Mitochondria and Endoplasmic Reticulum
Literature Review - Interactions
Between Mitochondria and Endoplasmic Reticulum and Mitochondria Calcium Dynamics
The interactions between mitochondria and endoplasmic reticulum in plants have great significance in the ability of the plant to self regulate, grow well and even fight off toxins. Calcium is very important for strengthening cell walls (Holdaway & Helper, 2003). The dynamic of calcium up-take has been studied since the 1800s due to the importance calcium has as a regulator for successful growth and development of plants (Hepler, 2005). The intricacies of the part Calcium plays in intracellular reactions which regulate growth and development has been carefully studied ever since the importance of the ion Ca2+ was determined.
In general Ca2+ signaling is necessary for regulating the mitochondria responses to both extra and intra-cellular stimuli. Satrústegui, Pard and del Arco (2007) theorize that more energy may be made available for the mitochondria to function when NADH/NAD ratios are increased. Ca2+ is necessary for the process to take place. The researchers explain “The main mechanism whereby Ca2+ modulates mitochondrial function involves Ca2+ entry in mitochondria via the Ca2+ uniporter or rapid uptake mode (RAM) mechanisms followed by the activation by Ca2+ of three dehydrogenases in the mitochondrial matrix. This causes an increase in mitochondrial NADH/NAD ratios which may result in increased energy available for mitochondrial functions.” (Satrústegui, Pard, & del Arco, 2007).
They also report that new methods with more sensitive detection abilities have been important in detecting plasma membrane channels (also known as Ca2+ release channels) close to the endoplasmic reticulum (ER). They theorize that because the mitochondria are closely positioned to the ER it is only with very modern techniques that the “microdomains of very high Ca2+ concentrations” can be distinguished. For example one such technique is using “an aequorin chimera which can be directed more exactly for measurements. (Satrústegui, Pard & del Arco, 2007).
Mitochondria and Endoplasmic reticulum Interactions
Wang, Zhu, Ling, Zhang, Liu, Baluska and Samaj (2010) investigated the role of microfilaments on both the positioning and the properties of plant mitochondria. This is important because mitochondria movement is regulated by actin filaments. The mitochondria stores of Ca2+ and their regulation by Ca2+ and may be affected by changes in the cytoskeleton. They found that indeed that their model describes the Ca 2+ release in a gradient within the mitochondria from the tip to the base of live Arabidopsis root hairs. This in turn caused more Ca2+ to be released from the ER which affected changes in the Ca2+ flux along the root hairs. (Wang, Zhu, Ling, Zhang, Liu, Baluska & Samaj, 2010)
Van Gestel, Köhler and Verbelen (2002) observed that both F-actin and microtubules are part of the mechanism which positions the plant mitochondria in the cortical cytoplasm. Interestingly once the F-actin was totally disrupted they found that (unless inhibited with orzalin) the mitochondria “parked themselves into conspicuous parallel arrays of transverse or oblique to the cell axis or clustered around chloroplasts and around patches and strands of endoplasmic reticulum. The cells used were elongated Nicotiana tabacum L. plant cells which contained “mitochondria-localized green fluorescent protein.” Van Gestel, Köhler & Verbelen, 2002)
Zuppini, Navazio and Mariani (2004) induced stress on the ER of soybean cells using a treatment of cyclopiazonic acid. Their observed that the ER and mitochondria worked together to try to regulate the effect.
Chatre, Matheson, Jack, Hanton, and Brandizzi (2008) developed a model to efficiently target mitochondrial pre-proteins and protein by using MITS1. This research is very detailed analysis of proteins and the role of the mitochondria. They observed that “presence of a tryptophan residue toward the C-terminus of the protein is crucial for mitochondrial targeting, as mutation of this residue results in a redistribution of MITS1 to the endoplasmic reticulum and Golgi apparatus.” They concluded that not only is protein movement to plant mitochondria a function of the full-length protein but also of the N-terminal extension. They used Nicotiana tabacum with Agrobacterium tumefaciens (strain GV3101) as the toxin to perturb reaction.
Gerasimenko, Sherwood, Tepikin, Petersen and Gerasimenko (2006) offer important insight into the Ca2+ release mechanisms within the ER. Three main actors, the NAADP, cADPR and IP3 all are involved in releasing Ca2+ for the ER.
Mitochondria Calcium Transport
Filippin, Magalha, DiBenedetto, Colella and Pozzan (2003) investigated the best probe to use when observing Ca2+ concentrations in a subpopulation of mitochondria during interactions between the Mitochondria and the ER. The stable reactions encouraged a larger concentration. They suggest is the green fluorescent protein-based probe, pericam, as the most useful for single cell studies due to the easy manipulation in sub-cellular locations, its photosensitivity and Ca2+ affinity for the probe. In conclusion they discuss the need for better understanding the ER Ca2+ release sites in relation to the mitochondria for both plant and animal cells. (Filipin, Magalha, DiBenedetto, Colella & Pozzan 2003)
Several studies investigate the relationship of Ca2+ with toxins in plant cells. For example, Kimber and Sze discovered that purified Helminthosporium maydis T “have shown the ATP-dependent methylamine uptake reflected activity of a proton-pumping ATPase enrichedin tonoplast vesicles and perhaps some ER vesicles.” The difference between their research and previous research is that they attempted to find the lowest toxin concentration that would affect show distinguishable change in primary and/or secondary sites of action. They were able to observe the toxin concentrations in the Ca2+ gradient and decreased Ca2+ “transport driven by different redox substrates as well as ATP in T mitochondria.” (Kimber and Sze 1974).
Heatherington and Brownlee (2004) successfully analyzed the kinases of Ca2+ signaling in order to better understand the “potential nodes of cross-talk for multiple signaling pathways that integrate Ca2+ signals into all aspects of plant growth and development.” They have managed to take the very complicated processes and successfully made progress in discerning the mechanisms. They used legume root hairs.
The understanding of the relationship between mitochondria and the endoplasmic reticulum (ER) plus the important processes in which Ca2+ is involved have been built upon for over forty years. The contemporary techniques of analyzing components in plant cells now allow for the differentiation between the mitochondria and the ER. Because of this the understanding of the concentrations of the stores of Ca2+ as well as the processes have become more accurate as the instrumentation continues to become more sophisticated.
There is no right or wrong way to study the concentration of the ion Ca2+ and its important functions and processes in regulating growth and development of plants. The complicated role it plays must be analyzed in pieces with the best measuring device available. Maybe one day we will be able to put all the pieces of the scientific research on mitochondria, ER and Ca2+ in order to understand in a global sense how plants are regulated and how they grow and develop.
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