Plastids differentiate into various functional types (chloroplasts, leucoplasts, chromoplasts, etc.) that have distinct proteomes depending on the specific tissue. Most plastid proteins are encoded by the nuclear genome, synthesized as higher molecular mass preproteins with an N-terminal transit peptide, and then posttranslationally imported from the cytosol. Evidence for tissue-specific regulation of import into plastids, and subsequent modulation of plastid proteomes, has been lacking. We quantified protein import into isolated pea (Pisum sativum) leaf chloroplasts and root leucoplasts and identified two transit-peptide motifs that specifically enhance preprotein import into root leucoplasts. Using a plastid preprotein expressed in both leaves and roots of stable transgenic plants, we showed that losing one of the leucoplast motifs interfered with its function in root leucoplasts but had no effect on its function in leaf chloroplasts. We assembled a list of all Arabidopsis (Arabidopsis thaliana) plastid preproteins encoded by recently duplicated genes and show that, within a duplicated preprotein pair, the isoform bearing the leucoplast motif usually has greater root protein abundance. Our findings represent a clear demonstration of tissue-specific regulation of organelle protein import and suggest that it operates by selective evolutionary retention of transit-peptide motifs, which enhances import into specific plastid types.

质体可分为多种功能类型（叶绿体，白质体，色质体等），这些功能类型取决于特定的组织而具有不同的蛋白质组。大多数质体蛋白由核基因组编码，合成为具有N末端转运肽的更高分子量的前蛋白，然后从细胞质中翻译后导入。缺乏用于组织特异性调节进入质体以及随后调节质体蛋白质组的证据。我们量化了蛋白质导入孤立豌豆（Pisum sativum）叶的叶绿体和根部的白质体，并鉴定出两个转运肽基序，这些基团可以特异性地增强前蛋白向根部的白质体中的导入。使用在稳定的转基因植物的叶和根中均表达的质体前蛋白，我们发现失去一种叶绿体基序会干扰其在根叶白质中的功能，但对其叶绿体中的功能没有影响。我们汇总了所有拟南芥（Arabid opsis thaliana）质体前蛋白，由最近复制的基因编码，并显示在复制的前蛋白对中，带有白质体基序的同工型通常具有更大的根蛋白丰度。我们的发现清楚地表明了细胞器蛋白质进口的组织特异性调节，并表明它通过转运肽基序的选择性进化保留来起作用，从而增强了向特定质体类型的进口。