Ethephon-Induced Ethylene Enhances Protein Degradation in Source Leaves, but Its High Endogenous Level Inhibits the Development of Regenerative Organs in Brassica napus.

Bok-Rye Lee,Van Hien La,Tae-Hwan Kim,Sang-Hyun Park,Rashed Zaman

doi:10.3390/plants10101993

Abstract

To investigate the regulatory role of ethylene in the source-sink relationship for nitrogen remobilization, short-term effects of treatment with different concentrations (0, 25, 50, and 75 ppm) of ethephon (2-chloroethylphosphonic acid, an ethylene inducing agent) for 10 days (EXP 1) and long-term effects at 20 days (Day 30) after treatment with 100 ppm for 10 days (EXP 2) on protein degradation and amino acid transport in foliar sprayed mature leaves of Brassica napus (cv. Mosa) were determined. In EXP 1, endogenous ethylene concentration gradually increased in response to the treated ethephon concentration, leading to the upregulation of senescence-associated gene 12 (SAG12) expression and downregulation of chlorophyll a/b-binding protein (CAB) expression. Further, the increase in ethylene concentration caused a reduction in protein, Rubisco, and amino acid contents in the mature leaves. However, the activity of protease and expression of amino acid transporter (AAP6), an amino acid transport gene, were not significantly affected or slightly suppressed between the treatments with 50 and 75 ppm. In EXP 2, the enhanced ethylene level reduced photosynthetic pigments, leading to an inhibition of flower development without any pod development. A significant increase in protease activity, confirmed using in-gel staining of protease, was also observed in the ethephon-treated mature leaves. Ethephon application enhanced the expression of four amino acid transporter genes (AAP1, AAP2, AAP4, and AAP6) and the phloem loading of amino acids. Significant correlations between ethylene level, induced by ethephon application, and the descriptive parameters of protein degradation and amino acid transport were revealed. These results indicated that an increase in ethylene upregulated nitrogen remobilization in the mature leaves (source), which was accompanied by an increase in proteolytic activity and amino acid transport, but had no benefit to pod (sink) development.

Highlights

Ethylene production increases drastically during several plant developmental events, such as germination, leaf and flower senescence and abscission, and fruit ripening [1,2,3,4,5]
A large portion of remobilized N from senescent leaves is derived from the degradation of leaf proteins [17,18], and the amino acids released after protein degradation are transported via phloem to meet the N demand for grain filling [19,20]
Endogenous ethylene concentration was increased in an ethephon concentration-dependent manner, with a 1.5and 1.7-fold increase, respectively, in ethylene level in 50 ppm or 75 ppm ethephon-treated mature leaves compared with that in the control at day 10 (Figure 1B)

Summary

Introduction

Ethylene production increases drastically during several plant developmental events, such as germination, leaf and flower senescence and abscission, and fruit ripening [1,2,3,4,5]. Ethylene is involved in the regulation of plant responses to abiotic stress [6,7] and plays a critical role in the modulation of plant defense against different biotic stresses [8]. Among the diverse regulatory roles of ethylene, the most widely studied aspect has been its involvement in the leaf senescence process and regenerative development. Using 15N tracing, Girondé et al [16] have estimated that the amount of N distributed to the silique was mainly derived from remobilized N from the source leaves. The role of ethylene in developmentally regulated senescence has been extensively studied [3,11,12,13], its role in the regulation of N remobilization from mature leaves (source) for pod (sink) development is still not clearly established

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Conclusion