Cellular and Pectin Dynamics during Abscission Zone Development and Ripe Fruit Abscission of the Monocot Oil Palm

Abstract

The oil palm (Elaeis guineensis Jacq.) fruit primary abscission zone (AZ) is a multi-cell layered boundary region between the pedicel (P) and mesocarp (M) tissues. To examine the cellular processes that occur during the development and function of the AZ cell layers, we employed multiple histological and immunohistochemical methods combined with confocal, electron and Fourier-transform infrared (FT-IR) microspectroscopy approaches. During early fruit development and differentiation of the AZ, the orientation of cell divisions in the AZ was periclinal compared with anticlinal divisions in the P and M. AZ cell wall width increased earlier during development suggesting cell wall assembly occurred more rapidly in the AZ than the adjacent P and M tissues. The developing fruit AZ contain numerous intra-AZ cell layer plasmodesmata (PD), but very few inter-AZ cell layer PD. In the AZ of ripening fruit, PD were less frequent, wider, and mainly intra-AZ cell layer localized. Furthermore, DAPI staining revealed nuclei are located adjacent to PD and are remarkably aligned within AZ layer cells, and remain aligned and intact after cell separation. The polarized accumulation of ribosomes, rough endoplasmic reticulum, mitochondria, and vesicles suggested active secretion at the tip of AZ cells occurred during development which may contribute to the striated cell wall patterns in the AZ cell layers. AZ cells accumulated intracellular pectin during development, which appear to be released and/or degraded during cell separation. The signal for the JIM5 epitope, that recognizes low methylesterified and un-methylesterified homogalacturonan (HG), increased in the AZ layer cell walls prior to separation and dramatically increased on the separated AZ cell surfaces. Finally, FT-IR microspectroscopy analysis indicated a decrease in methylesterified HG occurred in AZ cell walls during separation, which may partially explain an increase in the JIM5 epitope signal. The results obtained through a multi-imaging approach allow an integrated view of the dynamic developmental processes that occur in a multi-layered boundary AZ and provide evidence for distinct regulatory mechanisms that underlie oil palm fruit AZ development and function.

Disruption of the Auxin Gradient in the Abscission Zone Area Evokes Asymmetrical Changes Leading to Flower Separation in Yellow Lupine

“…It is well established that in many species, abscission is correlated with a burst of ROS in the AZ area [14,[89][90][91]. We have also observed a strong accumulation of these compounds followed by the separation of lupine flowers [19].…”

INFLORESCENCE DEFICIENT IN ABSCISSION-like is an abscission-associated and phytohormone-regulated gene in flower separation of Lupinus luteus

“…Significant differences to the natural active AZ in comparison to non-activated AZ are indicated as **P < 0.01. Significant differences to artificially activated AZ in comparison to inactive AZ are indicated as aa P < 0.01 However, nuclei remained intact and there was no visible evidence of cell death after separation of oil palm fruit (Roongsattham et al 2016). Another progressive cell degradation symptoms are the presence of oval-shaped chloroplasts containing large starch grains and thylakoidassociated plastoglobules (Fig.…”

A Salutary Role of Reactive Oxygen Species in Intercellular Tunnel-Mediated Communication

“…Interestingly, a recent study on oil palm fruit abscission zone shows that abscission zone is multi-cell layered structure and PD is frequently observed within the layer, but less frequently observed between layers (Roongsattham et al, 2016 ), suggesting that inter-layer communication via PD is becoming less likely prior to separation. With progression toward abscission, the PD size within layer is enlarged (Roongsattham et al, 2016 ), fusing membrane vesicles, highly branched PD and its associated cavities in the vicinity of middle lamella are frequently observed (Osborne and Sargent, 1976 ; Henry, 1979 ; Bar-Dror et al, 2011 ; Roongsattham et al, 2016 ).…”

“…Interestingly, a recent study on oil palm fruit abscission zone shows that abscission zone is multi-cell layered structure and PD is frequently observed within the layer, but less frequently observed between layers (Roongsattham et al, 2016 ), suggesting that inter-layer communication via PD is becoming less likely prior to separation. With progression toward abscission, the PD size within layer is enlarged (Roongsattham et al, 2016 ), fusing membrane vesicles, highly branched PD and its associated cavities in the vicinity of middle lamella are frequently observed (Osborne and Sargent, 1976 ; Henry, 1979 ; Bar-Dror et al, 2011 ; Roongsattham et al, 2016 ). The physiological function of these branched PD in the abscission layer is yet known, but clues from the presence of branched PD in the trichome-epidermal boundary (Faulkner et al, 2008 ) where there is only unidirectional transport (Christensen et al, 2009 ) (Figure 1 ) and also in the cells undergoing sink-source transition that allows carbon export (Volk et al, 1996 ; Oparka et al, 1999 ) suggest that the branched PD in the abscission zone may be involved in the unidirectional transport between the separated tissue and the main body.…”