Abstract
Unbound tetrapyrroles, i.e. protochlorophyllide (Pchlide), chlorophyllide and chlorophylls, bring the risk of reactive oxygen species (ROS) being generated in the initial stages of angiosperm deetiolation due to inefficient usage of the excitation energy for photosynthetic photochemistry. We analyzed the activity of superoxide dismutases (SODs) in etiolated wheat (Triticum aestivum) leaves and at the beginning of their deetiolation. Mn-SOD and three isoforms of Cu/Zn-SODs were identified both in etiolated and greening leaves of T. aestivum. Two Cu/Zn-SODs, denoted as II and III, were found in plastids. The activity of plastidic Cu/Zn-SOD isoforms as well as that of Mn-SOD correlated with cell aging along a monocot leaf, being the highest at leaf tips. Moreover, a high Pchlide content at leaf tips was observed. No correlation between SOD activity and the accumulation of photoactive Pchlide, i.e. Pchlide bound into ternary Pchlide:Pchlide oxidoreductase:NADPH complexes was found. Cu/Zn-SOD I showed the highest activity at the leaf base. A flash of light induced photoreduction of the photoactive Pchlide to chlorophyllide as well as an increase in all the SODs activity which occurred in a minute time-scale. In the case of seedlings that were deetiolated under continuous light of moderate intensity (100 μmol photons m-2 s-1), only some fluctuations in plastidic Cu/Zn-SODs and Mn-SOD within the first four hours of greening were noticed. The activity of SODs is discussed with respect to the assembly of tetrapyrroles within pigment-protein complexes, monitored by fluorescence spectroscopy at 77 K.
Highlights
It is commonly known that plants absorb light to drive photosynthesis
Cu/Zn-superoxide dismutase (SOD) I showed the highest activity in all the lanes, Cu/Zn-SOD II form and the least manganese SOD (Mn-SOD) and Cu/Zn-SOD III
Our study showed the presence of the same SOD forms, i.e. one Mn-SOD and three isoforms of Cu/Zn-SOD, in etiolated, green and greening leaves of T. aestivum
Summary
Angiosperm plants require light for the biosynthesis of chlorophyll (Chl), the main photosynthetic pigment as reviewed in [1,2,3]. Due to evolution, they have retained only a light-dependent protochlorophyllide oxidoreductase (EC 1.3.1.33; LPOR) to catalyze the reduction of protochlorophyllide (Pchlide) to chlorophyllide (Chlide) [4,5]. They have retained only a light-dependent protochlorophyllide oxidoreductase (EC 1.3.1.33; LPOR) to catalyze the reduction of protochlorophyllide (Pchlide) to chlorophyllide (Chlide) [4,5] This photoenzyme is not activated in the absence of light, which stops Chl biosynthesis and strongly influences seedling development.
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