RESUMEN
High-resolution profiles of surface pH and growth along vertically growing maize (Zea mays) primary root tips were determined simultaneously by pH-sensitive microelectrodes and marking experiments. Methodological tests were carried out that proved the reliability of our kinematic growth analysis, while questioning the validity of an alternative technique employed previously. A distal acidic zone around the meristematic region and a proximal one around the elongation zone proper were detected. This pattern as such persisted irrespective of the bulk pH value. The proximal acidic region coincided with maximum relative elemental growth rates (REGR), and both characters reacted in a correlated manner to auxin and cyanide. The distal acidic band was unrelated to growth, but was abolished by cyanide treatment. We conclude that: (a) the pattern of surface pH as such is a regulated feature of growing root tips; (b) the correlation of extracellular pH and growth rate suggests a functional relationship only along proximal portions of the growing root tip; and (c) the distal acidic band is not caused by pH buffering by root cap mucilage, as suggested previously, but rather is controlled by cellular activity.
RESUMEN
Using Ca(2+)-selective microelectrodes, the concentration of free calcium ([Ca(2+)]) in the cytosol has been measured in root hair cells of Medicago sativa L. in the presence of nodulation (Nod) factors. Growing root hairs of M. sativa displayed a steep apical [Ca(2+)] gradient, i.e. 604-967 nM in the tip compared with 95-235 nM in the basal region. When tested within the first 5 to 10 µm of the tip, addition of NodRm-IV(C16:2,S) decreased the cytosolic [Ca(2+)], whereas an increase was observed when tested behind the tip. Overall, this led to a partial dissipation of the [Ca(2+)] gradient. The Ca(2+) response was specific: it was equally well observed in the presence of NodRm-IV(Ac,C16:2,S), reduced with NodRm-IV(C16:0,S), but not with chitotetraose, the nonactive glucosamine backbone. In contrast to growing root hairs, non-growing root hairs without a tip-to-base cytosolic [Ca(2+)] gradient responded to NodRm-IV(C16:2,S) with an increase in cytosolic [Ca(2+)] at the tip as well as at the root hair base. We suggest that the response to Nod factors depends on the stage of development of the root hairs, and that changes in cytosolic [Ca(2+)] may play different roles in Nod-factor signaling: changes of cytosolic [Ca(2+)] in the apical part of the root hair may be related to root hair deformation, while the increase in [Ca(2+)] behind the tip may be essential for the amplification of the Nod signal, for its propagation and transduction to trigger downstream events.
RESUMEN
Using monoclonal tubulin and actin antibodies, Al-mediated alterations to microtubules (MTs) and actin microfilaments (MFs) were shown to be most prominent in cells of the distal part of the transition zone (DTZ) of an Al-sensitive maize (Zea mays L.) cultivar. An early response to Al (1 h, 90 µM) was the depletion of MTs in cells of the DTZ, specifically in the outermost cortical cell file. However, no prominent changes to the MT cytoskeleton were found in elongating cells treated with Al for 1 h in spite of severe inhibition of root elongation. Al-induced early alterations to actin MFs were less dramatic and consisted of increased actin fluorescence of partially disintegrated MF arrays in cells of the DTZ. These tissue- and development-specific alterations to the cytoskeleton were preceded by and/or coincided with Al-induced depolarization of the plasma membrane and with callose formation, particularly in the outer cortex cells of the DTZ. Longer Al supplies (>6 h) led to progressive enhancements of lesions to the MT cytoskeleton in the epidermis and two to three outer cortex cell files. Our data show that the cytoskeleton in the cells of the DTZ is especially sensitive to Al, consistent with the recently proposed specific Al sensitivity of this unique, apical maize root zone.