Chloroplast Structure and Function in Tissue Cultures of a C 4 Plant

Photorespiration is of great interest because of its relationship to plant productivity. It has been estimated that in C3 plants, the process of photorespiration oxidizes up to 50% of the newly synthesized photosynthetic products, whereas in C4 plants, photorespiration is either not present or occurs in much reduced amounts (20). In addition to the lack of apparent photorespiration and significantly greater net assimilation rate, C4 plants also possess several other structural and physiological features which distinguish them from C3 plants. These include: a different chloroplast ultrastructure, reduced discrimination of 13C relative to '2C, higher temperature optimum for photosynthesis, higher light saturation point, and significantly lower transpiration to photosynthesis ratios (13,19).One of the most interesting features of C4 plants is that virtually all of them have a characteristic leaf anatomy consisting of at least two chloroplast-containing cell layers radially arranged around the vascular bundles. This concentric arrangement of chlorenchyma cells in C4 plant leaves has been called "Kranz" anatomy (13), and many structure-function postulations have been made concerning the importance of Kranz anatomy to the over-all physiology of C4 plants. There is general agreement that operation of the complete cycle of C4 reactions is closely associated with, if not dependent upon, possession of Kranz anatomy by C4 plants. This is particularly true for the apparent lack of photorespiration. As envisioned, the concentric arrangement of chloroplast-containing cells in C4 plants acts either to inhibit photorespiratory CO2 loss by high CO2 concentrations in the bundle sheath cells, or to promote refixation of evolved CO2 as it diffuses outwardly through the mesophyll cells. A discussion and models for such structure-function relationships in C4 plants have recently been given (13).Regardless of the exact mechanism, most reports agree on the singular importance of Kranz anatomy in minimizing or eliminating detectable CO2 evolution in the light by C4 plants (1,3,7,15,16 (10), and photorespiratory responses in senescing Portulaca oleracea leaves which are virtually indistinguishable from those of mature C3 plant leaves (9). In spite of these examples, operation of the C4 pathway, particularly the lack of detectable photorespiration in C4 plants, is still thought by many to be dependent on an intact Kranz anatomy. MATERIALS AND METHODSIn the present experiments, P. oleracea L. stem tissue cultures, grown under a controlled light (about 80 ,einsteins m-2 sec-') and temperature regime (20 C) were used in all experiments. Cultures, conditions, and media were otherwise as reported by Laetsch and Kortschak (14). These tissues were similar to mature P. oleracea leaves on the basis of cell ultrastructure and 14C02 labeling patterns (10, unpublished observations). In addition, leaves and tissue cultures of a C3 plant, Streptanthus tortuosus, were used to enable comparison of photorespiratory activity in C3 plant tissues to ...

Section: Methodsmentioning

confidence: 94%

Section: Discussionmentioning

confidence: 99%

mentioning

confidence: 99%

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Photorespiration in C₃ and C₄ Plant Tissue Cultures

Kennedy¹

1976

“…Chloroplasts in Zea mays leaves are dimorphic, as they are in many other C4 plants (24). In green leaves, the elongated, ellipsoidal BS chloroplasts contain single thylakoids which extend through the entire length of the plastids and rudimentary grana.…”

Section: Methodsmentioning

confidence: 99%

Accumulation of δ-Aminolevulinic Acid and Its Relation to Chlorophyll Synthesis and Development of Plastid Structure in Greening Leaves

Klein¹,

Harel²,

Ne’eman³

et al. 1975

Levulinic acid inhibited the greening of etiolated maize (Zea mays) and bean (Phaseolus vulgaris) leaves and caused accumulation of 5-aminolevulinic acid (ALA). ALA accumulation in maize was equivalent to the decrease in chlorophyll, over a wide range of experimental conditions. It was saturated at low light intensities and was not limited by the supply of substrates during the early hours of greening. During 20 hours in light, levulinic acid had little effect on the structural development of thylakoids in bundle sheath chloroplasts but significantly reduced the number and size of thylakoids in grana of mesophyll chloroplasts. Recrystallization of prolamellar bodies and their reformation was inhibited. Mitochondria appeared not to be affected.The accumulation of ALA in bean leaves differed from that in maize in regard to its time course and the effect of levulinic acid concentration and light intensity. The amount of ALA accumulated exceeded that expected from the degree of inhibition of chlorophyll synthesis. Levulinic acid caused abnormalities in the structural development of bean chloroplasts and marked swelling of mitochondria.Chloramphenicol and cycloheximide inhibited ALA accumulation, whlile inhibitors of RNA synthesis had no effect. The extent of inhibition depended on the time the inhibitor was applied during the greening process. The use of ALA accumulation as a tool for studying the control of chlorophyll synthesis is discussed.

“…Photoheterotrophic, green cultures of the C4 species, Gisekia pharnaceoides (25), Portulaca oleracea (12,13) and Froelichia gracilis (15) have been initiated and used in photosynthesis studies, but these cultures were grown with 2 to 2.5% sucrose in the culture medium.…”

mentioning

confidence: 99%

Characteristics of Five New Photoautotrophic Suspension Cultures Including Two Amaranthus Species and a Cotton Strain Growing on Ambient CO₂ Levels

Xu¹,

Blair²,

Rogers³

et al. 1988

Suspension cultures of cotton (Gossypium hirsutum), Amaranthus cruentus, A. powellii, Datura innoxia, and a Nicotiana tabacum-N. glutinosa fusion hybrid were adapted to grow photoautotrophically under continuous light. The cotton strain grew with an atmosphere of ambient CO2 (about 0.06 to 0.07% in the culture room) while the other strains required elevated CO2 levels (5%). Photoautotrophy was indicated by the requirement for CO2 and for light for growth. The strains grew with doubling times near 14 days and had from 50 to 600 micrograms of chlorophyll per gram of fresh weight. The cells grew in small to moderate sized clumps with cell sizes from 40 to 70 micrometers (diameter). Like most photoautotrophic cultures described so far the ribulose 1,5-bisphosphate carboxylase (RuBPcase) activity levels were well below those of mature leaves. The phosphoenolpyruvate carboxylase levels were not elevated in the C4 Amaranthus species. The cells showed high dark respiration rates and had lower net CO2 fixation under high 02 conditions. Dark CO2 fixation rates ranged from near 10 to 30% of that in light.Fluorescence emission spectra measurements show that the cell antenna pigments systems of the four strains examined are similar to that of chloroplasts of green plants. The cotton strain which was capable of growth under ambient CO2 conditions showed the unique properties of a high RuBPcase activation level in ambient CO2 and a stable ability to show net CO2 fixation in 21% 02 conditions. The first higher plant tissue culture described as being photoautotrophic, i.e. growing with CO2 and light as the sole carbon and energy source, respectively, was that of Bergmann (3) lower RuBPcase2 activity which often leads to RuBPcase to PEPcase activity ratios near one. These differing characteristics are not really surprising since photoautotrophic cultured cells are growing and dividing unlike a mature leaf. Thus, the cultured cells should be compared to developing leaves which do have high respiration rates and a lower ratio of RuBPcase to PEPcase activity (1,14). Probably because of these characteristics all photoautotrophic cultures described thus far require elevated CO2 levels, usually 1 to 5%, for growth and have CO2 compensation concentrations higher than that ofmature leaves (reviewed in 11, 20).To date, no species with the C4 photosynthesis pathway has been grown photoautotrophically. Photoheterotrophic, green cultures of the C4 species, Gisekia pharnaceoides (25), Portulaca oleracea (12, 13) and Froelichia gracilis (15) have been initiated and used in photosynthesis studies, but these cultures were grown with 2 to 2.5% sucrose in the culture medium.Photoautotrophic cultures can be used for many purposes including the selection of mutants resistant to photosynthetic herbicides, for screening for herbicidal activity, and for herbicide mechanism ofaction studies. The photoautotrophic cultures may be valuable for producing desired compounds if chloroplasts are involved in the biosynthesis. Studies of chloroplast dev...