Accumulation of phototoxic thiophenes in Tagetes erecta (Asteraceae) elicited by Fusarium oxysporum

Abstract

Infection of 2-week-old seedlings of Tagetes erecta var “Crackerjack” by Fusarium oxysporum Schl. f. sp. callistephi (Beach) Snyder et Hansen, race 2 (moderately virulent) and F. oxysporum Schl. f. sp. radicis-lycopersici Jarvis and Shoemacker (highly virulent), resulted in different patterns of thiophene kinetics in the infected tissues. The detected thiophenes 5-(4-hydroxy-1-butenyl)-2-2′-bithienyl (BBT-OH), 5-(4 acetoxy-1-butenyl)-2,2′-bithienyl (BBT-OAc) and alpha-terthienyl (α-T), accumulated in roots and stems which had been partially damaged by the moderate pathogen, reaching maximum concentrations 12 days after inoculation with up to 12 times the concentration in uninoculated control plants. However, severe infection with the highly virulent F. oxysporum strain resulted in a decline in thiophene levels in the damaged tissues. The naturally occurring thiophenes BBT-OH, BBT-OAc, α-T, 5-(3-buten-1-ynil)-2,2′-bithienyl (BBT) and 5-(3,4-diacetoxy-1-butinyl)-2, 2′-bithienyl (BBT-20Ac) completely inhibited in vitro spore germination of both fungi at a concentration of 5 μg ml −1 in the presence of near UV light (4 W m −2). α-T was strongly photo-toxic towards mycelial growth of both fungi in shake culture with an EC 50 of 0·03 and 0·06 μg ml −1 for the highly virulent and the moderately virulent F. oxysporum strains, respectively. In the dark, only the latter strain was inhibited by high concentrations of the compound. Tritiated α-T was not significantly metabolized by F. oxysporum f. sp. radicis-lycopersici when grown in shake cultures containing 0·3 μg of 3H −α-T per millilitre. The role of thiophenes as phytoalexins is discussed in relation to these findings.