The role of gibberellic acid in the hydrolysis of endosperm reserves in Zea mays

Comparative Histological and Transcriptional Analysis of Maize Kernels Infected with Aspergillus flavus and Fusarium verticillioides

Aspergillus flavus and Fusarium verticillioides infect maize kernels and contaminate them with the mycotoxins aflatoxin, and fumonisin, respectively. Genetic resistance in maize to these fungi and to mycotoxin contamination has been difficult to achieve due to lack of identified resistance genes. The objective of this study was to identify new candidate resistance genes by characterizing their temporal expression in response to infection and comparing expression of these genes with genes known to be associated with plant defense. Fungal colonization and transcriptional changes in kernels inoculated with each fungus were monitored at 4, 12, 24, 48, and 72 h post inoculation (hpi). Maize kernels responded by differential gene expression to each fungus within 4 hpi, before the fungi could be observed visually, but more genes were differentially expressed between 48 and 72 hpi, when fungal colonization was more extensive. Two-way hierarchal clustering analysis grouped the temporal expression profiles of the 5,863 differentially expressed maize genes over all time points into 12 clusters. Many clusters were enriched for genes previously associated with defense responses to either A. flavus or F. verticillioides. Also within these expression clusters were genes that lacked either annotation or assignment to functional categories. This study provided a comprehensive analysis of gene expression of each A. flavus and F. verticillioides during infection of maize kernels, it identified genes expressed early and late in the infection process, and it provided a grouping of genes of unknown function with similarly expressed defense related genes that could inform selection of new genes as targets in breeding strategies.

Signaling from the embryo conditions Vp1-mediated repression of alpha-amylase genes in the aleurone of developing maize seeds

The VP1 transcription factor functions as both a repressor and an activator of gene expression in the developing aleurone. Vp1 activation of the anthocyanin pathway exhibits strict cell autonomy in aleurone. In contrast, Vp1-mediated repression of hydrolase genes in aleurone cells during seed development is determined by a combination of cell autonomous and cell non-autonomous signals. To analyze signaling between the embryo and aleurone during seed development, a T-B3La chromosome translocation was used to create seed that has non-concordant embryo and endosperm genotypes. We show that de-repression of an Amy-GUS reporter gene in developing vp1 mutant aleurone cells strongly depends on the presence of a viviparous embryo. Genetic ablation of the developing embryo in vp1 mutant and Vp1 seeds through the introduction of an early embryo mutation caused a similar enhancement of Amy-GUS expression in the aleurone, suggesting that the quiescent embryo present in normal seed is a critical source of inhibitory signals. Analysis of an ABA deficient vp1 vp5 double mutant indicates that ABA synthesized in the embyro interacts additively with Vp1 to prevent precocious induction of alpha-amylase genes in the aleurone of the developing seed. A lack of ABA synthesis, however, does not account for the strongly synergistic interaction between a viviparous vp1 embryo and mutant aleurone suggesting that a quiescent embyro is a source of other inhibitory signals.

Integrated control of seed maturation and germination programs by activator and repressor functions of Viviparous-1 of maize

The Viviparous-1 (VP1) transcriptional activator of maize is required for abscisic acid induction of maturation-specific genes late in seed development leading to acquisition of desiccation tolerance and arrest in embryo growth. Here, we show that VP1 also inhibits induction of the germination-specific alpha-amylase genes in aleurone cells of the developing seed and thereby appears to be involved in preventing precocious hydrolyzation of storage compounds accumulating in the endosperm. In developing seeds of the somatically instable vp1-m2 mutant, hydrolase activity was derepressed specifically in endosperm sectors underlying vp1 mutant aleurone. A barley alpha-amylase promoter-GUS reporter construct (Amy-GUS) was induced in developing vp1 mutant aleurone cells but not in wild-type aleurone cells. Moreover, transient expression of recombinant VP1 and vp1 mutant aleurone cells strongly inhibited expression of Amy-GUS and thus effectively complemented this aspect of the mutant phenotype. VP1 specifically repressed induction of Amy-GUS by the hormone gibberellic acid in aleurone of germinating barley seeds. Deletion of the acidic transcriptional activation domain of VP1 did not affect the inhibitory activity, indicating that VP1 has a discrete repressor function. Hence, physically combining activator and repressor functions in one protein may provide a mechanism to integrate the control of two normally consecutive developmental programs, seed maturation and seed germination.

The control of seed germination in Trollius ledebouri A model of seed dormancy

Treatment of Trollius ledebouri seeds with gibberellins A4+A7 promotes germination. The efficacy of the treatment is dependent upon the duration of imbibition in distilled water prior to GA4+7 application. 'Presoaking' increases both the final percentage germination attained and also its rate of achievement. No presoaking effect is exhibited by seeds induced to germinate by testa removal in the absence of GA4+7. Active washing of Trollius seeds enhances the presoaking effect and the eluent from washed seeds is inhibitory to germination. The results support the hypothesis that the presoaking effect exhibited by Trollius is the result of the leaching of a germination inhibitor from the seeds which is antagonistic to GA4+7. Additionally, treatment of Trollius seeds with the gibberellin-biosynthesis inhibitor (2-chloroethyl)-trimethylammonium chloride (CCC) prior to testa removal retards germination. The inhibitory effect of CCC on germination is overcome by GA4+7. Although CCC inhibits embryo growth during the presoaking of intact seeds, it does not affect the increased sensitivity of presoaked seeds to GA4+7. Therefore, although endogenous gibberellins may be involved in the germination process, they do not contribute to the presoaking phenomenon. The expansion of isolated endosperm tissue is not affected by CCC. However, the chemical markedly inhibits endosperm expansion in intact seeds and implicates the embryo as both the site of production of the germination inhibitor and of gibberellin. These results are discussed in relation to previous studies and a model is presented to account for the characteristics of germination in Trollius.