The root apex of Corn (Zea Mays) is made up of several populations of cells. One of these populations, known as the quiescent center (QC), show an average cycle time for its cells in the range of 170 hours, in contrast to the cycle time of 10-16 hours for root cap initials, the most rapidly dividing cells in the root. In addition, cells within QC are reported have the lowest rate of DNA synthesis, the lowest DNA content and the lowest RNA per cell. A new hypothesis suggests that the inactivity of cells in the QC in the maize root may be explained by differences in numbers and structural characteristics of the mitochondria in different cells. In order to study the mitochondria in the QC and surrounding meristem cells more closely, we examined maize root tissues at the ultrastructural level. Root tissues were fixed in glutaraldehyde and osmium, embedded in plastic resin, sectioned by ultramicrotomy, and the mitochondria viewed via transmission electron microscope. We will report results comparing and contrasting the mitochondria in the QC, with mitochondria in more rapidly dividing cells.
Seeds of corn (Zea mays, variety Merit) were imbibed for 2 hours in running water and germinated in the dark at 25 (C for 48 hours. After removing the root caps, root tips were fixed at 30 (C--in the microwave over-- in 6% glutaradehyde, buffered with 0.1 M sodium cacodylate (PH 7.2) for 4 minutes. After three washing periods (2.5 minutes each) with 0.1 M sodium cacodylate at room temperature, the samples were post-fixed in 1% osmium aqueous (Os O4) at 30 (C (in the microwave) for 4 minutes and rinsed in distilled water at room temperature, three times, 30 seconds each. Dehydration was carried out in a graded acetone series: 25%, 35%, 50%, 75%, 95% and pure acetone. For each different concentration of the acetone, the samples were microwaved twice for 40 seconds at 30 (C. Then the samples were infiltrated sequentially in 1:3, 1: 1, 3:1 epon/spur resin: acetone and finally transferred to pure epon/spur resin. After all the acetone in the tissues has been replaced by plastics, samples were polymerized in the over (50 (C to 60 (C) for 48 hours. Samples were thick sectioned (2-3 microns) by regular microtomy , until reaching approximately the median section. The sectioned block were then remounted and sectioned further on an ultramicrotome--MT 6000. (60 nanometer). The ultrathin sections were collected and stained using saturated Uranyl acetate and Lead tartrate. Photographs were taken using a transmission electron microscope. (170X--26500; FEI, Tecnai 12, USA) operating at 100 KV.
Mitochondria, double membrane bounded organelles, play very important roles in cell activities. They are the site of cellular respiration, and via the production of ATP are the powerhouse of all the plants and animals. To study mitochondria closely is one of the keys to understand the secrets of cell activities. From our work with the electron microscope, we can conclude that in normal roots of Zea Mays, most of the mitochondria in cells of the QC have a different shape and size, compared to the mitochondria elsewhere in the meristem. By comparing images of the same magnification, we have no trouble seeing that most mitochondria in the QC are smaller, fatter and shorter than the mitochondria in the adjacent rapidly dividing cells, which are larger and in elongated form (figure 3 vs. figure 4). In one particular QC cell, we found some flower shaped, large mitochondria, which were never found in any root cells. We hypothesize that these structural differences may be related to the inactivity of mitochondria in cells of the QC. However, if and how the differences in mitochondria structure cause the cells at QC divide infrequently, remains unknown. Further experiments and a closer examination are necessary. In addition, we find two more features of the cells of the quiescent center. One is in the numbers of ribosomes, the organelles responsible for making proteins. Compared Figure 1 & 3 to Figure 2 & 4, we can see that the density of the ribosome in the cells of QC is lower than the density of the ribosomes in meristem cells. In other words, we can draw the conclusion that cells at QC have fewer ribosomes than rapidly dividing cells. The other interesting observation is about the cell wall. Note that the cells at QC tend to have thicker walls than the meristem cells.