Root apices are composed of three distinct regions: an apical meristem where all cell divisions are taking place, a basal region of rapid cell elongation where cells extend parallel to the apical-basal axis of the root, and an interpolated transition zone (TZ). In contrast, shoot apices do not show such a clear zonation since cell divisions and rapid cell elongation occur side-by-side. In the root apex, the transition zone cells maintain an active cytoarchitecture, having central nuclei from which radiate both microtubules and actin filaments towards the peripheral plasma membrane. This specific cytoarchitecture is proposed to be optimally suited for the effective signal transmission between the plasma membrane and nucleus (Baluška et al. 2003a). Cells of the distal part of the transition zone, occupying approximately the region of 1,0 – 1,5 mm from the root tip, but not those of the meristem (approx. 0,0 – 1,0 mm from the root tip) and elongation region (approx. 2,0 – 7,0 mm from the tip), are characterized by the maximal degree of oxygen and IAA influx (Mancuso et al. 2005) into the root apex. Moreover, cells of the TZ are also specifically involved in the production of NO in response to environmental stimuli such as hypoxia. Immediately after the onset of hypoxia, NO is emitted specifically from the TZ of the root apex with a mean peak efflux of 925 ± 102 fmol NO cm^-2s^-1. Approximately 230 pmol NO cm^-2 are produced at the TZ. Interestingly, dramatic NO efflux was recorded only in the TZ of the root apex. Outside of the TZ, only small effluxes of NO (1/10 lower than those produced in the TZ) were detectable.
To further support the idea of the TZ as a kind of sensory zone, w e will present, simultaneous measurements of tissue oxygenation and single-cell electrical activity in the TZ, showing that action potentials were accompanied by immediate decreases in tissue oxygenation. This behaviour surprisingly resemble results recently published on the neural activity of cats (Thompson et al. 2003). Obviously, cells of the transition zone are accomplishing processes requiring very large amounts of oxygen. All these advancements converge towards a concept proposing that the transition zone of the root apex represents a highly specialized sensory and information processing region which encompasses neuronal features (Baluška et al. 2003b) . An attractive scenario would be that this root apex region acts as some kind of ‘brain-like’ command centre in higher plants (Baluška et al. 2004) . Future studies should answer this important question of plant biology.

1. Baluška F, Wojtaszek P, Volkmann D, Barlow PW (2003a) The architecture of polarized cell growth: the unique status of elongating plant cells. BioEssays 25, 569-576
2. Baluška, F, Šamaj, J, Menzel, D (2003b). Polar transport of auxin: carrier-mediated flux across the plasma membrane or neurotransmitter-like secretion? Trends in Cell Biology 13: 282-285
3. Baluška F, Mancuso S, Volkmann D, Barlow PW (2004) Root apices as plant command centres: the unique ‘brain-like’ status of the root apex transition zone. Biologia 59 (Suppl. 13): 7-19
4. Mancuso S, Marras AM, Volker M, Baluska F (2005) Non-invasive and continuous recordings of auxin fluxes in intact root apex with a carbon-nanotube-modified and self-referencing microelectrode. Anal Biochem, In press
5. Thompson JK, Peterson MR, Freeman RD (2003) Single-neuron activity and tissue oxygenation in the cerebral cortex. Science 299, 1070-1072