Per Gardeström - Leaf Mitochondria and Their Roles in Photosynthesis and Senescence

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Plants need a flexible metabolism in order to respond to the variable environmental conditions that they experience. We investigate leaf metabolism with the aim of obtaining a better understanding of the metabolic pathways involved, their regulation and compartmentation. We focus in particular on the interactions between mitochondria and chloroplasts. The project has two foci: (1) the interaction of respiratory and photosynthetic metabolic pathways in photosynthetically active cells and (2) the involvement of mitochondria and respiratory metabolism in the process of leaf senescence.
Per_Gardestrm_portrait

Respiration/photorespiration
Mitochondria fulfil essential functions in the energy metabo- lism of all cells. The conclusion from studies during the last few years is that mitochondrial oxidative phosphorylation is important for the supply of ATP to the cytosol of photosynthetic cells. However, mitochondrial activities in the light are closely related to photosynthetic metabolism, and many of the interactions between chloroplasts and mitochondria are coupled to photorespiration. In the light, a major function for leaf mitochondria is oxidation of glycine in the photores- piratory glycolate pathway. The flux through this pathway is considerable and involves redox reactions in chloroplasts, peroxisomes and mitochondria that strongly affect the redox situation of the cell. From estimations of subcellular redox states of NAD(H) and NADP(H) pools and ATP/ADP ratios, it is clear that the functions of leaf mitochondria are different in the light with from those in the dark. The TCA cycle appears to be reorganized in the light, so that it changes from being the main source of energy in the cell to providing a flexible mechanism that enables the cell to sustain optimal photosynthesis. This highlights the fact that the cell functions as a unit. Current studies aim to further explore the contributions of mitochondria to photosynthetic metabolism.

Senescence


	Rapid senescence is induced by darkening a few leaves while the rest of the plant remain in light (top right). The yellow leaf at 11 o ́clock was darkened for 6 days. Confocal microscopy pictures of leaf cells from whole darkened plants (top left) and individually darkened leaves (bottom left). Chloroplasts are red and mitochondria green.

Leaf senescence is an active and closely controlled process in which nutrients are reallocated from decaying leaves to other plant organs. Mitochondria and respiratory metabolism can contribute to this process by providing ATP and carbon skeletons. Analyses of ESTs from poplar indicate that respiration is important during the process of autumn leaf senescence. This is further supported by analysis of transcripts during the process, and determination of a detailed cellular timetable of autumn senescence.
We use Arabidopsis as a model to study dark-induced leaf senescence by comparing whole darkened plants (DP) and individually darkened leaves (IDL) (figure top right). In DP, chloroplasts are retained after a 6-day dark treatment and photosynthetic capacity is maintained, while the number of mitochondria and respiration decrease (figure top left). In IDL darkened for the same period of time, the photosynthetic capacity severely decreases aused due to chloroplast degra- dation, while mitochondria become bigger and respiration is maintained (figure bottom left). Based on metabolomic and transcriptomic results, we suggest that leaves from DP enter a stand-by metabolic state of low respiratory activity. In contrast, IDL show high respiratory activity with active mitochondrial contribution to the retrieval of nutrients from senescing leaves. In ongoing experiments we aim to elucidate the metabolic adjustments related to the rapid senescence in IDL.

Svensk samanfattning

Key publications

Hurry VM, Keerberg O, Pärnik T, Öquist G, Gardeström P (1996) Effect of cold hardening on the components of respiratory decarboxylation in the light and in the dark in leaves of winter rye. Plant Physiol 111:713-719

Igamberdiev AU, Gardeström P (2003) Regulation of NAD- and NADP-dependent isocitrate dehydrogenases by reduction levels of pyridine nucleotides in mitochondria and cytosol of pea leaves. BBA 1606:117-125

Bykova NV, Keerberg O, Pärnik T, Bauwe H, Gardeström P (2005) Interaction between photorespiration and respiration in transgenic plants with antisense reduction in glycine decarboxylase. Planta, 222: 130-140

Keskitalo J, Bergkvist G, Gardeström P, Jansson S (2005) A cellular timetable of autumn senescence. Plant Physiol 139: 1635-1648

Keech O, Pesquet, Ahad A, Askne A, Malmberg G, Nordvall D, Vodnala S, Tuominen H, Dizengremel P, Hurry V, Gardeström P (2007) The different fates of mitochondria and chloroplasts during dark-induced senescence in Arabidopsis leaves, Plant Cell & Env 30: 1523-1534

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July 2010

Friday, July 30, 2010
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