Studies on some plant species have shown that increasing the growth temperature gradually or pretreating with high temperature can lead to obvious photosynthetic acclimation to high temperature. To test whether this acclimation arises from heat adaptation of ribulose 1,5-bisphosphate carboxylase/oxygenase (Rubisco, EC 126.96.36.199) activation mediated by Rubisco activase (RCA), gene expression of RCA large isoform (RCA(L)) and RCA small isoform (RCA(S)) in rice was determined using a 4-day heat stress treatment [40/30 degrees C (day/night)] followed by a 3-day recovery under control conditions [30/22 degrees C (day/night)]. The heat stress significantly induced the expression of RCA(L) as determined by both mRNA and protein levels. Correlative analysis indicated that RCA(S) protein content was extremely significantly related to Rubisco initial activity and net photosynthetic rate (Pn) under both heat stress and normal conditions. Immunoblot analysis of the Rubisco-RCA complex revealed that the ratio of RCA(L) to Rubisco increased markedly in heat-acclimated rice leaves. Furthermore, transgenic rice plants expressing enhanced amounts of RCA(L) exhibited higher thermotolerance in Pn and Rubisco initial activity and grew better at high temperature than wild-type (WT) plants and transgenic rice plants expressing enhanced amounts of RCA(S). Under normal conditions, the transgenic rice plants expressing enhanced amounts of RCA(S) showed higher Pn and produced more biomass than transgenic rice plants expressing enhanced amounts of RCA(L) and wild-type plants. Together, these suggest that the heat-induced RCA(L) may play an important role in photosynthetic acclimation to moderate heat stress in vivo, while RCA(S) plays a major role in maintaining Rubisco initial activity under normal conditions.
"Also correlative analysis indicated that RCAS protein content was significantly related to Rubisco initial activity and net photosynthetic rate under both heat stress and normal conditions. Immunoblot analysis of the Rubiscoe RCA complex revealed that the ratio of RCAL to Rubisco increased markedly in heat-acclimated rice leaves (Wang et al., 2010). RCAL was upregulated in response to long-term salt stress. "
[Show abstract][Hide abstract] ABSTRACT: Regulation of Rubisco (D-ribulose-1,5-bisphosphate carboxylase/oxygenase activase (RCA) gene expression and polypeptide content were determined in Brachypodium distachyon leaves, stems and ear elements at different developmental stages under optimal growth conditions as well as under drought and salt stress conditions. B. distachyon leaf contains a much greater amount of Rubisco activase small (RCAS) isoform than the large one (RCAL) under optimal growth conditions. Increased levels of the RCAL isoform compared with the RCAS isoform were found in leaves and in green stems under salt and drought stress, respectively. Transcriptional levels of RCA are almost identical in different leaf positions. Short-term drought and salt stresses did not cause the impairment of RCA gene expression in early seedlings. But gradually increasing drought stress significantly decreased gene expression in early seedling samples. Amounts of the RCAS isoform were found to be more in different leaves of the plant compared with the RCAL isoform and their ratio was constant under normal condition. In green stems gene expression of RCA decreased under salt and drought stresses, although as it was in green leaves protein amounts of RCAL isoform increased compared with the RCAS isoform. All of the above described results clearly indicate that the accumulation of each RCA isoform is differentially regulated by developmental and environmental cues.
"Both the a and b isoforms are capable of activating Rubisco; however, they have slightly different maximal activities (Shen et al., 1991). In rice (Oryza sativa), the a isoform has been shown to play an important role in photosynthetic acclimation to moderate heat stress in vivo, whereas the b isoform has been shown to play a major role in maintaining the initial activity of Rubisco under normal conditions (Wang et al., 2010). More significantly, light modulation of Rubisco in Arabidopsis (Arabidopsis thaliana) requires a capacity for redox regulation of the a isoform via thioredoxin-f (Zhang and Portis, 1999; Zhang et al., 2001, 2002). "
[Show abstract][Hide abstract] ABSTRACT: Rubisco activase (RCA) catalyzes the activation of Rubisco in vivo and plays a crucial role in regulating plant growth. In maize, only β-form RCA genes have been cloned and characterized. In this study, a genome-wide survey revealed the presence of an α-form RCA gene and a β-form RCA gene in the maize genome, herein referred to as ZmRCAα and ZmRCAβ, respectively. An analysis of genomic DNA and cDNA sequences suggested that alternative splicing of the ZmRCAβ pre-mRNA at its 3'UTR could produce two distinctive ZmRCAβ transcripts. Analyses by electrophoresis and matrix-assisted laser desorption/ionization-tandem time-of-flight mass spectrometry (MALDI-TOF-MS) showed that ZmRCAα and ZmRCAβ encode larger and smaller polypeptides of approximately 46 kD and 43 kD, respectively. Transcriptional analyses demonstrated that the expression levels of both ZmRCAα and ZmRCAβ were higher in leaves and during grain filling and that expression followed a specific cyclic day/night pattern. In 123 maize inbred lines with extensive genetic diversity, the transcript abundance and protein expression levels of these two RCA genes were positively correlated with grain yield. Additionally, both genes demonstrated a similar correlation with grain yield compared to three C4 photosynthesis genes. Our data suggest that, in addition to the β-form RCA-encoding gene, the α-form RCA-encoding gene also contributes to the synthesis of RCA in maize, and support the hypothesis that RCA genes may play an important role in determining maize productivity.
"NM_001175017.1). Evidence has been presented for the expression of a high-molecular-mass Rca polypeptide, possibly the a-isoform, in some maize cultivars during heat stress (Sánchez de Jiménez et al., 1995; Ristic et al., 2009), similar to the up-regulation of a-Rca expression in rice (Wang et al., 2010). Detailed analysis of the kinetic and thermal properties of the various a-Rca and b-Rca forms is necessary to determine how patterns of Rca isoform expression might exert control over enzyme activity. "
[Show abstract][Hide abstract] ABSTRACT: Rubisco's catalytic chaperone, Rubisco activase (Rca), uses the energy from ATP hydrolysis to restore catalytic competence to Rubisco. In Arabidopsis, inhibition of Rca activity by ADP is fine-tuned by redox regulation of the α-isoform. To elucidate the mechanism for Rca regulation in species containing only the redox-insensitive β-isoform, the response of activity to ADP was characterized for different Rca forms. When assayed in leaf extracts, Rubisco activation was significantly inhibited by physiological ratios of ADP/ATP in species containing both α- and β-Rca (Arabidopsis thaliana and Camelina sativa) or just the β-Rca (Nicotiana tabacum). However, Rca activity was insensitive to ADP inhibition in an Arabidopsis transformant, rwt43, that expresses only Arabidopsis β-Rca, but not in a transformant of Arabidopsis that expresses a tobacco-like β-Rca. ATP hydrolysis by recombinant Arabidopsis β-Rca was much less sensitive to inhibition by ADP than recombinant tobacco β-Rca. Mutation of 17 amino acids in the tobacco β-Rca to the corresponding Arabidopsis residues reduced ADP sensitivity. In planta, Rubisco generally deactivated at low irradiance except in the Arabidopsis rwt43 transformant containing an ADP-insensitive Rca. Induction of CO2 assimilation after transition from low to high irradiance was much more rapid in the rwt43 transformant compared with plants containing ADP-sensitive Rca forms. The faster rate of photosynthetic induction by the rwt43 transformant compared with wild-type Arabidopsis suggests that manipulation of Rca regulation might provide a strategy for enhancing photosynthetic performance in certain variable light environments.
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