A point mutation in the gene encoding the Rubisco large subunit interferes with holoenzyme assembly

A missense mutation of plastid RPS4 is associated with chlorophyll deficiency in Chinese cabbage (Brassica campestris ssp. pekinensis)

BACKGROUND

Plastome mutants are ideal resources for elucidating the functions of plastid genes. Numerous studies have been conducted for the function of plastid genes in barley and tobacco; however, related information is limited in Chinese cabbage.

RESULTS

A chlorophyll-deficient mutant of Chinese cabbage that was derived by ethyl methanesulfonate treatment on isolated microspores showed uniformly pale green inner leaves and slow growth compared with that shown by the wild type "Fukuda 50' ('FT'). Genetic analysis revealed that cdm was cytoplasmically inherited. Physiological and ultrastructural analyses of cdm showed impaired photosynthesis and abnormal chloroplast development. Utilizing next generation sequencing, the complete plastomes of cdm and 'FT' were respectively re-mapped to the reference genome of Chinese cabbage, and an A-to-C base substitution with a mutation ratio higher than 99% was detected. The missense mutation of plastid ribosomal protein S4 led to valine substitution for glycine at residue 193. The expression level of rps4 was analyzed using quantitative real-time PCR and found lower in than in 'FT'. RNA gel-blot assays showed that the abundance of mature 23S rRNA, 16S rRNA, 5S rRNA, and 4.5S rRNA significantly decreased and that the processing of 23S, 16S rRNA, and 4.5S rRNA was seriously impaired, affecting the ribosomal function in cdm.

CONCLUSIONS

These findings indicated that cdm was a plastome mutant and that chlorophyll deficiency might be due to an A-to-C base substitution of the plastome-encoded rps4 that impaired the rRNA processing and affected the ribosomal function.

Development of an activity-directed selection system enabled significant improvement of the carboxylation efficiency of Rubisco

Photosynthetic CO₂ fixation is the ultimate source of organic carbon on earth and thus is essential for crop production and carbon sequestration. Ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) catalyzes the first step of photosynthetic CO₂ fixation. However, the extreme low carboxylation efficiency of Rubisco makes it the most attractive target for improving photosynthetic efficiency. Extensive studies have focused on re-engineering a more efficient enzyme, but the effort has been impeded by the limited understanding of its structure-function relationships and the lack of an efficient selection system towards its activity. To address the unsuccessful molecular engineering of Rubisco, we developed an Escherichia coli-based activity-directed selection system which links the growth of host cell solely to the Rubisco activity therein. A Synechococcus sp. PCC7002 Rubisco mutant with E49V and D82G substitutions in the small subunit was selected from a total of 15,000 mutants by one round of evolution. This mutant showed an 85% increase in specific carboxylation activity and a 45% improvement in catalytic efficiency towards CO₂. The small-subunit E49V mutation was speculated to influence holoenzyme catalysis through interaction with the large-subunit Q225. This interaction is conserved among various Rubisco from higher plants and Chlamydomonas reinhardtii. Knowledge of these might provide clues for engineering Rubisco from higher plants, with the potential of increasing the crop yield.

Photosynthetic characterization of Rubisco transplantomic lines reveals alterations on photochemistry and mesophyll conductance

Improving Rubisco catalysis is considered a promising way to enhance C3-photosynthesis and photosynthetic water use efficiency (WUE) provided the introduced changes have little or no impact on other processes affecting photosynthesis such as leaf photochemistry or leaf CO2 diffusion conductances. However, the extent to which the factors affecting photosynthetic capacity are co-regulated is unclear. The aim of the present study was to characterize the photochemistry and CO2 transport processes in the leaves of three transplantomic tobacco genotypes expressing hybrid Rubisco isoforms comprising different Flaveria L-subunits that show variations in catalysis and differing trade-offs between the amount of Rubisco and its activation state. Stomatal conductance (g s) in each transplantomic tobacco line matched wild-type, while their photochemistry showed co-regulation with the variations in Rubisco catalysis. A tight co-regulation was observed between Rubisco activity and mesophyll conductance (g m) that was independent of g s thus producing plants with varying g m/g s ratios. Since the g m/g s ratio has been shown to positively correlate with intrinsic WUE, the present results suggest that altering photosynthesis by modifying Rubisco catalysis may also be useful for targeting WUE.

Rubisco activity and regulation as targets for crop improvement

Rubisco (ribulose-1,5-bisphosphate (RuBP) carboxylase/oxygenase) enables net carbon fixation through the carboxylation of RuBP. However, some characteristics of Rubisco make it surprisingly inefficient and compromise photosynthetic productivity. For example, Rubisco catalyses a wasteful reaction with oxygen that leads to the release of previously fixed CO(2) and NH(3) and the consumption of energy during photorespiration. Furthermore, Rubisco is slow and large amounts are needed to support adequate photosynthetic rates. Consequently, Rubisco has been studied intensively as a prime target for manipulations to 'supercharge' photosynthesis and improve both productivity and resource use efficiency. The catalytic properties of Rubiscos from diverse sources vary considerably, suggesting that changes in turnover rate, affinity, or specificity for CO(2) can be introduced to improve Rubisco performance in specific crops and environments. While attempts to manipulate plant Rubisco by nuclear transformation have had limited success, modifying its catalysis by targeted changes to its catalytic large subunit via chloroplast transformation have been much more successful. However, this technique is still in need of development for most major food crops including maize, wheat, and rice. Other bioengineering approaches for improving Rubisco performance include improving the activity of its ancillary protein, Rubisco activase, in addition to modulating the synthesis and degradation of Rubisco's inhibitory sugar phosphate ligands. As the rate-limiting step in carbon assimilation, even modest improvements in the overall performance of Rubisco pose a viable pathway for obtaining significant gains in plant yield, particularly under stressful environmental conditions.

Structure and function of Rubisco

Ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) is the major enzyme assimilating CO(2) into the biosphere. At the same time Rubisco is an extremely inefficient catalyst and its carboxylase activity is compromised by an opposing oxygenase activity involving atmospheric O(2). The shortcomings of Rubisco have implications for crop yield, nitrogen and water usage, and for the global carbon cycle. Numerous high-resolution crystal structures of different forms of Rubisco are now available, including structures of mutant enzymes. This review uses the information provided in these structures in a structure-based sequence alignment and discusses Rubisco function in the context of structural variations at all levels--amino acid sequence, fold, tertiary and quaternary structure--with an evolutionary perspective and an emphasis on the structural features of the enzyme that may determine its function as a carboxylase.

Rubisco: structure, regulatory interactions, and possibilities for a better enzyme

Ribulose-1,5-bisphosphate (RuBP) carboxylase/oxygenase (Rubisco) catalyzes the first step in net photosynthetic CO2 assimilation and photorespiratory carbon oxidation. The enzyme is notoriously inefficient as a catalyst for the carboxylation of RuBP and is subject to competitive inhibition by O2, inactivation by loss of carbamylation, and dead-end inhibition by RuBP. These inadequacies make Rubisco rate limiting for photosynthesis and an obvious target for increasing agricultural productivity. Resolution of X-ray crystal structures and detailed analysis of divergent, mutant, and hybrid enzymes have increased our insight into the structure/function relationships of Rubisco. The interactions and associations relatively far from the Rubisco active site, including regulatory interactions with Rubisco activase, may present new approaches and strategies for understanding and ultimately improving this complex enzyme.

Phylogenetic relationships within the Gentianales based on NDHF and RBCL sequences, with particular reference to the Loganiaceae

Phylogenetic relationships in the Gentianales with focus on Loganiaceae sensu lato are evaluated using parsimony analyses of nucleotide sequence data from the plastid genes rbcL and ndhF. Inter- and intrafamilial relationships in the Gentianales, which consist of the families Apocynaceae (including Asclepiadaceae), Gelsemiaceae, Gentianaceae, Loganiaceae, and Rubiaceae, are studied and receive increased support from the combination of rbcL and ndhF data, which indicate that the family Rubiaceae forms the sister group to the successively nested Gentianaceae, Apocynaceae, and Loganiaceae, all of which are well supported. The family Gelsemiaceae forms a distinct, supported group sister to Apocynaceae. The Loganiaceae sensu stricto form a strongly supported group consisting of 13 genera: Antonia, Bonyunia, Gardneria, Geniostoma, Labordia, Logania, Mitrasacme, Mitreola, Neuburgia, Norrisia, Spigelia, Strychnos, and Usteria. These genera form two well-supported lineages. Several members of Loganiaceae sensu Leeuwenberg and Leenhouts, i.e., Androya, Peltanthera, Plocosperma, Polypremum, and Sanango are clearly not members of the Gentianales. The earlier exclusion of Buddlejaceae (including Buddleja, Emorya, Gomphostigma, and Nicodemia) as well as the reclassification of the genera Nuxia and Retzia to Stilbaceae of the Lamiales are all well supported.