The first challenge for the geminated seeds is that they need to break out the soil to emerge for photosynthetic growth. For most dicots, a hook like structure formed at the upper part of hypocotyls, named as "apical hook”, is very critical for seedling emergence. Apical hook can not only ensure that the seedling has a hard "drill" to break through the soil, but also protect the fragile cotyledons and the apical meristem from damage during the emergence.
The preliminary study of apical hook formation can be traced back to Charles Darwin's pioneering studies and was described in his last plant book "The Power of Movement in Plants". In the following 140 years, although apical hook has attracted the research interest of numerous plant biologists, the mechanism underlying its formation is still a puzzle in the field of plant biology.
To address this question, researchers from the Institute of Genetics and Developmental Biology of the Chinese Academy of Sciences and the University of Minnesota, for the first time, have found that gravity is the initial signal that triggers the formation of apical hook. Relevant results were published on
Science Advances.
It has long been believed that plant shoots have negative gravitropic response to grow upwards while the roots have positive gravitropic response to grow downwards. The basis of the different gravitropism in shoots and roots is that the plant hormone auxin promotes cell expansion in shoots while it inhibits cell expansion in roots.
Interestingly, the researchers found that high concentrations of auxin in the hypocotyl inhibit cell expansion at the early stages of seedling development, contrary to the traditional belief that auxin promotes shoot cell growth. Thus, the young hypocotyls respond positively to gravity, like the roots, to bend downwards to form the hook. Therefore, the formation of apical hook is basically a positive gravitropic response of hypocotyls.
The researchers also uncovered the molecular mechanism by which gravity induced the formation of apical hook. At the inner side of the apical hook, gravity-induced auxin maximum activates PP2C.D1 phosphatase expression via the ARF7 transcription factor that then suppresses plasma membrane H+-ATPase-dependent acid growth leading to hook formation.
Together, this study not only decodes a long-standing scientific question of apical hook formation, but also proposed a molecular framework for auxin-mediated inhibition of cell expansion, significantly extending people's understanding of how the cell size is regulated in plants.
This research was sponsored by the Kuancheng Wang Education Fund and the National Institutes of Health.
Proposed model for hypocotyl elongation and apical hook development 522 during etiolated seedling development. (Image by IGDB)
