Ustiloxin A is Produced Early in Experimental Ustilaginoidea virens Infection and Affects Transcription in Rice

Ustiloxin is a kind of 13-membered cyclic peptides found in mature rice false smut generated by Ustilaginoidea virens infecting rice spikelet. So far, six kinds of ustiloxins have been identified from false smut balls (FSBs) in which ustiloxin A is the main component. The toxins can not only inhibit the growth of rice, wheat, and corn, but also poison people and animals. However, so far, there have been few studies of the content of ustiloxin except that in mature FSB. The effect of ustiloxins on the process of infection has not been clarified. In this study, the technique of artificial inoculation coupled with UPLC-ESI-MS was introduced to investigate the content of ustiloxins in the course of infection. The initial formation time of ustiloxin A, B, C, D, F, and G was no later than 5, 5, 9, 7, 7, and 9 days post inoculation (dpi) prior to FSB's formation, respectively. The content of ustiloxin A per spikelet was increased rapidly from 6.0 ng at 5 dpi to 14,157.1 ng at 25 dpi. Meanwhile, the content of ustiloxin A per dry weight (DW) of the FSBs also peaked at 1321.2 μg/g at 25 dpi. Interestingly, both the contents of ustiloxin A per dry weight and per spikelet were significantly reduced from 25 to 30 dpi. Transcriptome sequencing revealed that a total of 146 transcripts (103 upregulated and 43 downregulated) were significantly changed in rice spikelets after 3-h acute exposure to 100 ng ustiloxin A. In addition, several of the significantly altered genes were validated by RT-qPCR.