Influence of phosphate starvation on phosphohydrolases during development of tomato seedlings

In tomato seedlings (Lycopersicon esculentum Mill. cv, Lukullus), phosphate mobilizing enzymes (acid phosphatase, phytase and ribonuclease) responded to the absence of an exogenous phosphate source with a remarkable increase in their specific activities. The definite beginning of a stress response on the level of enzyme activity was revealed at day 10 after sowing. The increase was tightly controlled by the decline of the free cellular phosphate level, Thus, in phosphate-deficient roots derived from 14-d-old seedlings, the enzyme activities were up to 32-fold higher than in the control plants, Only 7% of the free cellular phosphate content of control roots was measured in this part of the plants. However, phosphate-starved plants do not show visible deficiency symptoms at this stage. In addition, we found that phosphohydrolases reached their maximum specific activity early in germination, independent of the exogenous phosphate supply. Furthermore, acid phosphatase and ribonuclease isoforms exhibited different patterns depending on the nutrient supply, as well as on the developmental stage. The results of this work allow us to compare the responses of whole tomato plants following phosphate deprivation with those of a homologous suspension cell culture recently examined.