1-Aminocyclopropane-1-Carboxylate Oxidase Induction in Tomato Flower Pedicel Phloem and Abscission Related Processes Are Differentially Sensitive to Ethylene

Abstract

Ethylene has impact on several physiological plant processes, including abscission, during which plants shed both their vegetative and reproductive organs. Cell separation and programmed cell death are involved in abscission, and these have also been correlated with ethylene action. However, the detailed spatiotemporal pattern of the molecular events during abscission remains unknown. We examined the expression of two tomato ACO genes, LeACO1, and LeACO4 that encode the last enzyme in ethylene biosynthesis, 1-aminocyclopropane-1-carboxylate oxidase (ACO), together with the expression of other abscission-associated genes involved in cell separation and programmed cell death, during a period of 0–12 h after abscission induction in the tomato flower pedicel abscission zone and nearby tissues. In addition, we determined their localization in specific cell layers of the flower pedicel abscission zone and nearby tissues obtained by laser microdissection before and 8 h after abscission induction. The expression of both ACO genes was localized to the vascular tissues in the pedicel. While LeACO4 was more uniformly expressed in all examined cell layers, the main expression site of LeACO1 was in cell layers just outside the abscission zone in its proximal and distal part. We showed that after abscission induction, ACO1 protein was synthesized in phloem companion cells, in which it was localized mainly in the cytoplasm. Samples were additionally treated with 1-methylcyclopropene (1-MCP), a competitive inhibitor of ethylene actions, and analyzed 8 h after abscission induction. Cell-layer-specific changes in gene expression were observed together with the specific localization and ethylene sensitivity of the hallmarks of cell separation and programmed cell death. While treatment with 1-MCP prevented separation of cells through inhibition of the expression of polygalacturonases, which are the key enzymes involved in degradation of the middle lamella, this had less impact on the occurrence of different kinds of membrane vesicles and abscission-related programmed cell death. In the flower pedicel abscission zone, the physical progressions of cell separation and programmed cell death are perpendicular to each other and start in the vascular tissues.

The pattern of 1‐aminocyclopropane‐1‐carboxylate oxidase induction in the tomato leaf petiole abscission zone is independent of expression of the ribonuclease‐LX‐encoding LeLX gene

“…The analysis also revealed that the ACO1 protein is localised to the same area (Chersicola et al . 2017). In that study, ACO1 was found mainly in the phloem companion cells, which was thus suggested as the side for synthesis of ethylene.…”

“…(2017). Parapalast Plus‐embedded tissue sections were placed on Superfrost Ultra Plus slides (Menzel‐Gläser).…”

“…We have recently demonstrated (Chersicola et al . 2017) that in the tomato flower pedicel, the expression of LeACO1 and LeACO4 is localised to the vascular tissues. Moreover, the ACO1 protein is mainly located in the cytoplasm of the phloem companion cells (Chersicola et al .…”

“…2012; Chersicola et al . 2017). Both of these processes are induced by ethylene (Trobacher 2009; Meir et al .…”

“…Moreover, the ACO1 protein is mainly located in the cytoplasm of the phloem companion cells (Chersicola et al . 2017). However, it is not known where LeACO genes are expressed, and where the ACO protein is localised during abscission of the tomato leaf petiole.…”