Abstract
The bacterial blight caused by Xanthomonas oryzae pv. oryzae (Xoo) is the most devastating bacterial disease of rice worldwide. A number of dominant major disease resistance (MR) genes and recessive MR genes against Xoo have been cloned and molecularly characterized in the last two decades. However, how these MR genes mediated-resistances occur at the cytological level is largely unknown. Here, by ultrastructural examination of xylem parenchyma cells, we show that resistances to Xoo conferred by dominant MR genes and recessive MR genes resulted in different types of programmed cell death (PCD). Three dominant MR genes Xa1, Xa4, and Xa21 and two recessive MR genes xa5 and xa13 that encode very different proteins were used in this study. We observed that Xa1-, Xa4-, and Xa21-mediated resistances to Xoo were associated mainly with autophagy-like cell death featured by the formation of autophagosome-like bodies in the xylem parenchyma cells. In contrast, the xa5- and xa13-mediated resistances to Xoo were associated mainly with vacuolar-mediated cell death characterized by tonoplast disruption of the xylem parenchyma cells. Application of autophagy inhibitor 3-methyladenine partially compromised Xa1-, Xa4-, and Xa21-mediated resistances, as did Na2HPO4 alkaline solution to xa5- and xa13-mediated resistances. These results suggest that autophagy-like cell death is a feature of the dominant MR gene-mediated resistance to Xoo and vacuolar-mediated cell death is a characteristic of the recessive MR gene-mediated resistance.
Highlights
Plant resistance against pathogens can be genetically classified into two classes based on the strength of resistance: qualitative or complete resistance conferred by major disease resistance (MR) genes and quantitative or partial resistance mediated by multiple genes or quantitative trait loci (Kou and Wang, 2010; Zhang and Wang, 2013)
We found that autophagy-like cell death is the major characteristic in dominant MR gene-mediated resistance and that vacuolar-mediated cell death is the main feature in recessive MR gene-mediated resistance
At 5 days after infection (DAI), the brown hypersensitive response (HR)-like lesions appeared on all the inoculation sites of rice leaves of the near-isogenic rice lines (NILs) carrying MR genes, while the inoculation sites of susceptible IR24 leaves formed approximately 0.5-cm-long chlorotic water-soaked symptoms (Figure 1A)
Summary
Plant resistance against pathogens can be genetically classified into two classes based on the strength of resistance: qualitative or complete resistance conferred by major disease resistance (MR) genes and quantitative or partial resistance mediated by multiple genes or quantitative trait loci (Kou and Wang, 2010; Zhang and Wang, 2013). Cell Death Rice R Genes pattern-triggered immunity (PTI) or plant-derived damageassociated molecular PTI or basal resistance, and effectortriggered immunity (ETI) or gene-for-gene resistance (Jones and Dangl, 2006; Thomma et al, 2011; Monaghan and Zipfel, 2012). In general, PTI is quantitative resistance and ETI is qualitative resistance in many plant–pathogen pathosystems (Zhang and Wang, 2013). Oryzae (Xoo), which causes the most devastating bacterial disease of rice worldwide, are a unique pathosystem for rice qualitative resistance against Xoo (Zhang and Wang, 2013). The rice MR genes resistant to Xoo can be an ETI or a PTI or other mechanisms that cannot be explained by ETI and PTI (Zhang and Wang, 2013; Ke et al, 2017)
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