PilZ Domain Proteins of the Plant Pathogen Xanthomonas oryzae pv. oryzae Function Differentially in Virulence

EDITORIAL

In this issue of Applied and Environmental Microbiology (AEM), we publish a paper by Fenghuan Yang, Fang Tian, Huamin Chen, William Hutchins, Ching-Hong Yang, and Chenyang He from the State Key Laboratory for Biology of Plant Diseases and Insect Pests at the Chinese Academy of Agricultural Sciences and the Department of Biological Sciences at the University of Wisconsin—Milwaukee, reporting on the investigation of the virulence of Xanthomonas oryzae pv. oryzae, a bacterium that causes leaf blight of rice (1). X. oryzae has been used as a model microorganism to gain an understanding of the pathogenic mechanisms of bacterial plant pathogens (2–4), and the authors evaluate mutant strains of X. oryzae pv. oryzae that displayed increased virulence in their experimental assay to a variety of rice (Oryza sativa subsp. japonica) that is susceptible to this pathogen (1).

Cyclic dimeric-GMP (c-di-GMP) is an intracellular signaling molecule implicated in the control of cellular properties that can influence the virulence of pathogens (5, 6). The so-called PilZ domain of certain proteins can bind c-di-GMP and thereby have a regulatory impact on c-di-GMP-mediated processes (7). The authors examined PilZ domain proteins of X. oryzae and demonstrated that two of these proteins, PXO_00049 and PXO_02374, had high binding affinities for c-di-GMP (1). Gene deletions of PXO_00049 or PXO_02374 yielded mutant strains of X. oryzae that displayed increased virulence (1), a finding that is in marked contrast to previous studies with X. oryzae pv. oryzae and other pathogens that have demonstrated that mutation of PilZ domain proteins can decrease virulence under experimental conditions (4, 8, 9). The findings reported by Yang et al. (1) (i) make important contributions to understanding the biology and pathogenesis of X. oryzae pv. oryzae and (ii) provide a basis for future studies designed to develop strategies to control this plant pathogen.

X. oryzae is included on the U.S. Select Agents and Toxins List (10). These “agents and toxins have been determined to have the potential to pose a severe threat to both human and animal health, to plant health, or to animal and plant products,” and the U.S. Federal Select Agent Program “oversees the possession, use and transfer of biological select agents and toxins, which have the potential to pose a severe threat to public, animal or plant health or to animal or plant products” (10). Thus, work on X. oryzae in the United States is subject to regulation by the U.S. Federal Select Agent Program. However, Yang et al. (1) have declared in their publication that all biological work on the X. oryzae strains described in their paper was conducted in China. To the best of our understanding, China does not impose governmental regulations on research with X. oryzae. Historically, X. oryzae was not considered to be indigenous to the United States; however, strains of this organism that are less virulent than those that are endemic in China do occur in the United States (2, 11). Although work on X. oryzae is not subject to governmental regulations in China, the authors employed biosafety protocols to mitigate risk and prevent dissemination of the pathogen: (i) containment procedures were employed, (ii) X. oryzae-inoculated plants were restricted to independent greenhouses, and (iii) all X. oryzae-inoculated plants were destroyed by autoclaving (1).

The publication of work generating enhanced virulence of a select agent might be viewed as controversial, given DURC (dual use research of concern) considerations. As described in the United States Government Policy for Oversight of Life Sciences Dual Use Research of Concern (12), DURC is defined as “life sciences research that, based on current understanding, can be reasonably anticipated to provide knowledge, information, products, or technologies that could be directly misapplied to pose a significant threat with broad potential consequences to public health and safety, agricultural crops and other plants, animals, the environment, materiel, or national security.” However, governmental agencies and university biosafety committees in the United States may support such publication when the benefits of the research are seen as being of greater value than the potential risks, and previous editorials have addressed this topic (13, 14).

It should be noted that X. oryzae is not one of the subset of select agents that falls within the scope of U.S. DURC biosecurity considerations (12). Nonetheless, the present work describing variants with enhanced virulence raises issues relating to their potential intentional release into the environment.

Select agent status for plant pathogens has been questioned because this status impedes research that could facilitate a more rapid understanding of pathogenesis and the development of control measures (15). It should likewise be noted that, while increased virulence of X. oryzae pv. oryzae due to experimental gene deletion might be considered a striking finding (1), gene deletion under natural conditions may also result in increased virulence (16). Thus, increased virulence due to gene deletion is not a phenomenon restricted to experimental research but rather may constitute a natural process.

The paper by Yang et al. (1) was considered for publication by AEM like all others submitted for evaluation. Reviewers were asked to evaluate the paper for scientific rigor and significance and to consider whether the research represented DURC. The manuscript was also evaluated for DURC by senior editors and the chair of the Journals Board at the American Society for Microbiology, which publishes AEM, to determine whether any potential risks of publishing the paper would outweigh the benefits. Based on the peer reviews, the authors' response to the reviews, and our own internal review, it has been decided to move forward with publication because (i) publication will make the findings available to the X. oryzae research community and thus augment the development of strategies to control this plant pathogen and (ii) the benefits of this information being disseminated to the public far outweigh the potential risks associated with publication. The production of rice is of economic and societal importance in Asia, and it has been estimated that bacterial leaf blight of rice can lead to 20 to 75% loss in crop productivity (2). Thus, the importance of research on a causative agent of leaf blight of rice should not be underestimated.

The views expressed in this Editorial do not necessarily reflect the views of the journal or of ASM.