La germinazione dei semi

Seed germination is a plant developmental process defined by the protrusion of the radicle from the seed coat. The seed is a quiescent organ, which represents an evolutionary successful method of sexual reproduction in vascular plants. The earliest seed plants, "progymnosperms", emerged in the late Devonian about 370 million years ago. Once embryogenesis is completed, the mature seed is dehydrated and subsequently becomes dormant. Seed dormancy could be considered simply as a block to the completion of germination of an intact viable seed under favourable conditions. Therefore seed germination begins with rehydration (imbibition) of seeds and is controlled by multiple environmental and endogenous factors like temperature, light, and the phytohormones gibberellins (GA) and abscissic acid (ABA). GA and ABA respectively promotes and inhibits this process. Germination of seeds of most annuals needs a pulse of R light and is mediated mainly by the photoreceptor phyB. Recently it has been partially uncovered the transduction pathway leading to seed germination in Arabidopsis thaliana. PIL5 (Phytochrome Interacting Factor3- Like 5) plays a pivotal role by negatively regulating this process. PIL5 activates transcription of the target genes GA- INSENSITIVE (GAI) and REPRESSOR OF GA1-3 (RGA), whereas indirectly regulates GA and ABA metabolism. The DAG1 Dof (DNA binding with One Finger) protein has been previously characterized as a transcription factor primarily involved in the negative control of seed germination. Recently it has been demonstrated that DAG1 is part of the phyB/PIL5 signal transduction pathway leading to seed germination. In fact, DAG1 negatively regulates GA biosynthesis by directly binding the AtGA3ox1 promoter