Resistance levels and fitness of protoporphyrinogen oxidase (PROTOX) inhibitor-resistant transgenic rice in paddy fields

We previously confirmed that the transgenic rice line, M4, was about 200-fold more resistant to oxyfluorfen than the wild-type (WT) rice in whole-plant bioassays in pots. The transgenic rice line was also cross-resistant to other protoporphyrinogen oxidase (PROTOX)-inhibiting herbicides, acifluorfen, carfentrazone, and oxadiazon. The objectives of this research were to (a) verify the resistance of transgenic rice plants to commercial doses of PROTOX-inhibiting herbicides under paddy field conditions, (b) compare the growth, yield, and grain quality of transgenic and WT rice under weed-free conditions in a paddy field, and (c) determine the responses of transgenic and WT rice plants to chilling and drought stress. In the field, M4 was resistant to PROTOX inhibitors oxyfluorfen, acifluorfen, carfentrazone, pyraflufen, and oxadiazon in transplanted and direct-seeded rice culture. The transgenic and WT plants had similar plant heights and number of tillers. However, the yield of M4 at T4 and T5 generations was 7–8% less than that of WT plants. This was due to reduced number of spikelets per panicle and reduced grain weight. Head rice yield, immature kernels, damaged kernels, palatability, and protein and amylose contents were similar between M4 and WT rice. There was no difference in chilling injury between WT and M4, but M4 was more tolerant to drought stress than WT plants. PROTOX inhibitor-resistant rice is agronomically viable. It will expand the herbicide options in rice production. Follow-up research is required to elucidate the mechanisms underlying the slight yield difference and differential drought response between WT and transgenic rice.