Abstract
Melanose spot was formed as the results by the self-defense reaction of citrus plants against the penetration of hyphae of Diaporthe citri (Faw.) Wolf. A series of experiments was conducted to clarify the mechanism of the abnormal division of cells which occur at just under the necrotic cells in which invaded hyphae of D. citri were arrested.
A factor (cell-division inducer: CD inducer) related to the induction of cell division was detected from the necrotic tissue of melanose spots formed on the fruit. The CD inducer were extracted by 50% aqueous ethanol from the necrotic tissues. The CD inducer shows three types of activity against citrus plant tissues, namely, cell division, cell elongation and cell enlargement. These anatomical changes were differentially regulated by the concentration of CD inducer. The cell elongation was induced with high concentration such as n (100mg fresh weight equivalent/ml), while cell division was induced at concentration as low as n/100. There was a difference in concentration of the CD inducer around of the cells, and the cell elongated toward the higher concentration of the CD inducer, and new cell wall was formed between the higher- and lower-sides of concentration. When concentrations of the CD inducer around the cell were almost uniform, the cells spherically enlarged.
The cell-division activity was not detected in uninfected tissues of citrus peel and leaf, suggesting that the CD inducer was newly formed in citrus plant tissues after D. citri invasion, probably as a wound hormone.
Since the anatomical changes of the tissues caused by the CD inducer were similar to those of melanose spot incited by D. citri, it is suggested that the cell division just under the necrotic cells of melanose spot is due to the action of the CD inducers produced in the necrotic cells post-infectiously, and leads to formation of mechanical barrier against the pathogen.
