Chlamydomonas reinhardtii Mutants Abnormal in Their Responses to Phosphorus Deprivation1

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Abstract
P-starved plants scavenge inorganic phosphate (Pi) by developing elevated rates of Pi uptake, synthesizing extracellular phosphatases, and secreting organic acids. To elucidate mechanisms controlling these acclimation responses in photosynthetic organisms, we characterized the responses of the green algaChlamydomonas reinhardtii to P starvation and developed screens for isolating mutants (designated psr[phosphorus-stress response]) abnormal in their responses to environmental levels of Pi. Thepsr1-1 mutant was identified in a selection for cells that survived exposure to high concentrations of radioactive Pi. psr1-2 andpsr2 were isolated as strains with aberrant levels of extracellular phosphatase activity during P-deficient or nutrient-replete growth. The psr1-1 andpsr1-2 mutants were phenotypically similar, and the lesions in these strains were recessive and allelic. They exhibited no increase in extracellular phosphatase activity or Pi uptake upon starvation. Furthermore, when placed in medium devoid of P, the psr1 strains lost photosynthetic O2evolution and stopped growing more rapidly than wild-type cells; they may not be as efficient as wild-type cells at scavenging/accessing P stores. In contrast, psr2 showed elevated extracellular phosphatase activity during growth in nutrient-replete medium, and the mutation was dominant. The mutant phenotypes and the roles of Psr1 and Psr2 in P-limitation responses are discussed.