The roles of plant phenolics in defence and communication during Agrobacterium and Rhizobium infection

Abstract

Phenolics are aromatic benzene ring compounds with one or more hydroxyl groups produced by plants mainly for protection against stress. The functions of phenolic compounds in plant physiology and interactions with biotic and abiotic environments are difficult to overestimate. Phenolics play important roles in plant development, particularly in lignin and pigment biosynthesis. They also provide structural integrity and scaffolding support to plants. Importantly, phenolic phytoalexins, secreted by wounded or otherwise perturbed plants, repel or kill many microorganisms, and some pathogens can counteract or nullify these defences or even subvert them to their own advantage. In this review, we discuss the roles of phenolics in the interactions of plants with Agrobacterium and Rhizobium.

Figure 1

Phenolics in the Agrobacterium–plant interaction. Black arrows indicate how Agrobacterium uses phenolics to initiate pathogenesis in host plants for opine synthesis and nutrition, and red arrows indicate how it inactivates the excess amounts of the same phenolics using its own specific O‐demethylase system. Blue arrow indicates the involvement of the phenolic‐sensing bacterial VirA protein in negative chemotaxis. Green arrows show how plants synthesize and use some phenolics, such as salicylic acid, to interfere with VirA and hence pathogenesis, and also to degrade the quoromones that give ecological advantage to Agrobacterium over other bacteria in competition for the opines synthesized by the infected plant.

Figure 2

The use of phenolics by Agrobacterium and Rhizobium for survival and infection of the host plant. Black arrows indicate how Agrobacterium uses phenolics to initiate a complex process of pathogenesis, culminating with opine synthesis. In addition to their nutritional value, opines help Agrobacterium's competition with nonpathogenic bacteria, such as A. radiobacter, by increasing its population density and biofilm formation through quorum sensing. Agrobacterium also uses the attKLM operon to regulate its population density during times of nutritional starvation and to synthesize alternative sources of nutrients and energy by degrading γ‐butyrolactones produced by other rhizospheric bacteria. Green arrows indicate the use of phenolics by Rhizobium leguminosarum bv. viciae for the induction of nod genes followed by the process of symbiosis. Under stress conditions, phenolics also regulate the increase in population density, biofilm formation and effective nodulation by repressing the quorum‐sensing rhi operon. An increase in population density as a result of quorum sensing provides a competitive edge to rhizobia over other rhizospheric bacteria. Bold lines indicate the quorum‐quenching mechanisms, whereas triangles represent the steps of activation. TCA, tricarboxylic acid.

Figure 3

The xenobiotic detoxification reactions by Agrobacterium. (a) Conversion of the excess and toxic amounts of the vir‐inducing ferulic acid into its relatively less toxic caffeic acid by the VirH2 demethylase. (b–d) Oxidation and mineralization of vanillate, vanillyl alcohol and vanillin into sources of carbon and energy. (e) Biotransformation of hydantoin by hydantoinase into d‐N‐carbamoyl amino acid and further into d‐amino acids by d‐N‐carbamoylase. The d‐amino acid is finally converted into its easily utilizable l‐form of amino acid by racemase.

Figure 4

Reactions showing how rhizospheric phenolics are utilized as sources of carbon and energy by Rhizobium spp. for saprophytic and symbiotic survival in soil and host. (a) Mineralization of catechins and substituted chloro‐aromatics by catechol‐1,2‐dioxygenase into phloroglucinolcarboxylic acid and protocatechuate. (b) Degradation of flavonoids by C‐ring cleavage into utilizable forms.