The PR genes displayed differential expression patterns. PR1 was expressed at high levels in water-infiltrated control leaves and was strongly up-regulated in Xoo-infiltrated leaves (81- and 202-fold induction at 12 and 24 hpi, respectively). PR2 was expressed at a moderate level in water-infiltrated leaves and was up-regulated in Xoo-infiltrated leaves mildly at 12 hpi (5-fold induction) and strongly at 24 hpi (125-fold induction), respectively. Expression of PR4 and PR5 was not obvious in control leaves, but was mildly induced in Xoo-infiltrated leaves at 12 hpi and strongly induced at 24 hpi (100-fold induction) (Figure 3; Figure S1). These results support the hypothesis that the necrosis observed in Xoo-infiltrated leaves is a type of HR.