Maize Thylakoids Research Articles

Partial characterization of a minor chlorophyll-protein found in primary thylakoids of intermittently illuminated maize

A mild solubilization with sodium dodecyl sulphate of intermittently illuminated maize (Zea mays L. Mvsc 429) thylakoids allows the separation of a minor chlorophyll-protein in the position of the light harvesting chlorophyll-protein monomer of green plants by polyacrylamide gel electrophoresis. It contains mainly chlorophyll a, its chlorophyll b content may come from the slightly contaminating light harvesting chlorophyll a/b-protein. It represents about 15% of the chlorophyll in protochloroplasts. The new chlorophyll-protein has an absorption maximum at 672 nm, and only one fluorescence emission peak at 680 nm. A 34 kD polypeptide is the most abundant one in the polypeptide pattern of the complex. The function of the new chlorophyll-protein is unknown at present. Its relationship to other chlorophyll-proteins is discussed.

Cation-mediated regulation of excitation energy distribution in chloroplasts lacking organized Photosystem II complexes

Despite the total loss of Photosystem II activity, thylakoids isolated from the green nuclear maize mutant hcf∗-3 contain normal amounts of the light-harvesting chlorophyll a b pigment-protein complex (LHC). We interpret the spectroscopic and ultrastructural characteristics of these thylakoids to indicate that the LHC present in these membranes is not associated with Photosystem II reaction centers and thus exists in a ‘free’ state within the thylakoid membrane. In contrast, the LHC found in wild-type maize thylakoids shows the usual functional association with Photosystem II reaction centers. Several lines of evidence suggest that the free LHC found in thylakoids isolated from hcf∗-3 is able to mediate cation-dependent changes in both thylakoid appression and energy distribution between the photosystems: (1) Thylakoids isolated from hcf∗-3 and wild-type seedlings exhibit a similar Mg 2+-dependent increase in the short/long wavelength fluorescence emission peak ratio at 77 K. This Mg 2+ effect is lost following incubation of thylakoids isolated from either source with low concentrations of trypsin. Such treatment results in the partial proteolysis of the LHC in both membrane types. (2) Thylakoids isolated from both hcf∗-3 and wild-type seedlings show a similar Mg 2+ dependence for the enhancement of the maximal yield of room temperature fluorescence and light scattering; both Mg 2+ effects are abolished by brief incubation of the thylakoids with low concentrations of trypsin (3) Mg 2+ acts to reduce the relative quantum efficiency of Photosystem I-dependent electron transport at limiting 650 nm light in thylakoids isolated from hcf∗-3. (4) The pattern of digitonin fractionation of thylakoid membranes, which is dependent upon structural membrane interactions and upon LHC in the thylakoids, is similar in thylakoids isolated from both hcf∗-3 and wild-type seedlings. We conclude that the surface-exposed segment of the LHC, but not the LHC-Photosystem II core association, is necessary for the cation-dependent changes in both thylakoid appression and energy distribution between the two photosystems, and that the LHC itself is able to transfer excitation energy directly to Photosystem I in a Mg 2+-dependent fashion in the absence of Photosystem II reaction centers. The latter phenomenon is equivalent to a cation-induced change in the absorptive cross-section of Photosystem I.

Light-induced structural changes of thylakoids in normal and carotenoid deficient chloroplasts of maize

Changes of membrane thickness and loculi were studied after red (650 nm) and far-red (707 nm) light in thylakoids of maize with different stacking and pigment compositions.