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Population heterogeneity of higher-plant mitochondria in structure and function
Abstract:
Mitochondria of rapidly developing mungbean seedlings were fractionated into four populations: two density classes, each from a 1500S and a 150S pellet. Each of the four populations exhibited cytochrome c oxidase (COX) activity and contained mitochondrial DNA and cardiolipin; plastid and glyoxysome content were found to be relatively low. Five mitochondrial membrane proteins, COXII/III, ATPase alpha/beta and porin, and a matrix enzyme, manganese superoxide dismutase (MnSOD), were detected by immunoblots in all four populations. Another matrix enzyme, pyruvate dehydrogenase was detected only in the two respiratory-competent 1500S populations. The two 150S populations contained a previously unidentified organelle that lacked demonstrable respiratory capability. This organelle, which we have tentatively referred to as "slow-sedimenting (ss-) mitochondrion", was small in size (below light-optics resolution, 70-300nm, majority < or =200nm) and possessed a peculiar looking boundary membrane, ribosomes, and an occasional prominent electron-dense spot. Characteristically, ss-mitochondria were almost always in contact with a filament-aligned membrane-like structure of varying length. Cristae structure, while undetected in small ss-mitochondria, appeared in larger individuals. Typical mitochondria were found in the denser 1500S population, while the lighter 1500S population consisted of 300-800 nm mitochondria exhibiting a varying degree of size-dependent inner membrane folding. Using electron microscopy (EM) immunolocalization and serial sectioning, we have identified in situ organelles resembling in size and in fine structure the ss-mitochondria, which also exhibit a size-dependent folding of the inner membrane. These results suggest that small ss-mitochondria may undergo a progressive development in situ. Taken together, our findings demonstrate the existence of a pattern of structure-function-coordinated gross heterogeneity among mitochondria. This pattern of mitochondrial heterogeneity, characterized both in isolated mitochondria and in situ, implies that small ss-mitochondria may represent a type of "nascent mitochondria" derived from a yet unidentified mitochondria-propagation mode operating during rapid seedling growth. Mitochondrial division by binary fission, characterized by the appearance of dumbbell-shaped intermediates, was also detected.
Insights
Mungbean seedlings contain diverse mitochondria, including small, nascent forms potentially developing into mature organelles. This heterogeneity suggests a novel mitochondrial propagation mode during rapid growth.
Area of Science:
- Plant Biology
- Cell Biology
- Mitochondrial Biology
Background:
- Mitochondria are crucial for cellular energy production and exhibit heterogeneity in various organisms.
- Understanding mitochondrial development and diversity is key to comprehending plant growth and adaptation.
Purpose of the Study:
- To investigate the heterogeneity of mitochondria in rapidly developing mungbean seedlings.
- To characterize distinct mitochondrial populations and their functional capabilities.
- To explore the potential developmental pathway of small, less-respiratory mitochondria.
Main Methods:
- Differential centrifugation to isolate mitochondrial populations.
- Enzyme activity assays (cytochrome c oxidase, pyruvate dehydrogenase).
- Immunoblotting to detect specific mitochondrial proteins (COX, ATPase, MnSOD).
- Electron microscopy (EM) for ultrastructural analysis and in situ identification.
- Immunolocalization and serial sectioning for organelle identification.
Main Results:
- Four distinct mitochondrial populations were isolated, differing in density and respiratory competence.
- All populations contained mitochondrial DNA and key proteins, but pyruvate dehydrogenase was absent in less dense fractions.
- A novel, small organelle, termed "slow-sedimenting (ss-) mitochondrion," was identified, lacking significant respiratory activity but possessing ribosomes and a unique membrane.
- ss-mitochondria showed size-dependent cristae development and were observed in situ, suggesting a developmental progression.
- Mitochondrial division by binary fission was also observed.
Conclusions:
- Mungbean seedlings exhibit significant mitochondrial heterogeneity, with distinct populations varying in size, structure, and function.
- Small ss-mitochondria may represent nascent or developing forms, indicating a progressive in situ development.
- These findings suggest a novel mitochondria-propagation mode contributing to rapid seedling growth.
- Mitochondrial heterogeneity is coordinated with structure and function, impacting cellular processes during development.