Phosphate-starvation response in plant cells: de novo synthesis and degradation of acid phosphatases.

Induction of phosphatase activity is an important component of the plant cell response to phosphate deficiency. Suspension cell cultures of Brassica nigra contain two major inducible acid phosphatase (APase) isozymes; vacuolar phosphoenolpyruvate (PEP) APase and cell wall nonspecific APase. Polyclonal antibodies raised against purified PEP-APase crossreacted specifically with both isozymes. Furthermore, anti-(PEP-APase) IgG detected proteins from a wide range of higher plants, suggesting that the major plant APase isozymes have diverged from a common ancestral form. Quantification on immunoblots indicated that in B. nigra suspension cells experiencing transition from Pi sufficiency to deficiency or vice versa, the amount of total antigenic APase protein correlated closely with total enzyme activity. This was also shown in intact plant roots. Therefore, the activity was governed by the synthesis and degradation of APases. Increases in the amounts of both major APase isozymes occurred simultaneously following Pi deprivation of B. nigra suspension cells, suggesting the involvement of a common regulatory mechanism.