P.E Singh et al. (1997) Phytoparasitica 25(2):111-116 Abscisic Acid is a Potent Inhibitor of Growth and Sporidial Formation in Neovossia indica Cultures: Dual Mode of Action via Loss of Polyamines and Cellular Turgidity P.P. Singh, 1 Amarjit S. Basra 2 and EES. Pannu 1 The inhibition of polyamine biosynthesis in Neovossia indica (Mitra) Mundkur by D,L- ~-dlfluoromethylornithme (DFMO) caused an effective reduction of mycelial growth and sporidial production under in vitro conditions, which was reversed by omithine application. Abscisic acid proved to be similarly effective, not only in inhibiting mycelial growth and sporidial formation but also the germination of teliospores, which constitute the primary inoculum of the pathogen. ABA-mediated inhibition resulted in decreased polyamine levels and loss of cellular turgidity of mycelial cultures. Scanning electron microscopy of ABA-treated cultures revealed extremely shrunken hyphae, in marked contrast to the turgid controls. It is suggested that the manipulation of ABA levels and/or tissue sensitivity in wheat could be a strategy to combat 'Kamal' bunt, the disease caused by N. indica. KEY WORDS: Abscisic acid; polyamines; Neovossia indica; Karnal bunt; morphogenesis; disease resistance. INTRODUCTION There is increasing evidence that both the pathogen and the host have the capacity to synthesize various growth regulators, and alterations in their levels and/or sensitivity as a result of plant-pathogen interaction are related to disease susceptibility or resistance reaction (3,6,9,10). The research work on these subjects has provided new insights into our understanding of pathogenesis and of the manipulation of disease resistance. 'Karnal' bunt, a serious disease of wheat caused by the fungus Neovossia indica (Mitra) Mundkur, has proved difficult to control, being seed-, soil- and airborne in nature (2). Wheat plants infected with the bunt fungus undergo major morpho-physiological alterations, mediated possibly by hormonal interactions between the pathogen and the host (10). A marked change in endogenous cytokinin levels has been observed (17). Healthy wheat plants treated exogenously with zeatin riboside or isopentenyl adenosine developed dwarf bunt-like symptoms (17). The involvement of other hormones such as auxins (8) and gibberellins (19) is less well understood. Although abscisic acid (ABA) has been implicated in plant responses to many fungal infections (6), its role in the development of infection by bunt fungi is not known. The fungicide triadimefon, which also has plant growth-regulating properties, appears to operate via its effects on ABA concentration in treated tissues (1,4). Interesting work on the involvement of polyamines, especially in regard to the possibility that control of fungal plant diseases might be achieved through inhibition of fungal polyamine biosynthesis, has been published (5,11). RecewedAug. 28, 1996;recewedin finalformJan. 23. 1997. 1 Dept. of Plant Pathology and 2Dept. of Botany, PunjabAgriculturalUniversity, Ludhiana 141 004, India [Fax: +91-161-400945]. Phytoparasitica 25:2, 1997 111