The vast majority of land plants regulate gas exchange through their stomata – tiny pores usually found on the underside of leaves. The liverwort plant group Marchantiidae is an exception, as it lacks stomata and instead breathes through air pore complexes. This is an important evolutionary adaptation, and yet the mechanisms that regulate air pore complex development in Marchantiidae remain unknown. In this issue (p. 1472), Victor Jones and Liam Dolan identify the zinc finger protein MpWIP as necessary for the morphogenesis of the air pore complex in the epidermis of Marchantia polymorpha. The gene was first identified through a mutagenesis screen, in which overexpression led to the presence of ectopic rhizoids on the dorsal epidermis. Using a construct containing the MpWIP promoter fused to a reporter gene, the authors show that MpWIP is expressed both ventrally and dorsally and that the dorsal expression pattern is within the developing air pore complex cells. To determine whether MpWIP is required for air pore development, the authors use artificial microRNAs to generate plants with reduced expression of MpWIP, which results in defects in air pore complex morphology. Based on chimeric dominant repressor and activator versions of MpWIP expressed separately in transgenic plants, the authors provide some evidence for the possible role of MpWIP as a transcriptional repressor. This study identifies, for the first time, a gene that regulates the development of the air pore complex, which is an important evolutionary innovation in liverworts as an alternative to stomata.
