Abstract

Phloem sieve elements are the specialized cells of higher plants responsible for conducting photoassimilates from regions of production (sources) to regions of utilization or storage (sinks). The translocation of photoassimilates is believed to be the result of the A'Tf'-dependent membrane transport of sugars across the plasma membrane-lined sieve elements or associated companion cell and the subsequent generation of hydrostatic pressure due to osmosis. During the development of the sieve element, many cellular organelles are lost from the cell, including the nucleus and the vacuole. Mitochondria, however, are retained in the mature sieve element. Here we show that many of the particles visible in the sieve and develops from a plant tissue culture are strongly labeled by Rhodamine 123, a probe that is specifically accumulated by mitochondria in plant and animal cells. These observations suggest that sieve element mitochondria are metabolically active and that they may contribute to the generation of the ATP required for phloem loading.