At-P5R, a gene encoding the last enzyme of the proline (Pro) biosynthetic pathway in Arabidopsis thaliana, is developmentally regulated. To characterize the cis elements responsible for this developmental regulation, a series of 5' deletions of the At-P5R promoter were transcriptionally fused to a beta-glucuronidase (GUS)-coding region and transformed into Arabidopsis. The complete promoter of At-P5R directs strong GUS activity in root tips, the shoot meristem, guard cells, hydathodes, pollen grains, ovules, and developing seeds, all of which contain rapidly dividing cells and/or are undergoing changes in osmotic potential. This expression pattern is consistent with the function of Pro as an energy, nitrogen, and carbon source and as an osmoticum in response to dehydration. Promoters longer than 212 base pairs (bp) showed the same expression pattern, whereas those shorter than 143 bp did not direct any detectable GUS activity in any organs. This suggests that a 69-bp promoter region located between -212 and -143 bp is necessary to establish the tissue-specific expression of At-P5R during development. The Pro content measured in different organs suggests that, in addition to transcriptional control of the biosynthetic pathway, the transport of Pro may play a role in its distribution within Arabidopsis. Several aspects of the relationship between Pro metabolism and plant physiology are discussed.