Vp1 and ABI3

Vp1, or ABI3 (ABA insensitive 3 ), from Arabidopsis (Arabidopsis thaliana (L.) Heynh.) are members of a large group of transcription factors that act as intermediates in regulating ABA-responsive genes during seed maturation and germination. Vp1 also inhibits induction of germination-specific AMY genes in the aleurone (Hoecker et al., 1995). Jones et al. (1997, 2000) studied Vp1 in A. fatua in mature dry seed and during imbibition, and expression of Vp1 was found to be higher in more dormant genotypes (Jones et al., 1997). After-ripened seeds also had lower expres sion of Vp1 during imbibition while seeds with secondary dormancy had higher expression. Similarly in wheat, Vp1 expression in mature embryos of dormant and non-dormant wheat cultivars was positively correlated with the level of dormancy and sensitivity of the embryo to ABA (Nakamura and Toyama, 2001). However, sorghum (Sorghum bicolor (L.) Moench) embryos from a genotype susceptible to preharvest sprouting expressed a slightly higher level of Vp1 mRNA than those from a line resistant to preharvest sprouting during embryogenesis (Carrari et al., 2001). The timing of Vp1 expression may, however, be important as it peaked earlier in seed development in the line susceptible to preharvest sprouting (Carrari et al., 2001). During germination in the presence of fluridone, which inhibits ABA biosynthesis, Vp1 expression was correlated with ABA sensitivity but not dormancy (Carrari et al., 2001). However, inhibition of ABA biosynthesis may not be reliable in causing changes in Vp1 expression to correlate with dormancy. In Arabidopsis expression of Vp1, together with Leafy cotyledon1 (Led), Fusca3 (Fus3) and Em revealed no correlation with dormancy in various ecotypes and mutants (Baumbusch et al., 2004). They suggested that ABI3 (Vpl) gene expression is regulated by cold and light, and not directly related to dormancy.

Preharvest sprouting can occur if there are cool and wet conditions during grain development of modern wheat varieties. In embryos excised from grains grown under such conditions, the majority of Vp1 transcripts were not correctly spliced and produced transcripts of different sizes (McKibbin et al., 2002). Consequently, the resultant non-functional proteins were unable to repress germination. So mis-splicing of this gene complicates expression studies as transcripts may not be functional.

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