The Innate Immunity of Maize and the Dynamic Chemical Strategies Regulating Two-Component Signal Transduction in Agrobacterium tumefaciens

Abstract

Members of the benzoxazinone family of heterocycles are major secondary meta bolites found in grasses of the family Poaceae (also known as Graminae), including the cereals maize, wheat, and rye. Generally, they have been recognized as "resistance factors" that function as part of a nonspecific arsenal of metabolic defense agents, since they have been implicated in antimicro bial, antifungal, insecticidal, antifeeding, and muta genic activites, as well as involvement in a tritrophic interaction (1, 2 ). The costs associated with the main tenance and adaptive evolution of entire metabolic pathways, particularly those associated with general resistance factors, are clearly some function of the sum of their benefits. Yet, the biology of such compounds is less welldefined than that of more specific gene forgene relationships, ones in which complements of resistance and avirulence genes determine compatible/ incompatible relationships in pathogenesis (3 ).One complicating factor in understanding benzo xazinone activity is the spectrum of possible chemical mechanisms and the intrinsic hydrolytic instability of these species. For example, the primary benzoxazinone found in maize and wheat, DIMBOA (2,4dihydroxy 7methoxy1,4benzoxazin3one, 2, Scheme 1), possesses many chemical reactivities with proposed biological relevance. On one hand, the ringchain tautomeric aldehyde of DIMBOA is considered to be an electrophile implicated in reversible reaction with protein nucleophiles, particularly in the hydrophobic binding cavities accessible to the aromatic ring. In a simple model for this mode of action, DIMBOA was shown to form imine adducts with the εNH 2 group of Nacetyl lysine (4 ). DIMBOA was also shown to inhibit chymotrypsin activity by addition of the aldehyde to

Benzoxazinoid Metabolites Regulate Innate Immunity against Aphids and Fungi in Maize

“…2 and 4). The lack of HDMBOA aglycone in this fraction can be explained by the highly unstable nature of this metabolite (Maresh et al, 2006). Although we cannot exclude that apoplastic BX accumulation during the early stages of aphid or fungal infestation occurs entirely without tissue damage, our experiments with chitosan demonstrate that this response can occur independently of tissue damage in maize.…”

“…Subsequent hydrolysis into biocidal aglycones can provide chemical defense against pests and diseases (Cambier et al, 2001;Rostás, 2007). Both HDMBOA and DIMBOA are degraded into MBOA and BOA (Maresh et al, 2006), indicating that DIMBOA (red), and not HDMBOA, has an additional function in the regulation of Bx gene expression and callose deposition. I, Intracellular space; A, apoplast.…”

“…6). Since DIMBOA and HDMBOA-glc are both degraded into MBOA and BOA (Maresh et al, 2006;Macías et al, 2007), it can be concluded that extracellular DIMBOA, rather than its successive breakdown products, is responsible for the regulation of callose deposition and Bx gene expression (Fig. 9).…”

The plant signal salicylic acid shuts down expression of the vir regulon and activates quormone-quenching genes in Agrobacterium

“…Among grasses, defensive benzoxazinones, such as the exuded product 2-hydroxy-4,7-dimethoxybenzoxazin-3-one (HDMBOA), also inhibit the VirA/G system (32,33). Recent work suggests that the Agrobacterium vir genes are subject to feedback control (11).…”

Agrobacterium-Mediated Plant Transformation: Biology and Applications

“…On the other hand, plant factors can also act as inhibitors of the A. tumefaciens sensory machinery. A major organic exudate of maize seedling roots, 2-hydroxy-4,7-dimethoxybenzoxazin-3-one (HDMBOA), has been demonstrated to provide innate immunity and hinder successful A. tumefaciens infection of maize plants (Zhang et al, 2000); an o-imidoquinone decomposition intermediate, (3Z)-2,2-dihydroxy-N-(4-methoxy-6-oxocyclohexa-2,4-dienylidene) acetamide, is a specific inhibitor of signal perception in A. tumefaciens (Maresh et al, 2006;Lin et al, 2008). These inhibitors may account for the recalcitrance of some plants to Agrobacterium-mediated transformation.…”