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

Marko Chersicola,Aleš Kladnik,Tanja Mrak,Kristina Gruden,Marina Dermastia,Magda Tušek Žnidarič

doi:10.3389/fpls.2017.00464

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.

Highlights

The small volatile hydrocarbon ethylene is a gaseous plant hormone that affects diverse physiological processes during plant growth and development, including abscission (Rudus et al, 2013)
The transcription dynamics of two ethylene biosynthetic genes expressed in tomato flower and fruit tissues, LeACO1 and LeACO4 (The Tomato Genome Consortium, 2012; Tomato eFP Browser) were followed during pedicel abscission (Figure 1)
A detailed localization of LeACO1 transcript in specific cell layers showed that 8 h after abscission induction its level remained very low in the abscission zone, but was high on the proximal and distal sides of it (Figure 1B)

Summary

Introduction

The small volatile hydrocarbon ethylene is a gaseous plant hormone that affects diverse physiological processes during plant growth and development, including abscission (Rudus et al, 2013). In many studied systems, including tomato (Kimura and Sinha, 2008; Klee and Giovannoni, 2011; Ito and Nakano, 2015), the timing of abscission is determined by tissue sensitivity to ethylene, as ethylene can induce or accelerate abscission (Roberts et al, 2002). LeACO5 has been shown to be expressed in the tomato flower pedicel distal to the abscission zone area (Nakano et al, 2013), as well as in the abscission zone (Meir et al, 2010). Expression of LeACO1 and LeACO4 in specific cell layers of the flower pedicel abscission zone and the localization of the ACO protein within the abscission zone are not known. The subcellular localization of the ACO protein remains unclear (Reinhardt et al, 1994; Ramassamy et al, 1998; Chung et al, 2002)

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Discussion

Conclusion