Ammonium sensing in plant roots
Ying Liu, Markus Meier
Ammonium, as a major inorganic nitrogen source, triggers multiple physiological and morphological responses in plants, such as specific changes in gene expression, metabolism, redox status, and root system architecture (Liu and von Wirén, 2017, J. Exp.Bot.). Many of these responses are dependent on AMT-type ammonium transporters and are linked to a signal sensing event rather than a nutritional effect, indicating that ammonium acts as a signaling molecule in plants. Changes in root system architecture serve as an adaptive response of plants to fluctuating nutrient availabilities in soils and also as a morphological read-out for nutrient sensing. This also applies to ammonium, which modifies root system architecture by inhibiting root elongation and stimulating lateral root branching.
In response to ammonium supply, H2O2 accumulates in the root elongation zone which represses meristematic cell division and longitudinal cell elongation, thereby shortening primary root growth. By manipulating the level of H2O2 in roots, the inhibitory effect on root growth can be uncoupled from ammonium. In this context, a novel ROS scavenging process counteracting the ammonium induced generation of H2O2 has been identified by genetic screening. Our work elucidates how cell fate determination in roots is regulated by combinatorial effects of ROS generation and ROS scavenging under ammonium supply.
Compared to the primary root, lateral roots constitute the majority of the total root system and show a higher plasticity to heterogeneous nutrient availabilities (Giehl and von Wirén, 2014, Plant Physiol.). Local ammonium supply strongly stimulates lateral root branching in an AMT-dependent manner in Arabidopsis. Our current work shows that local ammonium supply shapes a highly branched root system mainly by stimulating lateral root emergence, and this response is most likely triggered by an altered distribution of auxin in roots exposed to localized ammonium. This line of research thus contributes to a better understanding of how local nutrient availabilities shape root system architecture.