Effects of glucose starvation on mitochondrial subpopulations in the meristematic and submeristematic regions of maize root

Mitochondria isolated from 3-mm long maize (Zea mays L. var Dea) root tips were found to be heterogeneous on Percoll density gradients. The ultrastructure of these isolated mitochondria correlated well with that of mitochondria observed in situ and was consistent with the existence of mitochondria at different stages of maturation during cell development. The mitochondria of higher density presented an ultrastructure with many cristae and a dense matrix. These mitochondria showed classic respiratory properties, although with low ADP/O ratios. In contrast, the mitochondria of lower density showed few cristae and a clear matrix and did not seem to be fully functional because their rate of respiration was low and showed weak respiratory control. Lower- and higher- density mitochondria were shown to be differentially affected during the first stages of glucose starvation. The higher-density mitochondria from glucose-starved maize root tips retained the ultrastructure and most of the respiratory properties of nonstarved mitochondria, whereas lower- and intermediate-density mitochondria were absent in the mitochondrial preparations from glucose-starved maize root tips and were not observed in situ. Quantitatively, there was a decrease of the total mitochondrial pool when expressed as the amount of mitochondrial protein per root tip. However, this decrease affected low- and intermediate-density mitochondria, but not higher-density mitochondria. Thus, it was shown that a significant pool of functional mitochondria is maintained in maize root tips during the first stages of glucose starvation. The reasons for these apparently selective effects of glucose starvation on mitochondria are discussed in relation to effects on mitotic and differentiation processes.