Jasmonate-auxin Crosstalk Contributes To Arabidopsis Resistance Against The Necrotrophic Pathogen Alternaria Brassicicola

Auxin has been generally regarded as a hormone mainly regulating plant growth anddevelopment. Recent investigations demonstrated that auxin confers biotrophic pathogenecityand it plays a negative role in plant resistance against biotrophic pathogens. However, thepossible roles of auxin during plant interaction with necrotrophic pathogens are still elusive.We infected a series of auxin-related Arabidopsis mutants asa1-1、cyp79b2/b3、doc1-1、axr2-1and coi1-2mutants, which contain a point mutation of the jasmonate receptor geneCOI1with Alternaria brassicicola, a model fungus with a typical necrotrophic life style. Withthe measurement of lesion size and the expression of some crucial genes,we draw theconclusion as fellows:1.Our pathogen infection assays showed that, while the wild type (WT) Col-0plants aremore resistant to A. brassicicola than them. We demonstrated here that the expression of somecrucial genes, ASA1,ASB1,CYP96B2and CYP96B3, in auxin biosynthetic pathway wasstrongly up-regulated and the free IAA level was also drastically increased in Arabidopsisseedlings infected with A. brassicicola.2.We treated transgenic plants containing the promoters of PINs combined withPIN-GFP protein fusions, PIN1::PIN1-GFP, PIN2::PIN2-GFP, PIN3::PIN3-GFP,PIN4::PIN4-GFP and PIN7::PIN7-GFP, with exogenous JA. And we examined the effect ofJA on PAT in both acropetal and basipetal directions, two distinct auxin transport flowsspatially separated in root.Together, our results indicated that JA generally downregulatesPIN proteins and therefore negatively modulates auxin transport in the root.3.Auxin signaling repressors AUX/IAAs proteins were significantly degraded upon thepathogen infection, and their degradation was also partly dependent on COI1and ASA1,consistent with the up-regulated free IAA level. Some auxin-responsive genes expression wasconsequently stimulated using DR5:GUS reporter and qRT-PCR analysis.4.Further experiments showed that exogenous application of both IAA and MeJAsimultaneously could synergistically induce the expression of PDF1.2(Plant defensin1.2)andHEL (hevein-like protein or PR4) while IAA alone did not have significant effect.5.The elevation of free IAA level was partly dependent on jasmonate receptor COI1and mostly dependent on ASA1, indicating infection with the necrotrophic pathogen could induceauxin biosynthesis through both JA-dependent and JA-independent manners. Consistently,mutants defective in auxin biosynthesis, transport and signaling showed enhancedsusceptibility to A. brassicicola infection to different degrees. JA is involved in the auxinbiosynthetic pathway and transport pathway in plant resistance against necrotrophicpathogens. These results suggest that enhancement of JA-dependent defense might be part ofauxin-mediateddefense mechanism in resistance against necrotrophic pathogens.