Abstract
Alternaria alternata (Fries) Keissler is a facultative parasitic fungus which causes brown spot disease of senescing tobacco (Nicotiana tabacum L.) leaves. To test whether ammonia accumulation at the infection site plays a role in differentiation of infection structures and lifestyle transition of A. alternata, an artificial ammonia environment was set up to imitate and manipulate the host ammonia environment that pathogens would encounter both on leaf cuticle and in extracellular spaces. Light microscopy showed that, within 6 h after ammonia exposure, appressoria and penetration pegs could be detected at the tips of germ tubes incubated on glass slides and on host leaf surfaces, and foliar epidermal cells influenced by appressoria turned chlorotic and necrotic. Neither appressorium formation nor necrotic lesion was found in control experiments (ammonia free). Moreover, a compatible interaction between A. alternata and a resistant tobacco cultivar Jingyehuang was induced by ambient ammonia, resulting in expanding, ring-like lesions on its excised leaves within 60 h. In the absence of ammonia treatment, however, no spot symptoms could be found on the leaves of a susceptible cultivar K326 during the same period. These results suggest that A. alternata can respond to ambient ammonia and use it as a stimulator to invade the host by differentiating into infection structures and to switch to a necrotrophic lifestyle, next to secrete pathogenicity factors.
Zusammenfassung
Alternaria alternata (Fries) Keissler ist ein obligat parasitischer Pilz, der die Braunfleckenkrankheit an alternden Blättern des Tabaks (Nicotiana tabacum L.) verursacht. Zur Untersuchung, ob die Anreicherung von Ammoniak an der Infektionsstelle eine Rolle bei der Bildung pilzlicher Infektionsstrukturen und der Umstellung der Ernährungsweise von A. alternata spielt, wurden die Bedingungen artifiziell imitiert und manipuliert, denen das Pathogen auf der Blattcuticula und im Interzellularraum des Wirtes im Hinblick auf Ammoniak ausgesetzt ist. Lichtmikroskopische Beobachtungen zeigten, dass Appressorien und Infektionsschläuche während der ersten 6 Stunden nach der Zugabe von Ammoniak an der Spitze von Keimschläuchen auf Objektträgern und Blattoberflächen nachweisbar waren. Epidermale Blattzellen chlorotisierten und nekrotisierten dann als Reaktion auf die Appressorienbildung. Weder Appressorien noch nekrotische Läsionen bildeten sich in den nicht mit Ammoniak behandelten Kontrollen. Darüber hinaus wurde eine kompatible Interaktion zwischen A. alternata und der resistenten Tabaksorte Jingyehuang durch Ammoniakzugabe induziert. Sie resultierte in größer werdenden, ringförmigen Läsionen auf abgeschnittenen Tabakblättern innerhalb der ersten 60 Stunden nach der Behandlung. Im gleichen Zeitraum konnten keine Symptome auf Blättern der nicht mit Ammoniak behandelten, anfälligen Sorte K326 beobachtet werden. Die Ergebnisse deuten darauf hin, dass A. alternata durch Ammoniak stimuliert wird und den Wirt durch die Bildung von Infektionsstrukturen und die Umstellung auf eine nekrotrophe Ernährungsweise mit einhergehender Sekretion von Pathogenitätsfaktoren infiziert.
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Duan, W.J., Zhang, X.Q., Yang, T.Z. et al. A novel role of ammonia in appressorium formation of Alternaria alternata (Fries) Keissler, a tobacco pathogenic fungus. J Plant Dis Prot 117, 112–116 (2010). https://doi.org/10.1007/BF03356345
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DOI: https://doi.org/10.1007/BF03356345
Key words
- Alternaria alternata (Fries) Keissler
- ammonia
- appressorium differentiation
- brown spot disease
- tobacco-Alternaria interaction
- Nicotiana tabacum L.