Abscisic Acid and Stress Signals Induce Viviparous1 (Vp1) Expression in Seed and Vegetative Tissues of Maize

Viviparous1 (Vp1) encodes a B3 domain-containing transcription factor that is a key regulator of seed maturation in maize. However, the mechanisms of Vp1 regulation are not well understood. To examine physiological factors that may regulate Vp1 expression, transcript levels were monitored in maturing embryos placed in culture under different conditions. Expression of Vp1 decreased after culture in hormone-free medium, but was induced by salinity or osmotic stress. Application of exogenous ABA also induced transcript levels within one hour, in a dose-dependent manner. The Vp1 promoter fused to GUS or GFP reproduced the endogenous Vp1 expression patterns in transgenic maize plants, and also revealed previously unknown expression domains of Vp1. The Vp1 promoter is active in the embryo and aleurone cells of developing seeds, and upon drought stress, was also found in phloem cells of vegetative tissues, including cobs, leaves and stems. Sequence analysis of the Vp1 promoter identified a potential ABRC, consisting of an ACGT-containing ABRE (ABA response element) and a CE1 (coupling element 1)-like motif. Electrophoretic mobility shift assay (EMSA) confirmed that the ABRE and putative CE1 components specifically bound proteins in embryo nuclear protein extracts. Treatment of embryos in hormone-free MS medium blocked the ABRE-protein interaction, while exogenous ABA or mannitol treatment restored this interaction. Our data support a model for a VP1-dependent positive feedback mechanism regulating Vp1 expression during seed maturation.