The Study On Secondary Modification Of Plasmodesmata During The Sink-Source Transition In Vicia Faba Leaves

The leaves undergoing sink-source transition of Viciafaba L were used to as a experimental system researching the secondary modification process of plasmodesmata. The localization of ATPase, peroxidase and other hydrolases including cellulase, pectinase and acid phosphatase in the mesophyll cells were studied by using ultracytochemical (lead or copper precipitate) and immunocytochemical (colloid gold particles) methods.The 6(5)CF, which is a kind of phloem transport tracer, was used to determine the sink, source and the transition region of sink-source of in Viciafaba L leaves. After transported in the veins, the fluorescence was detected and imaged using the fluorescence microscope. The results were revealed that the base region of leaf was identified as a sink where the fluorescence was extraordinarily intense and obvious in every class vein. However, the fluorescence was not observed in the tip region of leaf and this region was determined as source region. And the region of the leaf where fluorescence was only limited in the class II and I veinal network was identified as the transition region of sink-source. Further observation by electronic microscope with ultrathin sections have shown that the plasmodesmata posses differently complex branched structures, such as "H", "Y" shape, and multi-branch with central gravities in the source region of sink-source transiting leaves, but only simple and no branched plasmodesmata were found in the sink region of the same leaf. Statistical data from observation of ultrathin sections showed that the simple plasmodesmata were dominated in sink region of the sink-source transiting leaves as the results in the immature leaves. At the source of the sink-source transiting leaf, approximately 80% of the plasmodesmata were branched or other more complex shaped. All above experimental evidence suggested that plasmodesmata between the mesophyll cells undergo secondary modification from the simple to branched form during the sink-source transition in Viciafaba leaves.ATPase, peroxidase and other hydrolase including cellulase, pectinase and acid phosphatase were localized by using ultracytochemical and immunochemical methods in the mesophyll cells of the sink and source region of the sink-source transiting leaf and the young leaf. It was found that these enzymes showed only weak activity in the cell wall and plasmodesmata in the sink region of leaf, while in the source region of the transiting leaves, these enzymes appeared intense reaction products which distributied in the branched plasmodesmata and plasmodesmatal vicinal wall. Our results have provided direct experimental evidence which enzymes, such as ATPase, peroxidase, cellulase, pectinase and acid phosphatase involved in the process of the secondary modification of plasmodesmata. During the sink-source transition, these enzymes functioned mainly in the source region of the leaf undergoing sink-source transiton.