Chlororespiration is a respiratory process that takes place within plants. Inside plant cells there is an organelle called the chloroplast which is surrounded by the thylakoid membrane. This membrane contains an enzyme called NAD(P)H dehydrogenase which transfers electrons in a linear chain to oxygen molecules. This electron transport chain (ETC) within the chloroplast also interacts with those in the mitochondria where respiration takes place. Photosynthesis is also a process that Chlororespiration interacts with. If photosynthesis is inhibited by environmental stressors like water deficit, increased heat, and/or increased/decreased light exposure, or even chilling stress then chlororespiration is one of the crucial ways that plants use to compensate for chemical energy synthesis.
Chlororespiration – the latest model
Initially, the presence of chlororespiration as a legitimate respiratory process in plants was heavily doubted. However, experimentation on Chlamydomonas reinhardtii, discovered Plastoquinone (PQ) to be a redox carrier. The role of this redox carrier is to transport electrons from the NAD(P)H enzyme to oxygen molecules on the thylakoid membrane. Using this cyclic electron chain around photosystem one (PS I), chlororespiration compensates for the lack of light. This cyclic pathway also allows electrons to re-enter the PQ pool through NAD(P)H enzyme activity and production, which is then used to supply ATP molecules (energy) to plant cells.
In the year 2002, the discovery of the molecules; plastid terminal oxidase (PTOX) and NDH complexes have revolutionised the concept of chlororespiration. Using evidence from experimentation on the plant species Rosa Meillandina, this latest model observes the role of PTOX to be an enzyme that prevents the PQ pool from over-reducing, by stimulating its reoxidation. Whereas, the NDH complexes are responsible for providing a gateway for electrons to form an ETC. The presence of such molecules are apparent in the non-appressed thylakoid membranes of higher order plants like Rosa Meillandina.
The relation between chlororespiration, photosynthesis and respiration
Experimentation with respiratory oxidase inhibitors (for instance, cyanide) on unicellular algae has revealed interactive pathways to be present between chloroplasts and mitochondria. Metabolic pathways responsible for photosynthesis are present in chloroplasts, whereas respiratory metabolic pathways are present in mitochondria. In these pathways, metabolic carriers (like phosphate) exchange NAD(P)H molecules between photosynthetic and respiratory ETCs. Evidence using mass spectrometry on algae and photosynthetic mutants of Chlamydomonas, discovered that oxygen molecules were also being exchanged between photosynthetic and chlororespiratory ETCs. The mutant Chlamydomonas alga species, lacks photosystems one and two (PS I and PS II), so when the alga underwent flash-induced PS I activity, it resulted in no effect on mitochondrial pathways of respiration. Instead, this flash-induced PS I activity caused an exchange between photosynthetic and chlororespiratory ETCs, which was observed using polarography. This flash of PS I activity is triggered by an over-reduction of the PQ pool and/or lack of the pyridine nucleotide in the thylakoid membrane. A reduction in such molecules then stimulates NADPH and PTOX molecules to trigger chlororespiratory pathways.
Furthermore, in the absence of light (and thus photosynthesis), chlororespiration plays an integral role in enabling metabolic pathways to compensate for chemical energy synthesis. This is achieved through the oxidation of stromal compounds, which increases the PQ pool and allows for the chlororespiratory ETC to take place.
Stimulation of chlororespiration
Heat and light as stimulants
Quiles' experiment
An experiment on oat plants by scientist Maria Quiles, revealed that extreme light intensity can inhibit photosynthesis and result in the lack of PS II activity. This reduction leads to an increase in NAD(P)H and PTOX levels which then causes the stimulation of chlororespiration.
Oat leaves were incubated and Chlorophyll fluorescence emission was used to examine the effect of extreme light intensity. As the emission of the Chlorophyll fluorescence increased the PQ pool decreased. This stimulated the cyclic electron flow, causing NAD(P)H and PTOX levels to ultimately incline and initiate the process of chlororespiration within the thylakoid membrane of oat plants.
