The case for cytosolic NO₃^- heterostasis: A critique of a recently proposed model

A model recently proposed by Siddiqi & Glass (Plant, Cell, and Environment 25, 1211-1217, 2002) attempts to reconcile discrepancies in the measurement of cytosolic nitrate concentrations ([NO₃^-]_cyt) in plant root cells, specifically between low (∼4 mM) homeostatic values reported in studies using ion-specific microelectrodes on the one hand, and wide fluctuations in [NO₃^-]_cyt reported in other studies, especially those using compartmental analysis by tracer efflux (CATE). Although Siddiqi & Glass concede that cytosolic NO₃^- homeostasis, as determined by microelectrodes, is at odds with certain experimental observations, they nevertheless promote a model that takes microelectrode readings at face value, and assert that the variations seen using CATE methodology are artefacts attributable to contributions from substantial, rapidly exchanging, and highly variable NO₃^- pools putatively residing in organelles such as plastids and the endoplasmic reticulum. We show here that such a model is not tenable, drawing upon experimental evidence from previous studies, and from a more comprehensive model that examines the characteristics and consequences of subcompartmented cytoplasmic exchange in root cells.