Dominant and Recessive Major R Genes Lead to Different Types of Host Cell Death During Resistance to Xanthomonas oryzae in Rice

Abstract

The bacterial blight caused by <i>Xanthomonas oryzae</i> pv. <i>oryzae</i> (<i>Xoo</i>) is the most devastating bacterial disease of rice worldwide. A number of dominant major disease resistance (<i>MR</i>) genes and recessive <i>MR</i> genes against <i>Xoo</i> have been cloned and molecularly characterized in the last two decades. However, how these <i>MR</i> genes mediated-resistances occur at the cytological level is largely unknown. Here, by ultrastructural examination of xylem parenchyma cells, we show that resistances to <i>Xoo</i> conferred by dominant <i>MR</i> genes and recessive <i>MR</i> genes resulted in different types of programmed cell death (PCD). Three dominant <i>MR</i> genes <i>Xa1, Xa4</i>, and <i>Xa21</i> and two recessive <i>MR</i> genes <i>xa5</i> and <i>xa13</i> that encode very different proteins were used in this study. We observed that <i>Xa1</i>-, <i>Xa4</i>-, and <i>Xa21</i>-mediated resistances to <i>Xoo</i> were associated mainly with autophagy-like cell death featured by the formation of autophagosome-like bodies in the xylem parenchyma cells. In contrast, the <i>xa5</i>- and <i>xa13</i>-mediated resistances to <i>Xoo</i> 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 <i>Xa1</i>-, <i>Xa4</i>-, and <i>Xa21</i>-mediated resistances, as did Na₂HPO₄ alkaline solution to <i>xa5</i>- and <i>xa13</i>-mediated resistances. These results suggest that autophagy-like cell death is a feature of the dominant <i>MR</i> gene-mediated resistance to <i>Xoo</i> and vacuolar-mediated cell death is a characteristic of the recessive <i>MR</i> gene-mediated resistance.