Elsevier

Plant Physiology and Biochemistry

Volume 73, December 2013, Pages 412-419
Plant Physiology and Biochemistry

Research article
Ethylene-induced changes in lignification and cell wall-degrading enzymes in the roots of mungbean (Vigna radiata) sprouts

https://doi.org/10.1016/j.plaphy.2013.10.020Get rights and content

Highlights

  • Primary root elongation, lateral root numbers were inhibited by ethephon.

  • ETH enhanced the lignin content by regulating activities of key lignin biosynthesis enzymes.

  • The change of roots morphology was correlated with lignin content.

  • Cell wall-degrading enzymes, which affect root growth, were regulated by ETH.

Abstract

As an important regulator, ethylene inhibits root growth and development in plants. To determine the mechanism of ethylene on root elongation growth and lateral root formation, ethylene-induced lignification and cell wall-degrading enzymes in the roots of mungbean sprouts were tested. We initially observed that primary root elongation and lateral root numbers were inhibited, while lignin content was enhanced by ethephon (ETH). Cell wall remolding proteins, polygalacturonase (PG) and carboxymethyl cellulose (Cx) activities were reduced, but α-expansins and xyloglucan endotransglucosylases/hydrolases (XTH) were enhanced by ETH. The promotion in lignin production was correlated with changes in activities of key lignin biosynthesis enzymes and hydrogen peroxide (H2O2) content. These actions induced by ETH were altered via treatment with an ethylene perception antagonist (Ag+). We subsequently demonstrated that the role of endogenous ethylene in regulating root elongation growth and lateral root formation were correlated with lignification and cell wall-degrading enzymes, respectively. These results suggested that the ethylene-regulated inhibition of primary root elongation growth was caused by an increase in lignification that reinforced the cell wall and shortened root length, and the suppression of lateral root formation was linked to activities of PG, Cx, α-expansins and XTH.

Introduction

Plant root systems display considerable plasticity in response to environmental and endogenous signals [1]. Phytohormones such as ethylene play important roles in regulating the plant life cycle including seed germination, root initiation, fruit ripening, and senescence [2], especially during root development. In Arabidopsis thaliana roots, ethylene and its precursor 1-aminocyclopropane-1-carboxylic acid (ACC) cause growth responses such as an increase in root width and a rapid but reversible downregulation of cell elongation [3], [4], [5]. Ethylene not only regulates root length by affecting cell elongation, but also regulates root cell division [6]. However, cells affected by ethylene are located in the root elongation zone, rather than in the root meristem. Lateral root development is initiated when the root pericycle cells undergo cell division [7]. Studies on A. thaliana seedlings have used mutants altered in ethylene signaling and synthesis to explore the role of ethylene in lateral root formation, and found that enhanced ethylene synthesis or signaling could negatively or positively impact lateral root formation [8]. Such evidence implies that the contradictory effects on lateral root formation are affected by different concentrations of ethylene.

Root growth inhibition is due to the depression of apical cell elongation, which is intimately associated with cell wall elasticity. Lignification, the metabolic process of sealing a plant cell wall by lignin deposition, occurs during the course of normal tissue development and is an important step during root growth [9]. Lignin, a complex phenolic polymer found in plant cell walls, is a product of the phenylpropanoid pathway, and phenylalanine amine (PAL), cinnamyl alcohol dehydrogenase (CAD) and peroxidase (POD) are key enzymes in lignin synthesis. Cinnamate is produced during the deamination of phenylalanine by the catalytic action of PAL in the above pathway. Cinnamyl alcohol dehydrogenase catalyzes coniferaldehyde into coumaryl alcohol. Lignin formation is accomplished under the catalysis of POD by association with hydrogen peroxide (H2O2) [10], [11], [12].

Cell growth is correlated with the degradation of cell wall components [13], [14]. Cell wall differentiation is affected by changes in enzymatic activity [15], for example, pectin and xyloglucan in the cell wall are degraded by polygalacturonase (PG) and carboxymethyl cellulase (Cx), respectively. In addition, a class of enzymes known as xyloglucan endotransglucosylases/hydrolases (XTH) catalyzes the endocleavage of xyloglucan polymers and the subsequent transfer of the newly generated reducing ends to other polymeric or oligomeric xyloglucan molecules [16]. Plant cell expansion is mainly regulated by cell wall extensibility and results from selective loosening and rearrangement of the load-bearing cellulose/xyloglucan network [17]. Expansins are a class of proteins that catalyze the long-term expansion of cell walls and are considered principal proteins that affects cell expansion by cutting hydrogen bonds between cellulose and hemicellulose in plants, thereby extensibility of the cell wall is enhanced [18]. Such reactions are responsible for the reduction in strength and elasticity of the cell wall, and promote lateral root primordium breakthrough of root cortex tissue.

Although some studies have investigated the role of ethylene in root growth and development, the involvement of ethylene, lignification and activities of cell wall-degrading enzymes in the same physiological process led us to explore whether these factors act in coordination to modulate root growth in mungbean sprouts. The present study was undertaken to investigate changes in lignification and activities of cell wall-degrading enzymes during elongation growth of primary roots and formation of lateral roots in mungbean sprouts when treated with ethephon (ETH) and ethylene-action inhibitor silver nitrate (Ag+). The results will help clarify the role of ethylene in primary root elongation and lateral root formation.

Section snippets

Effects of ETH on root growth

Root growth of 5-day-old mungbean sprouts were significantly inhibited by ETH (Table 1). The primary root length and lateral root numbers were significantly inhibited by 7.7–58.7% and 11.5–95.3%, respectively, with increasing concentrations of ETH (0.015–1.5 mM) compared with the control. A similar trend was also observed in FW and DW, with FW reduced by 7.55–21.54% for the same range of ETH concentrations and significant inhibition of DW at 0.15 mM and 1.5 mM ETH. Interestingly, we found that

Discussion

Primary root elongation growth and lateral root formation are two key factors affecting acquisition efficiency of sufficient water and nutrients [19]. Ethylene, a simple gaseous phytohormone, is involved in numerous physiological aspects of plant growth and development, and has been proven to cause a typical triple response in roots of dark-grown seedlings [20]. Biochemical and genetic data have demonstrated that ethylene interacts with auxin in the regulation of root growth [21]. For instance,

General procedures

Mungbean (Vigna radiata L.) seeds were soaked in hot water for 3 min. After cooling to room temperature, seeds were imbibed in distilled water for 6 h, and then germinated in an incubator for five days at 23 ± 1 °C and 85 ± 5% humidity in the dark. The seeds were sprayed with distilled water every 6 h and with the treatments at 18 h and 36 h (that is, treated twice). The treatments consisted of germinated-seeds treated with 0.015, 0.15, or 1.5 mM ETH for 10 min. Additional experiments with

Acknowledgments

This research was financially supported by the National Modern Agriculture Industry Technology System (Grant CARS-09), Ministry of Agriculture, China.

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    The first two authors contributed equally to this work.

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