The FUS3 MAPK signaling pathway of the citrus pathogen Alternaria alternata functions independently or cooperatively with the fungal redox-responsive AP1 regulator for diverse developmental, physiological and pathogenic processes.
Alternaria alternata, the fungus that causes citrus brown spot, invades its hosts primarily through the production and action of a host-selective ACT toxin that kills citrus cells prior to invasion. In this study, we show that, in the tangerine pathotype of A. alternata, a mitogen-activated protein kinase (MAPK)-mediated signaling pathway governs a number of biological functions, either separately or in a cooperative manner, with the AaAP1 gene encoding a transcription regulator. The reported MAPK is encoded by the AaFUS3 gene, which we show to be necessary for conidial development, resistance to copper fungicides, melanin biosynthesis, and particularly, for elaboration of the penetration process. In contrast, AaFUS3 negatively controls salt tolerance and production of several hydrolytic enzymes. AaFUS3 has no apparent role in the biosynthesis of host-selective toxin or in resistance to oxidative stress. Both AaAP1 and AaFUS3 are required for fungal resistance to 2,3,5-triiodobenzoic acid (TIBA), 2-chloro-5-hydroxypyridine (CHP), diethyl maleate (DEM), and many pyridine-containing compounds. A strain with mutations in both AaAP1 and AaFUS3 displayed an increased sensitivity to these compounds. Expression of the AaAP1 and AaFUS3 genes and phosphorylation of AaFUS3 were also induced by CHP, DEM, or TIBA. Expression of two genes coding for a putative MFS transporter was coordinately regulated by AaAP1 and AaFUS3. The AaAP1::sGFP (synthetic green fluorescent protein) fusion protein became localized in the nucleus in response to CHP or TIBA. Inactivation of the AaAP1 gene, however, promoted phosphorylation of AaFUS3. Taken together, our results indicate that A. alternata utilizes specialized or synergistic regulatory interactions between the AP1 and MAPK signaling pathways for diverse physiological functions.
Lin
Ching-Hsuan
CH
Citrus Research and Education Center, Department of Plant Pathology, Institute of Food and Agricultural Sciences (IFAS), University of Florida, 700 Experiment Station Road, Lake Alfred, Florida 33850, USA.
Yang
Siwy Ling
SL
Wang
Nan-Yi
NY
Chung
Kuang-Ren
KR
D000528:Alternaria / Q000472:pathogenicity / Q000502:physiology*; D000935:Antifungal Agents / Q000633:toxicity; D002467:Cell Nucleus / Q000737:chemistry; D002957:Citrus / Q000382:microbiology; D003300:Copper / Q000633:toxicity; D004271:DNA, Fungal / Q000737:chemistry / Q000235:genetics; D025141:Drug Resistance, Fungal; D005656:Fungal Proteins / Q000378:metabolism*; D017353:Gene Deletion; D015966:Gene Expression Regulation, Fungal; D008543:Melanins / Q000096:biosynthesis; D020928:Mitogen-Activated Protein Kinases / Q000378:metabolism*; D008969:Molecular Sequence Data; D010935:Plant Diseases / Q000382:microbiology*; D017422:Sequence Analysis, DNA; D015398:Signal Transduction; D013172:Spores, Fungal / Q000254:growth & development; D013312:Stress, Physiological; D014157:Transcription Factors / Q000378:metabolism*
D000935:Antifungal Agents; D004271:DNA, Fungal; D005656:Fungal Proteins; D008543:Melanins; D014157:Transcription Factors; D003300:Copper; D020928:Mitogen-Activated Protein Kinases