β-Aminobutyric Acid-Induced Resistance Against Plant Pathogens.

Abstract

The potential of a plant to resist attack by a pathogen is enhanced after an initial inoculation with a necrotrophic pathogen. Not only the initially inoculated tissue, but also the remote organs such as leaves and roots, become resistant. Such immunization, named systemic acquired resistance (SAR), was first observed by Chester (4) with Tobacco mosaic virus (TMV) in tobacco and was further demonstrated by the pioneering work of Ross (50) and Kuc (28,29). Efforts devoted to discovering the signal translocating during SAR revealed (26,35,40) the central role of salicylic acid (SA) in activating the defense mechanisms leading to SAR. Functional analogs of SA, such as 2,6-dichloroisonicotinic acid (INA) or benzo[1,2,3] thiadiazole-7-carbothionic acid S-methyl ester (BTH or acibenzolar-Smethyl), were developed which activate the resistance mechanisms downstream of SA (16,26,35,40,57). Since the observations of Chester, little attention was given to the role of induced resistance in general, and of amino acids in particular, in plant defense (29). Kuc et al. were the first to notice in 1957 (31) and 1959 (30) that Dphenylalanine, D-alanine, and DL-tryptophan injected into apple leaves increased resistance against scab without affecting the causal pathogen in vitro. As early as 1958, Van Andel (60) examined 50 amino acids for inducing resistance against Cladosporium cucumerinum in cucumber and found only D-serine, D-threonine, and L-threo-β-phenylserine highly active in vivo but not in vitro. Interestingly, she found no activity with DL-α-aminobutyric (AABA), DL-β-aminobutyric (BABA), γaminobutyric (GABA), or their iso isomers (60). In 1960, Oort and Van Andel first noted induced resistance to tomato late blight following BABA treatment (39). In 1963, two groups reported on the activity of aminobutrates. MacLennan et al. (34) showed that D-AABA and AIB (2-aminoisobutyric acid) were active in apple leaves against scab (but not L-AABA, BABA, or isoBABA). Papavizas and Davey (45) reported on the partial activity of AABA and high activity of BABA against Aphanomyces euteiches causing root rot in peas. They also showed high SAR activity with longer, straight-chain aminobutyrates such as DL-α-aminovaleric (DL-norvaline, five carbons) and DL-α-aminocaprylic acid (eight carbons). Harnack and Schwarz (19) showed that N-substituted glycine derivatives (especially ethyl and butyl) were systemically effective against Phytophthora infestans in tomato and Septoria apii in celery without being active in vitro. The interest in amino acid–mediated induced resistance was renewed about 30 years later when we discovered a strong activity of BABA against disease in potato (10), tomato (6), and tobacco (5), and revealed some of the defense mechanisms it activates in tomato (13) and tobacco (5). In recent years, substantial evidence has been accumulated, especially in Arabidopsis mutants, showing that BABA possesses a large spectrum of activity (7,22,41), as well as multiple forms of plant activation against disease (41,55,66,67).