Vascular defense responses in rice: Peroxidase accumulation in xylem parenchyma cells and xylem wall thickening

The rice bacterial blight pathogen Xanthomonas oryzae pv. oryzae is a vascu lar pathogen that elicits a defensive response through interaction with met abolically active rice cells. In leaves of 12-day-old rice seedlings, the e xposed pit membrane separating the xylem lumen from the associated parenchy ma cells allows contact with bacterial cells. During resistant responses, t he xylem secondary walls thicken within 48 h and the pit diameter decreases , effectively reducing the area of pit membrane exposed for access by bacte ria In susceptible interactions and mock-inoculated controls, the xylem wal ls do not thicken within 48 h. Xylem secondary wall thickening is developme ntal and, in untreated 65-day-old rice plants, the size of the pit also is reduced. Activity and accumulation of a secreted cationic peroxidase, PO-Cl , were previously shown to increase in xylem vessel walls and lumen. Peptid e-specific antibodies and immunogold-labeling were used to demonstrate that PO-Cl is produced in the xylem parenchyma and secreted to the xylem lumen and walls. The timing of the accumulation is consistent with vessel seconda ry wall thickening. The PO-CI gene is distinct but shares a high level of s imilarity with previously cloned pathogen-induced peroxidases in rice. PO-C I gene expression was induced as early as 12 h during resistant interaction s and peaked between 18 and 24 h after inoculation. Expression during susce ptible interactions was lower than that observed in resistant interactions and was undetectable after infiltration with water, after mechanical woundi ng, or in mature leaves. These data are consistent with a role for vessel s econdary wall thickening and peroxidase PO-Cl accumulation in the defense r esponse in rice to X. oryzae pv. oryzae.