Overlapping pathways of ABA-modulated gene expression during embryo maturation
--Connie White and Carol Rivin

Viviparous mutants of maize fail to complete normal embryo maturation and instead precociously germinate on the ear. We have used a differential screen to isolate novel cDNA clones whose expression is reduced or abolished in viviparous embryos. We obtained Vp1-requiring cDNA clones by preparing a mid-maturation phase cDNA library and screening it differentially with wildtype and vp1 cDNA. These messages were then characterized according to their sequence, their expression during normal development and their tissue specificity. The individual effects of Vp1, ABA and changing water relations on these mRNAs were examined by measuring message levels in ABA-synthesis deficient mutants (vp5) and wildtype embryos in embryo culture, where hormone levels and water uptake can be controlled experimentally. The accumulation of the maturation set of messages was examined in wildtype and mutant development in vivo and compared with that seen in cultured embryos. We also compared two mutant alleles of vp1 (vp1 and vp1-mcwhirter) that differ in their ability to suppress precocious germination. From these experiments, our clone set can be divided into groups that encompass four overlapping pathways of maturation expression:

Group 1: The most abundant class of messages reaches peak accumulation between 27 and 30 days after pollination. In addition to requiring the Vp1 gene product, these messages require high levels of ABA for induction. They include the Glb1 storage protein and a homologue to the wheat LEA gene Em. Other messages have sequences unrelated to previously reported genes. Several of the clones of this group detect a small amount of transcript in maturing tassel, but none in leaf, cob or husk.

Group 2: These messages reach peak expression earlier in embryogenesis than Group 1 and they require lower levels of ABA for induction. No homologous sequences were found in genetic databanks.

Group 3: Expressed later in embryogenesis than Group 1, this class requires a high level of ABA. Unlike Group 1 or Group 2 messages, the vp1-mcwhirter allele will not substitute for wildtype Vp1 in the expression of these mRNAs. The Glb2 storage protein message is in this group.

Group 4: Using a wheat homologue, we isolated a cDNA for maize LEA group 3, a protein associated with dehydration. This message is ABA-inducible, but unlike all the other clones we have examined, it does not also require the Vp1 gene product. This message peaks in the same time frame as Group 2. 

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