ON THE VALIDITY OF PLASMODESMOGRAMS

Because the operation of plasmodesmata is of paramount importance for the integrative action of tissues and organs, it is important to quantify the symplastic continuity. An attempt to visualize the intercellular communication pathways in plant tissues is the plasmodesmogram, a two-dimensional diagram of plasmodesmatal frequencies. Plasmodesmograms assume that the greater the frequency of plasmodesmata, the greater is the potential for symplasmic communication. In fact, however, the validity of plasmodesmograms hinges on the premise that plasmodesmata are uniform in structure and functioning. In view of recent findings that plasmodesmata are gatable channels with different functional diameters, a correlation between plasmodesmatal frequency and the rate of intercellular communication - including transport - is disputable and the concept of the plasmodesmogram needs to be re-examined. Its validity appears to vary with the developmental stage and the nature of the plant tissue. Our over-all conclusion is that plasmodesmograms are an acceptable device for coarse (qualitative) assessment of intercellular transport pathways in mature phloem tissues. Pathways identified by plasmodesmograms often correlate with the physiologically determined modes of photosynthate transport. Whether this applies to differentiated tissues, in general, remains to be elucidated. The value of plasmodesmograms for developmental phyisology seems to be limited, as developmental processes coincide with temporary or permanent closure of plasmodesmata.