Plant defense activators potentiate the generation of elicitor-responsive photon emission in rice

https://doi.org/10.1016/j.pmpp.2005.04.007Get rights and content

Abstract

Ultraweak photon emission from rice induced by an elicitor was measured to estimate the influence of pretreated plant defense activators on elicitor-responsive photon emission. Rice leaf segments treated with an elicitor transiently generated relatively high levels of elicitor-responsive photon emission. Pretreatment with plant defense activators increased elicitor-responsive photon emission. This increase was also observed in suspension-cultured rice cells. Prolonged pretreatment allowed the plant defense activators to accelerate photon generation and to act at lower doses. The activators themselves did not induce any marked photon emission in rice leaf segments or cells. The spectral compositions of the increased and nonincreased elicitor-responsive photon emissions from rice cells were almost the same. Therefore, plant defense activators probably potentiate the generation of elicitor-responsive photon emission itself. The elicitor-responsive expression of the PBZ1 gene was strongly enhanced by pretreatment of rice cells with plant defense activators, when elicitor-responsive photon emission was also enhanced. We could not identify a significant correlation between the degree of photon emission enhancement and the potentiation of PBZ1 expression with each treatment. However, the results indicate that the generation of elicitor-responsive photon emission is potentiated when rice cells are primed for disease resistance by plant defense activators.

Introduction

The induced resistance response of plants to pathogenic organisms has been of interest in the study of crop protection. Especially in last few decades, the induction of disease resistance in plants as a practical method of crop protection has involved the investigation of several chemicals that induce systemic disease resistance in plants, the so-called ‘plant defense activators’ [1], [2], [3]. Rapid and enhanced defense responses to pathogens/elicitors are characteristic cellular events in plants protected by plant defense activators. This phenomenon has been called ‘priming’ [4]. The priming effects of several defense responses have been reported, including phytoalexin accumulation [5], [6], defense-related gene/protein accumulation [5], [6], [7], [8], [9], and oxidative burst [10], [11]. Priming effects should be useful markers of resistance induction in plant cells [4].

We have already reported a new type of defense-related phenomenon in which plants generate relatively high levels of ultraweak photon emission in an unusual spectrum in response to pathogen attack or elicitor treatment [12], [13], [14]. Living organisms constitutively generate ultraweak photon emissions [15], [16], [17]. The major sources of these in living organisms are thought to be the generation of excited and chemiluminescent molecules and/or energy transfer from excited molecules to fluorescent molecules [15], [18], [19], [20], [21]. This pathogen/elicitor-responsive photon emission could be used as a noninvasive indicator of physiological changes, together with the generation of excited molecules involved in the defense response.

We report here that the generation of elicitor-responsive photon emissions in leaf segments and suspension-cultured cells of rice was potentiated by pretreatment with plant defense activators.

Section snippets

Apparatus for ultraweak photon emission measurement

A multisample photon counter (PCX-100; Hamamatsu K.K., Japan) was used to measure the time-dependent variation in ultraweak photon emission. This device is equipped with an R329P photomultiplier tube, which measures spectral responses ranging from 240 to 630 nm, and a special dark box system for 16 samples. Samples were placed in petri dishes of 60 mm diameter for measurements. Spectrometry of ultraweak photon emission was performed using an MSPC II multisample photon counter (Hamamatsu K.K.,

Elicitor-responsive photon emission from rice leaf segments

Rice leaf segments on distilled water generated ultraweak photon emissions as low as 10 counts s−1 cm−2, with no noticeable changes during the measurements. The PGPF elicitor and PDB themselves emitted photons as low as those emitted by leaf segments (Fig. 1A). Leaf segments floated on PGPF elicitor, however, transiently emitted high levels of photons at over 500 counts s−1 cm−2, with a peak at 6–8 h after application (Fig. 1B). These results indicate that rice leaf segments generate photon emissions

Acknowledgements

This study was supported by a research grant from Shizuoka Prefecture. The authors thank Prof. Mitsuo Hyakumachi for providing the PGPF isolate GP17-2. We are also grateful to Dr Eiichi Minani of the National Institute of Agricultural Resources and Dr Hanae Kaku of Meiji University for their helpful suggestions on the PBZ1 gene expression assay. We are also grateful to Prof. Naoto Shibuya of Meiji University for helpful suggestions about priming effects. We also greatly appreciate the

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