Hypoxia in shoot apical meristem

Complex multicellular organisms evolved on Earth in an oxygen-rich atmosphere1; their tissues, including stem-cell niches, require continuous oxygen provision for efficient energy metabolism2. Notably, the maintenance of the pluripotent state of animal stem cells requires hypoxic conditions, whereas higher oxygen tension promotes cell differentiation3. Here we demonstrate, using a combination of genetic reporters and in vivo oxygen measurements, that plant shoot meristems develop embedded in a low-oxygen niche, and that hypoxic conditions are required to regulate the production of new leaves. We show that hypoxia localized to the shoot meristem inhibits the proteolysis of an N-degron-pathway4,5 substrate known as LITTLE ZIPPER 2 (ZPR2)—which evolved to control the activity of the class-III homeodomain-leucine zipper transcription factors6–8—and thereby regulates the activity of shoot meristems. Our results reveal oxygen as a diffusible signal that is involved in the control of stem-cell activity in plants grown under aerobic conditions, which suggests that the spatially distinct distribution of oxygen affects plant development. In molecular terms, this signal is translated into transcriptional regulation by the N-degron pathway, thereby linking the control of metabolic activity to the regulation of development in plants.

Plants typically respond to waterlogging by producing new adventitious roots with aerenchyma and many wetland plants form a root barrier to radial O2 loss (ROL), but it was not known if this was also the case for lateral roots. We tested the hypothesis that lateral roots arising from adventitious roots can form a ROL barrier, using root-sleeving electrodes and O2 microsensors to assess ROL of Zea nicaraguensis, the maize (Zea mays ssp. mays) introgression line with a locus for ROL barrier formation (introgression line (IL) #468) from Z. nicaraguensis and a maize inbred line (Mi29). Lateral roots of Z. nicaraguensis and IL #468 both formed a ROL barrier under stagnant, deoxygenated conditions, whereas Mi29 did not. Lateral roots of Z. nicaraguensis had higher tissue O2 status than for IL #468 and Mi29. The ROL barrier was visible as suberin in the root hypodermis/exodermis. Modelling showed that laterals roots can grow to a maximum length of 74 mm with a ROL barrier, but only to 33 mm without a barrier. Presence of a ROL barrier in lateral roots requires reconsideration of the role of these roots as sites of O2 loss, which for some species now appears to be less than hitherto thought.