Credit: NPG

A recent study shows that development of male germline cells in maize is triggered by hypoxia. This provides a new elegant example of how environmental conditions and developmental decisions in plants are connected, and it challenges the previous model of anther development.

Kelliher and Walbot used confocal microscopy to track the fates of cells in developing maize anthers and showed that cells derived from the second meristem layer, known as L2-d cells, generate presumptive germ cells that then develop into premeiotic cells. Importantly, their work shows that the germinal lineage derives from many L2-d cells and thus is multiclonal rather than monoclonal, as was previously believed. Studies of mutants with defects in male reproduction showed that a secreted protein called MAC1 is crucial in germ-cell development; it limits the proliferation of L2-d cells and then directs the appropriate organization of the somatic cells surrounding the germ cells.

But what is the initial trigger for a mulitpotent L2-d cell to generate germ cells? The authors found that this requires a redox regulatory protein encoded by the male sterile converted anther 1 (msca1)locus. Through experimental manipulation of the redox status — for example, by administering gases to developing anthers — they show that hypoxic conditions trigger the germ-cell fate. This finding was confirmed by genetic evidence, as loss-of-function mutations in certain genes that control reactive oxygen species in the anther led to male sterility.

This mode of germline development is strikingly different to that seen in most animals, in which the germline is defined during embryogenesis. However, an intriguing parallel is that hypoxic conditions are also required in animal stem cell niches.