For sexual diploid organisms, the beginning of a new life is marked with the fusion of two haploid gametes (egg and sperm) from the parents. After fertilization, thousands of genes are turned on and off in harmony with exact timing and spacing, presenting a remarkable symphony of embryonic development. In animals, the zygotic genome is not expressed at first. Instead, the early stage of this process (e.g. the establishment of anterior-posterior and dorsal-ventral coordinates) is controlled by mRNA and proteins deposited in the egg cell by the mother, known as the maternal effect. By analogy, we would expect plants to follow the same rule and exhibit a comparable maternal controlled period before the activation of zygotic genome. This is indeed the current prevailing model for plant embryogenesis, according to which the maternal-to-zygotic transition should not happen until the globular stages (when the embryo has ~32 to >100 cells). Mixed evidences have been found for this model. A most noteworthy support for this model came from a recent study revealing that maternal genome contributed ~80% of the mRNA products in the early Arabidopsis embryo. In contrast, in the study we will discuss today, the authors (Nodine and Bartel) adopted a similar genomic strategy yet showed an opposite result. Continue reading →