Glycolate Oxidase Isozymes Are Coordinately Controlled by GLO1 and GLO4 in Rice

Glycolate oxidase (GLO) is a key enzyme in photorespiratory metabolism. Four putative GLO genes were identified in the rice genome, but how each gene member contributes to GLO activities, particularly to its isozyme profile, is not well understood. In this study, we analyzed how each gene plays a role in isozyme formation and enzymatic activities in both yeast cells and rice tissues. Five GLO isozymes were detected in rice leaves. GLO1 and GLO4 are predominately expressed in rice leaves, while GLO3 and GLO5 are mainly expressed in the root. Enzymatic assays showed that all yeast-expressed GLO members except GLO5 have enzymatic activities. Further analyses suggested that GLO1, GLO3 and GLO4 interacted with each other, but no interactions were observed for GLO5. GLO1/GLO4 co-expressed in yeast exhibited the same isozyme pattern as that from rice leaves. When either GLO1 or GLO4 was silenced, expressions of both genes were simultaneously suppressed and most of the GLO activities were lost, and consistent with this observation, little GLO isozyme protein was detected in the silenced plants. In contrast, no observable effect was detected when GLO3 was suppressed. Comparative analyses between the GLO isoforms expressed in yeast and the isozymes from rice leaves indicated that two of the five isozymes are homo-oligomers composed of either GLO1 or GLO4, and the other three are hetero-oligomers composed of both GLO1 and GLO4. Our current data suggest that GLO isozymes are coordinately controlled by GLO1 and GLO4 in rice, and the existence of GLO isozymes and GLO molecular and compositional complexities implicate potential novel roles for GLO in plants

Variation in net photosynthesis, rubisco activity and chloroplast ultrastructure among somatic hybrids of Solanum tuberosum and S. brevidens

The effect of ploidy, parental chloroplast type and parental nuclear genome dosage on net photosynthesis, Rubisco activity and chloroplast ultrastructure was studied among somatic hybrids of diploid S. brevidens and dihaploid S. tuberosum. An increase in nuclear ploidy resulted in an increase in net photosynthesis and specific leaf weight. There were no significant differences in net photosynthesis or Rubisco activity between the hybrids having different parental chloroplast type. Examination of the hexaploid hybrids indicated that Rubisco activity was affected by nuclear-organelle genome incompatibility, the most affected combination being tuberosum chloroplast type with brevidens nuclear genome. Examination of chloroplast ultrastructure revealed wide variation in the size of chloroplasts, starch granules, plastoglobuli and in grana stacking among the hybrids and between fusion parents

A nocturnal inhibitor of carboxylation in leaves

The diurnal variation in the activity of ribulose-l,5-bisphosphate carboxylase (RuBPCase), the major CO2-fixing enzyme in plants, has been shown to result from the influx and efflux of Mg2+ ions into and out of the chloroplast stroma. A recent re-examination of the phenomenon indicates that the inactivation of the enzyme, rather than being due to the efflux of Mg2+, is correlated in some plant species with an increase in the concentration of an organic phosphate ester in the chloroplast in the dark1-3. We have purified this potent inhibitor from dark-treated potato (Solanum tuberosum) leaves, and established that its structure is 2-carboxy-D-arabinitol-1-phosphate, a molecule that closely resembles an intermediate in the carboxylase reaction of RuBPCase