Transgenic rice plants overexpressing BBTI4 confer partial but broad-spectrum bacterial blight resistance

Plant Bowman-Birk type bran trypsin inhibitors (BBTI) belong to a family of serine protease inhibitors that inhibit trypsin activity and play roles in plant development and defense responses to both biotic and abiotic stresses. In this study, transgenic rice plants overexpressing BBTI4 (OXBBTI4) were generated. Reverse-transcription polymerase chain reaction and western blot (WB) analysis demonstrated that the BBTI4 mRNA and protein levels were significantly increased in OX-BBTI4. Notably, two BBTI4 protein forms with different molecular weight (18 kD and 28 kD) were revealed by WB analysis. In non-transgenic plants, BBTI4-28kD and BBTI4-18kD were mainly expressed in roots and leaves, respectively, while in transgenic OX-BBTI4 plants, both protein forms were expressed constitutively. Subcellular analysis revealed that BBTI4 is localized in the cytosol. Moreover, Xanthomonas oryzae pv. oryzae (Xoo) inoculation experiments demonstrated that transgenic OX-BBTI4 rice plants conferred partial but broad-spectrum Xoo resistance. In OX-BBTI4 transgenic rice plants, the expression of OsPR3 and OsPR10a proteins was induced and gradually increased after Xoo infection, while the expression of OsPR1a, OsPR1b and OsPR-pha remained unchanged. Taken together, these results suggest that BBTI4 may play a role in rice resistance to Xoo, and OsPR3 and OsPR10a may be involved in the OX-BBTI4-dependent partial Xoo resistance response.