A role for mitochondria in the establishment and maintenance of the maize root quiescent center

Abstract

Mitochondria in the oxidizing environment of the maize (Zea mays) root quiescent center (QC) are altered in function, but otherwise structurally normal. Compared to mitochondria in the adjacent, rapidly dividing cells of the proximal root tissues, mitochondria in the QC show marked reductions in the activities of tricarboxylic acid cycle enzymes. Pyruvate dehydrogenase activity was not detected in the QC. Use of several mitochondrial membrane potential (DeltaPsi(m)) sensing probes indicated a depolarization of the mitochondrial membrane in the QC, which suggests a reduction in the capacity of QC mitochondria to generate ATP and NADH. We postulate that modifications of mitochondrial function are central to the establishment and maintenance of the QC.

Figure 1.

Longitudinal sections of the maize root tip. A, Light micrograph indicating the locations of the QC, PM, and root cap (RC). B to F, Transmission electron micrographs of the maize root tip. B, Longitudinal section of the maize root tip/QC at low magnification. The RC was removed just prior to fixation. QC cells marked with an asterisk (*) were examined further at higher magnification (C and D). Note cell files converging in the QC. C, QC cell in longitudinal section. Note large number of round-to-oval-shaped mitochondria (arrow) encircling the nucleus (N). D, High-power view of C showing that QC mitochondria (arrow) have intact double membranes and cristae. E and F, Cells from the PM. Note the relatively thin walls (indicative of a relatively high rate of mitosis), and dumbbell-shaped mitochondria (arrow) clustered about the nuclei.

Figure 2.

Whole mounts of QCs treated with various probes. Scale bar = 100 μm (except in C where the scale bar = 25 μm). A, Treated with carboxy-H₂DCFDA (C-400). Bright staining in the center of the QC points to the relatively oxidized redox status of this region. B and C, Whole mounts of a QC treated with the mitochondrial marker dye, Mitotracker Orange. Fluorescing mitochondria occur at the margins, but not in the center, of the QC. C, High-magnification view of B showing a strip of cells extending from the outside edge of the QC to the center (*). Fluorescing mitochondria are evident clustered around nuclei (arrow). D and E, Whole mounts of two different QCs treated with the dye JC-9. Fluorescing mitochondria (numerous small white dots) occur in cells at the margins, but not in the center (*), of the QC. Bright, diffuse staining is artifact.