize (White and Rivin, Plant Physiol. 122:1089-1097, 2000; White et al., Plant Physiol. 122:1081-1088, 2000). In these studies, we found that ABA-deficient kernels are viviparous (germinating precociously on the ear) and desiccation-sensitive, but that mutants deficient in both ABA and GA exhibit the wild-type phenotypes of quiescence and desiccation tolerance. Thus, the early GA peak may either intercept the ABA signaling pathway to modulate ABA sensitivity, or participate in a negative regulatory mechanism to suppress maturation independently of ABA.
The wildtype behavior of ABA / GA double-deficiency embryos suggests that gene expression in this genotype is more like that in wildtypes than in ABA-deficient mutants. To test this proposition, we collected early maturation (stage 3) embryos from two types of ears: 1) ears segregating for vp5 (ABA-deficient kernels) and 2) d1 homozygotes (bioactive GA deficient) segregating for vp5 kernels. mRNA was isolated from sibling wildtype and vp5 homozygous embryos from each type of ear for comparison by microarray analysis. A loop-design hybridization scheme was used to compare the message profiles of the four genotypes, using the maize oligonucleotide array produced by the University of Arizona. Bioconductor and Limma software packages were used to identify genes with significantly different expression based on an adjusted P value p< 0.05.
In a comparison of wildtype and ABA-deficient sibling embryos at Stage 3 of embryogenesis, 75 moderate to highly expressed genes were found to be significantly different in expression between the normal and hormone-deficient condition. Of these genes, 70 were also found to be significantly different in a comparison of sibling embryo mRNAs from the double ABA/GA vs. single ABA-deficient ears, an 89% overlap in expression patterns indicating that gene expression in the double hormone mutants is very similar to wildtype on a broad scale. The differentially expressed genes included well-known maturation genes like the storage globulins and LEA proteins previously shown to be regulated by ABA and the Vp1 transcription factor, but a wide variety of other genes, not known to be ABA regulated, also appeared in this gene set.
Please Note: Notes
submitted to the Maize Genetics Cooperation Newsletter may be cited only with
consent of authors.