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.