Developmental regulation of protein import into plastids.

The plastid proteome changes according to developmental stages. Accruing evidence shows that, in addition to transcriptional and translational controls, preprotein import into plastids is also part of the process regulating plastid proteomes. Different preproteins have distinct preferences for plastids of different tissues. Preproteins are also divided into at least three age-selective groups based on their import preference for chloroplasts of different ages. Both tissue and age selectivity are determined by the transit peptide of each preprotein, and a transit-peptide motif for older-chloroplast preference has been identified. Future challenges lie in identifying other motifs for tissue and age selectivity, as well as in identifying the receptor components that decipher these motifs. Developmental regulation also suggests that caution should be exercised when comparing protein import data generated with plastids isolated from different tissues or with chloroplasts isolated from plants of different ages.