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.
Highlights
In plants, plastoquinone-9 (PQ-9) plays indispensable roles in plant growth and development
In Arabidopsis, an alternatively spliced transcript of FBN5, AtFBN5-A, could not interact with solanesyl diphosphate synthases (SPSs) (Kim et al, 2015). The product of this transcript was altered in 14 amino acids, including a 10 amino acid deletion at the C-terminus, indicating that these residues are indispensable for AtFBN5 function
The high amino acid sequence similarity of OsFBN5 and AtFBN5 suggests that OsFBN5 might function as a structural protein providing a scaffold for prenyl chain assembly during the synthesis of PQ-9 in rice, similar to Arabidopsis AtFBN5
Summary
Plastoquinone-9 (PQ-9) plays indispensable roles in plant growth and development. The redox state of the PQ-9 pool regulates many physiological and molecular processes during short-term and long-term photoacclimations. These processes include phosphorylation of thylakoid membrane proteins (Vener et al, 1997; Zito et al, 1999; Depège et al, 2003); chloroplast expression of photosystem (PS) I and II genes (Allen, 1995; Maxwell et al, 1995; Pfannschmidt et al, 2001); and expression of nuclearencoded genes such as ascorbate peroxidase (Karpinski et al, 1997), plastocyanin transcription factors (Adamiec et al, 2008), and others (Bräutigam et al, 2009; Akhtar et al, 2010). When tobacco (Nicotiana tabacum) and black nightshade (Solanum nigrum) were challenged with the pathogens tobacco mosaic virus (TMV) and Phytophthora infestans, respectively, the contents of PQ-9 were increased (Maciejewska et al, 2002; Bajda et al, 2009)
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