9:00 am - 10:00 am
A graduate exam seminar is a presentation of the student’s final research project for their degree.
This is an ALES MSc Final Exam Seminar by Shengjuan Li. This seminar is open to the general public to attend.
MSc with Dr. Jocelyn Ozga and Dr. Gavin Chen.
Google Meet Link: meet.google.com/rou-erwy-swc
Thesis Topic: Effect of modulation of auxin response on clubroot development caused by Plasmodiophora brassicae in Arabidopsis
Clubroot disease caused by the obligate biotrophic protist Plasmodiophora brassicae, is a serious soilborne disease in Brassicaceae species including cruciferous crops such as canola and the genetic model species Arabidopsis. This disease, identified by the formation of sizable root galls, is accompanied by alterations in the plant’s source-sink relations and hormonal balance, leading to stunting of above-ground growth and reduction in yields in crop plants. The plant hormone auxin is believed to be among the hormones utilized by the clubroot pathogen in gall development; therefore, it may be possible to suppress gall development due to P. brassicae by modulating auxin signaling in the plant host. In this study, Arabidopsis auxin receptor double mutant (tir1afb2) and quadruple mutant (tir1afb245) lines were assessed for their ability to suppress clubroot disease progression. The WT line displayed the most significant disease symptoms when inoculated with P. brassicae [Disease Index (DI) = 90.3 %], with the auxin receptor mutants exhibiting less severe disease symptoms [tir1afb2, DI = 67.7 %; tir1afb245, DI = 45.0 % at 32 days after inoculation (DAI)]. Less severe clubroot disease symptoms were also observed in all three separately generated tir1afb2 double mutant lines compared to the WT line, further confirming a reduction in clubroot progression in the tir1afb2 lines. Additional confirmation that reduced auxin response was associated with reduced clubroot disease progression was obtained by measuring the fresh weights of the root-shoot transition region among the lines tested. Root-shoot transition region fresh weights were less (reduced galling) in the auxin receptor mutant lines than the WT line.
The transcript abundance of auxin-response gene expression markers, AtARF3, AtARF19, AtGH3.3 and AtGH3.17, decreased with lost of auxin response (WT> tir1afb2C ≥tir1afb245) at 21 DAI, confirming reduced auxin signaling in the auxin receptor mutant lines when inoculated with P. brassicae. The transcript abundance of AtPR5, a defense gene marker, was higher in the tir1afb2C and tir1afb245 lines compared to their non-inoculated controls at 21 DAI with P. brassicae, but not in the WT line, suggesting that plant defense genes are stimulated in the auxin receptor mutant lines when challenged with P. brassicae infection.
Another possible approach to modify auxin response in the root is to transform Arabidopsis with an auxin receptor that does not naturally occur in this species. Using this approach, auxin receptor mutants expressing PsAFB6, which codes for a pea (Pisum sativum) auxin receptor that does not occur in Brassicaceae species including Arabidopsis, were also assessed for their ability to suppress clubroot disease progression. When PsAFB6 was expressed in auxin receptor tir1afb2 and tir1afb245 mutant backgrounds, there was a trend to reduce P. brassicae-induced disease development. In clubroot inoculated plants, expression of PsAFB6 in the tir1afb2 auxin receptor mutant background decreased the transcript abundance of the Aux/IAA genes (AtIAA9, AtIAA16, and AtIAA19), ARF genes (AtARF3, AtARF5, and AtARF19), and GH3 genes (AtGH3.3 and AtGH3.17), suggesting that expression of PsAFB6 reduced auxin response in this line, consistent with results of the auxin-inhibition root growth assays. In summary, reduction in auxin response reduced the progression of clubroot symptoms in Arabidopsis, supporting the hypothesis that auxin is utilized by the clubroot pathogen in gall development.