Carotenoid‐deficient young wheat etioplasts are able to bind precursor proteins on the plastid surface but are impaired in their translocation ability

Abstract

Young carotenoid‐deficient etioplasts, isolated from Norflurazon (NF)‐treated wheat seedlings, were used to study the role of coloured carotenoids in the binding and import reactions of different nuclear‐encoded plastid proteins. Plastids from control seedlings exhibited significantly higher import efficiencies than did plastids from NF‐treated plants. Etioplasts containing normal levels of carotenoids imported approximately 2000 and 800 molecules per plastid of the precursors of the small Rubisco subunit (pSS) and the Rieske FeS protein (pFeS), respectively. Plastids from NF‐treated plants imported approximately 100 and 70 pSS and pFeS molecules per plastid, respectively. In addition, a maximum binding capacity of NF‐treated plastids of 1200 protein molecules per plastid was observed for both pSS and pFeS when assayed at 25°C: and a maximum binding capacity of approximately 1300 molecules per plastid was noted at 4°C. For control plastids, a similar amount of binding, or approximately 1400 protein molecules per plastid, could only be observed if import was inhibited by low ATP concentrations at 4°C. When these plastids were washed and transferred to conditions promoting import at 25°C and 10 mM Mg‐ATP, close to 60% of the envelope‐associated precursor protein molecules were imported. These results indicate that control and NF‐treated young etioplasts contain similar amounts of binding sites for precursor proteins. However, only in the case of control plastids the binding was productive and lead to import and processing in the stroma upon transfer to conditions promoting import. Plastids isolated from wheat seedlings grown in weak red light and containing different amounts of carotenoids, were assayed for their ability to bind and import a protein with unusual import characteristics, the Chlamydomonas reinhardtii PsaF precursor of PSI (pPsaF) and transit peptide deletion constructs. The PsaF protein was imported in a transit peptide‐dependent manner into control etioplasts, whereas import of pPsaF into young wheat etioplasts isolated from NF‐treated plants was inhibited at low levels of plastid carotenoids.