Overexpression of BUNDLE SHEATH DEFECTIVE 2 improves the efficiency of photosynthesis and growth in Arabidopsis.

Bundle Sheath Defective 2, BSD2, is a stroma-targeted protein initially identified as a factor required for the biogenesis of ribulose 1,5-bisphosphate carboxylase/oxygenase (RuBisCO) in maize. Plants and algae universally have a homologous gene for BSD2 and its deficiency causes a RuBisCO-less phenotype. As RuBisCO can be the rate-limiting step in CO2 assimilation, the overexpression of BSD2 might improve photosynthesis and productivity through the accumulation of RuBisCO. To examine this hypothesis, we produced BSD2 overexpression lines in Arabidopsis. Compared with wild type, the BSD2 overexpression lines BSD2ox-2 and BSD2ox-3 expressed 4.8-fold and 8.8-fold higher BSD2 mRNA, respectively, whereas the empty-vector (EV) harbouring plants had a comparable expression level. The overexpression lines showed a significantly higher CO2 assimilation rate per available CO2 and productivity than EV plants. The maximum carboxylation rate per total catalytic site was accelerated in the overexpression lines, while the number of total catalytic sites and RuBisCO content were unaffected. We then isolated recombinant BSD2 (rBSD2) from E. coli and found that rBSD2 reduces disulfide bonds using reductants present in vivo, for example glutathione, and that rBSD2 has the ability to reactivate RuBisCO that has been inactivated by oxidants. Furthermore, 15% of RuBisCO freshly isolated from leaves of EV was oxidatively inactivated, as compared with 0% in BSD2-overexpression lines, suggesting that the overexpression of BSD2 maintains RuBisCO to be in the reduced active form in vivo. Our results demonstrated that the overexpression of BSD2 improves photosynthetic efficiency in Arabidopsis and we conclude that it is involved in mediating RuBisCO activation.

Cytological evidence of BSD2 functioning in both chloroplast division and dimorphic chloroplast formation in maize leaves.

BACKGROUND: Maize bsd2 (bundle sheath defective2) is a classical C4 mutant with defective C4 photosynthesis, accompanied with reduced accumulation of Rubisco (ribulose bisphosphate carboxylase oxygenase) and aberrant mature chloroplast morphology in the bundle sheath (BS) cells. However, as a hypothetical chloroplast chaperone, the effects of BSD2 on C4 chloroplast development have not been fully examined yet, which precludes a full appreciation of BSD2 function in C4 photosynthesis. The aims of our study are to find out the role ofBSD2 in regulating chloroplasts development in maize leaves, and to add new insights into our understanding of C4 biology. RESULTS: We found that at the chloroplast maturation stage, the thylakoid membranes of chloroplasts in the BS and mesophyll (M) cells became significantly looser, and the granaof chloroplasts in the M cells became thinner stacking in the bsd2 mutant when compared with the wildtype plant. Moreover, at the early chloroplast development stage, the number of dividing chloroplasts and the chloroplast division rate are both reduced in the bsd2 mutant, compared with wild type. Quantitative reverse transcriptase-PCR analysis revealed that the expression of both thylakoid formation-related genesand chloroplast division-related genes is significantly reduced in the bsd2 mutants. Further, we showed that BSD2 interacts physically with the large submit of Rubisco (LS) in Bimolecular Fluorescence Complementation assay. CONCLUSIONS: Our combined results suggest that BSD2 plays an essential role in regulating the division and differentiation of the dimorphic BS and M chloroplasts, and that it acts at a post-transcriptional level to regulate LS stability or assembly of Rubisco.

The BSD2 ortholog in Chlamydomonas reinhardtii is a polysome-associated chaperone that co-migrates on sucrose gradients with the rbcL transcript encoding the Rubisco large subunit.

The expression of the CO2 -fixation enzyme ribulose-bisphosphate carboxylase/oxygenase (Rubisco), which is affected by light, involves the cysteine-rich protein bundle-sheath defective-2 (BSD2) that was originally identified in maize bundle-sheath cells. We identified the BSD2 ortholog in Chlamydomonas reinhardtii as a small protein (17 kDa) localized to the chloroplast. The algal BSD2-ortholog contains four CXXCXGXG DnaJ-like elements, but lacks the other conserved domains of DnaJ. BSD2 co-migrated with the rbcL transcript on heavy polysomes, and both BSD2 and rbcL mRNA shifted to the lighter fractions under oxidizing conditions that repress the translation of the Rubisco large subunit (RbcL). This profile of co-migration supports the possibility that BSD2 is required for the de novo synthesis of RbcL. Furthermore, BSD2 co-migrated with the rbcL transcript in a C. reinhardtii premature-termination mutant that encodes the first 60 amino acids of RbcL. In both strains, BSD2 shared its migration profile with the rbcL transcript but not with psbA mRNA. The chaperone activity of BSD2 was exemplified by its ability to prevent the aggregation of both citrate synthase (CS) and RbcL in vitro following their chemical denaturation. This activity did not depend on the presence of the thiol groups on BSD2. In contrast, the activity of BSD2 in preventing the precipitation of reduced ß-chains in vitro in the insulin turbidity assay was thiol-dependent. We conclude that BSD2 combines a chaperone 'holdase' function with the ability to interact with free thiols, with both activities being required to protect newly synthesized RbcL chains.

Complete genome sequence of Bacillus subtilis BSD-2, a microbial germicide isolated from cultivated cotton.

Bacillus subtilis BSD-2, isolated from cotton (Gossypium spp.), had strong antagonistic activity to Verticillium dahlia Kleb and Botrytis cinerea. We sequenced and annotated the BSD-2 complete genome to help us the better use of this strain, which has surfactin, bacilysin, bacillibactin, subtilosin A, Tas A and a potential class IV lanthipeptide biosynthetic pathways.