TOUCH ME - 'Touch' genes in the micropylar endosperm

The micropylar region of endosperm (ME) is a physical barrier to radicle emergence in seeds of many different species, including tomato (Solanum lycopersicum) and Arabidopsis thaliana. ME is thought to be weakened through cell wall-modifying proteins, and this is supported by transcriptome data showing enrichment of cell wall-associated genes in ME. Gibberellin and ethylene have been suggested to be involved in induction of these genes in ME. However, mechanisms underlying this critical event for germination still remain elusive. In addition to hormonal regulation of ME weakening, recent data from high-throughput analyses suggested that it might be important for the radicle tip to 'touch' ME (or mechanosensing), in terms of ME-specific gene induction. This emerging hypothesis can be integrated with previous hypotheses about hormonal regulation of ME-specific gene expression in seeds.