Analysis of Boron and B-pectin Complex Content in the Abscission Zone during Floral and Fruit Abscission in Tobacco (Nicotiana tabacum)

Abstract

After fruit development has been triggered by pollination, the abscission zone (AZ) in the pedicel strengthens its adhesion to retain the fruit. In tobacco, unpollinated flowers are shed from the AZ, while enlargement of the same tissue occurs in pollinated flowers. The abscission rate depends on effective pollination. In this study, we focused on pectin and boron (B) distribution and their functions in the AZ. B is an essential micronutrient that forms cross-links in pectin, which are important for organ differentiation and maturation. B content and the formation rate of the borate-ester cross-linked rhamnogalacturonan II dimer (dRG-II-B) were measured in tobacco. Expression of NtGUT1, which is important for the formation of dRG-II-B, and the B transporter NtNIP3;1 were analyzed in the peduncle and AZ. To determine B content and dRG-II-B formation rate, the following samples were collected: bud peduncle, bud AZ, fruit peduncle, fruit AZ, and flower AZs, which were treated to prevent pollination or were artificially pollinated. Samples treated in HNO3 were assayed for total B content with ICPMS. Endo-polygalacturonase (EPG)-treated alcohol-insoluble residues were assayed for formation rates of dRGII-B with HPLC. Although there was more total B in fruit AZs than in branches, no differences in total B, watersoluble B, and B relevant to cross-linking with RG-II were found between buds and fruits. The formation rates of dRG-II-B were also the same in peduncles and AZs in both buds and fruits. These have the same characteristics of variation with or without pollination. Expression of both NtGUT1 and NtNIP3;1 was highest in AZs of flowers before opening, however, expression of NtGUT1 in flower AZs without pollination was not significantly different from after artificial pollination. These results suggest that the presence of relatively large amounts of B and pectin is important in structural enhancement of AZs after anthesis, but is not related to abscission with or without pollination.