Conserved Function of Fibrillin5 in the Plastoquinone-9 Biosynthetic Pathway in Arabidopsis and Rice

Abstract

Plastoquinone-9 (PQ-9) is essential for plant growth and development. Recently, we found that fibrillin5 (FBN5), a plastid lipid binding protein, is an essential structural component of the PQ-9 biosynthetic pathway in Arabidopsis. To investigate the functional conservation of FBN5 in monocots and eudicots, we identified OsFBN5, the Arabidopsis FBN5 (AtFBN5) ortholog in rice (Oryza sativa). Homozygous Osfbn5-1 and Osfbn5-2 Tos17 insertion null mutants were smaller than wild type (WT) plants when grown on Murashige and Skoog (MS) medium and died quickly when transplanted to soil in a greenhouse. They accumulated significantly less PQ-9 than WT plants, whereas chlorophyll and carotenoid contents were only mildly affected. The reduced PQ-9 content of the mutants was consistent with their lower maximum photosynthetic efficiency, especially under high light. Overexpression of OsFBN5 complemented the seedling lethal phenotype of the Arabidopsis fbn5-1 mutant and restored PQ-9 and PC-8 (plastochromanol-8) to levels comparable to those in WT Arabidopsis plants. Protein interaction experiments in yeast and mesophyll cells confirmed that OsFBN5 interacts with the rice solanesyl diphosphate synthase OsSPS2 and also with Arabidopsis AtSPS1 and AtSPS2. Our data thus indicate that OsFBN5 is the functional equivalent of AtFBN5 and also suggest that the SPSs–FBN5 complex for synthesis of the solanesyl diphosphate tail in PQ-9 is well conserved in Arabidopsis and rice.

Genome-Wide Identification and Expression Analyses of the Fibrillin Family Genes Suggest Their Involvement in Photoprotection in Cucumber

“…The genomic structure and SNP analyses of the same FBN genes in the Chinse long and Gy14 lines indicated that the sequences of CsaFBN1 and CsaFBN9 are the most diverse ( Figure 2 ). Phylogenetic analysis showed that the 10 cucumber FBN genes identified in this study were divided into 10 groups, as was reported in Arabidopsis and rice [ 15 , 23 ], although no gene belonging to Group 5 ( FBN5 ) was found in the two cucumber genomes ( Table 1 , Figure 1 ). Plant FBN1 , FBN2 and FBN4 genes have been identified in many plants and have been reported to respond to abiotic stress [ 19 , 20 , 21 ].…”

“…Among the 10 cucumber FBN genes, the deduced protein sequence of CsaFBN1 (Csa6M512870.1) was identical to chromoplast-specific carotenoid-associated protein CHRC (Q96398) in the corollas of cucumber [ 11 , 40 ]. Compared with the number of FBN genes in other plant species, determined using conserved domain searches, cucumber has fewer than other plant species, such as Arabidopsis, with 14 genes [ 15 ] but a similar number to others, such as rice, with 11 genes [ 23 ].…”

“…These groupings were consistent with the results from the BLASTP searches against Arabidopsis FBN genes ( Table 1 ). Among the 14 Arabidopsis FBN genes, FBN5 (AT5G09820), which is essential for the synthesis of plastoquinone-9 by interacting with SPS enzyme in Arabidopsis [ 24 ] and rice [ 23 ], had no close counterpart in cucumber ( Figure 1 ). This was because cucumber FBN homologues of Arabidopsis genes could not be found by domain and BLAST search.…”

“…The absence of FBN5 in the cucumber genome is a mystery ( Table 1 ). FBN5 is essential for plant growth, as it is a component of the SPS protein complex, which is essential for the formation of the SPP tail of plastoquinone-9 [ 23 , 24 ]. In Arabidopsis and rice, SPS enzymes require interaction with FBN5 for the synthesis of SPP.…”

“…In Arabidopsis and apple, FBN4 is involved in biotic stress response and is related to plastoquinone content [ 18 , 19 ]. Recently, Arabidopsis and rice FBN5 were shown to be an essential component, along with solenesyl diphosphate synthase (SPS), in the synthesis of the solenesyl diphosphate (SPP) tail of plastoquinone-9 [ 23 , 24 ]. This list of the representative functions of FBN show that FBNs are involved in photosynthesis and hormone-mediated photoprotection but their specific functions still need to be studied.…”