Chlamydomonas reinhardtii mutants abnormal in their responses to phosphorus deprivation

P-starved plants scavenge inorganic phosphate (Pi) by developing elevated r ates of Pi uptake, synthesizing extracellular phosphatases, and secreting o rganic acids. To elucidate mechanisms controlling these acclimation respons es in photosynthetic organisms, we characterized the responses of the green alga Chlamydomonas reinhardtii to P starvation and developed screens for i solating mutants (designated psr [phosphorus-stress response]) abnormal in their responses to environmental levels of Pi. The psr1-1 mutant was identi fied in a selection for cells that survived exposure to high concentrations of radioactive Pi. psr1-2 and psr2 were isolated as strains with aberrant levels of extracellular phosphatase activity during P-deficient or nutrient -replete growth. The psr1-1 and psr1-2 mutants were phenotypically similar, and the lesions in these strains were recessive and allelic. They exhibite d no increase in extracellular phosphatase activity or Pi uptake upon starv ation. Furthermore, when placed in medium devoid of P, the psr1 strains los t photosynthetic O-2 evolution and stopped growing more rapidly than wild-t ype cells; they may not be as efficient as wild-type cells at scavenging/ac cessing P stores. In contrast, psr2 showed elevated extracellular phosphata se activity during growth in nutrient-replete medium, and the mutation was dominant. The mutant phenotypes and the roles of Psr1 and Psr2 in P-limitat ion responses are discussed.