Regulation of gene expression by abscisic acid

The multiple pleiotropic effects associated with viviparous mutants in maize (D.S. Robertson, Genetics 40:745, 1955) suggest that these mutations affect expression of other genes, but the manner by which they do this has not been obvious. The xanthophyll-deficient mutants vp2, vp5, vp9, ps(=vp7) and w3 all appear to have lesions in structural genes involved in carotenoid biosynthesis, and all of these mutants have similar pleiotropic effects, including reduced levels of the phytohormone abscisic acid (ABA).

The vp mutant has normal xanthophyll synthesis, and ABA levels in vp/vp embryos are in the normal range. It also has multiple pleiotropic effects but, with the exception of vivipary, the pleiotropic effects of vp appear to be different from those of the xanthophyll-deficient mutants.

We have previously shown that ABA, if applied at the appropriate time, can induce embryo dormancy in xanthophyll-deficient mutants. Initiation of embryo dormancy is very specific for stage 4 of embryogenesis about 15 days after pollination (DAP). Thus, if ABA is directly involved in the regulation of gene expression, it seems probable that some of these regulatory events should occur at about the same time that embryo dormancy is inducible by ABA.

Kernel sectors from the inbred Tx5855 develop normally when cultured in vitro, and they produce sufficient ABA to induce embryo dormancy. Both xanthophyll and ABA synthesis is blocked if sectors are cultured on medium containing fluridone. These kernels are viviparous and embryos contain near-zero levels of ABA. We incubated both types of kernel sectors for 24h on media containing 35S-Met on a daily basis from 12-19 DAP extracted proteins from embryo and endosperm, and compared flurograms of SDS-PAGE gels. No differences were observed in 12-15 DAP embryo or endosperm between normal and fluridone-treated kernels. However, in 16-19 DAP kernels several differences were detected in both embryo and endosperm tissue for newly synthesized proteins in the 32 to 45 kD range. These differences became more obvious as the kernels aged. None of these bands have been identified yet.

Scutellum tissue of normal maize kernels has essentially no lipase activity at any stage of kernel development. However, scutellum of both vp and vp5 has detectable lipase activity at 16 DAP and high activity at 25 DAP Activity is about 3 times as high in vp as in vp5 at 25 DAP The vp mutant has normal ABA levels, but it is ABA insensitive, while ABA levels in 25 DAP field-grown vp5 are about 50% of normal. This suggests that both the vp gene product and ABA are required to suppress lipase activity during kernel development, and that the effect of ABA may be concentration-dependent.

Although these experiments are not definitive, they suggest that ABA has a direct or indirect effect on the expression of specific genes at specific stages of maize kernel development.

D.J. Hole, D.G. Bai, F Fong, CW. Magill and J.D. Smith

Please Note: Notes submitted to the Maize Genetics Cooperation Newsletter may be cited only with consent of the authors.

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