Relationship between Rubisco activase and Rubisco contents in transgenic rice plants with overproduced or decreased Rubisco content

<p>Overproduction of ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco; E.C. 4.1.1.39) in rice did not necessarily improve photosynthesis. The reason for this was that a partial deactivation of Rubisco occurred in <i>RBCS</i>-overexpressed rice plants. Since a negative correlation between the amounts of Rubisco activase (RCA) and Rubisco has been reported for plants with overproduced or decreased RCA, the possibility that RCA content declines in <i>RBCS</i>-overexpressed rice plants was considered. The relationship between RCA and Rubisco contents was examined in <i>RBCS</i>-overexpressed and <i>RBCS</i>-antisense rice plants. Whereas the ratio of RCA to Rubisco contents in <i>RBCS</i>-antisense plants increased three- to fourfold as compared with that of the wild-type levels, this ratio decreased 60–70% of the wild-type levels in <i>RBCS</i>-overexpressed rice plants. Thus, an apparent trade-off between the amounts of RCA and Rubisco was observed in <i>RBCS</i>-transgenic rice plants. However, the amounts of several Calvin–Benson cycle enzymes changed in a similar manner to that of RCA in both types of <i>RBCS-</i>transgenic rice plants. When the relationships between the amounts of these enzymes, including RCA, and those of total leaf-N minus Rubisco-N and trichloroacetic acid (TCA)-soluble N were examined, there were no differences between them irrespective of genotypes. These results indicate that the negative correlation between the amounts of RCA and Rubisco in <i>RBCS</i>-transgenic rice plants is the result of a change in N allocation to Rubisco in transgenic rice plants rather than a trade-off. Such a negative correlation was also found for other Calvin–Benson cycle enzymes. In addition, since the amounts of these Calvin–Benson cycle enzymes and RCA were highly correlated with their mRNA levels irrespective of genotype, it is suggested that changes in the amounts of these proteins are regulated at their transcript levels by a change in N allocation to Rubisco.</p

Autophagy is induced under Zn limitation and contributes to Zn-limited stress tolerance in Arabidopsis (<i>Arabidopsis thaliana</i>)

<p>Autophagy is a degradation system for cellular components conserved in eukaryotes. In Arabidopsis, it is known that autophagy is crucial for growth under dark-induced carbon starvation and N deficiency. However, little is known about the relationship between autophagy and other nutrients. Here, we focused on the relationship between autophagy and Zn nutrition. We found that autophagy-deficient (<i>atg</i>) mutants showed an early senescence phenotype under Zn limitation and limited growth recovery from Zn limitation. Furthermore, we confirmed the induction of autophagy under Zn limitation by expression analysis of <i>autophagy-related</i> genes (<i>ATG</i>s) and imaging analysis of autophagic bodies with green fluorescent protein-ATG8a (GFP-ATG8a). In <i>atg</i> mutants, although the Zn concentrations were similar to those of the wild-type plants, the transcript levels of <i>Zn deficiency-inducible</i> genes fluctuated more, and O₂^– and H₂O₂ levels increased more than in wild-type plants. These results suggest that autophagy is involved in intracellular Zn usage and suppresses the accumulation of reactive oxygen species (ROS) generated by Zn limitation.</p

Respiration accumulates Calvin cycle intermediates for the rapid start of photosynthesis in <i>Synechocystis</i> sp. PCC 6803

<div><p>We tested the hypothesis that inducing photosynthesis in cyanobacteria requires respiration. A mutant deficient in glycogen phosphorylase (∆GlgP) was prepared in <i>Synechocystis</i> sp. PCC 6803 to suppress respiration. The accumulated glycogen in ΔGlgP was 250–450% of that accumulated in wild type (WT). The rate of dark respiration in ΔGlgP was 25% of that in WT. In the dark, P700⁺ reduction was suppressed in ΔGlgP, and the rate corresponded to that in (2,5-dibromo-3-methyl-6-isopropyl-<i>p</i>-benzoquinone)-treated WT, supporting a lower respiration rate in ∆GlgP. Photosynthetic O₂-evolution rate reached a steady-state value much slower in ∆GlgP than in WT. This retardation was solved by addition of d-glucose. Furthermore, we found that the contents of Calvin cycle intermediates in ∆GlgP were lower than those in WT under dark conditions. These observations indicated that respiration provided the carbon source for regeneration of ribulose 1,5-bisphosphate in order to drive the rapid start of photosynthesis.</p></div

OsATG7 is required for autophagy-dependent lipid metabolism in rice postmeiotic anther development

<div><p>In flowering plants, the tapetum, the innermost layer of the anther, provides both nutrient and lipid components to developing microspores, pollen grains, and the pollen coat. Though the programmed cell death of the tapetum is one of the most critical and sensitive steps for fertility and is affected by various environmental stresses, its regulatory mechanisms remain mostly unknown. Here we show that autophagy is required for the metabolic regulation and nutrient supply in anthers and that autophagic degradation within tapetum cells is essential for postmeiotic anther development in rice. Autophagosome-like structures and several vacuole-enclosed lipid bodies were observed in postmeiotic tapetum cells specifically at the uninucleate stage during pollen development, which were completely abolished in a retrotransposon-insertional <i>OsATG7</i> (autophagy-related 7)-knockout mutant defective in autophagy, suggesting that autophagy is induced in tapetum cells. Surprisingly, the mutant showed complete sporophytic male sterility, failed to accumulate lipidic and starch components in pollen grains at the flowering stage, showed reduced pollen germination activity, and had limited anther dehiscence. Lipidomic analyses suggested impairment of editing of phosphatidylcholines and lipid desaturation in the mutant during pollen maturation. These results indicate a critical involvement of autophagy in a reproductive developmental process of rice, and shed light on the novel autophagy-mediated regulation of lipid metabolism in eukaryotic cells.</p></div