Signal Transduction
The putative auxin receptors, which are responsible for transducing auxin signals into physiological changes, recognize auxin and other auxinic molecules (i.e. 2,4-D) because of certain structural characteristics (Figures 3 and 9).
Fig. 9: Auxin receptor recognizes IAA and auxinic herbicides (Image credit: Tracy Sterling, Deana Namuth, Jeremy Steele, and Smitha Kasinadhuni)
These receptors are then thought to signal a cascade of events leading to physiological responses or plant death in the case of auxinic herbicides. The mechanism for the signal-transduction pathway may involve direct interaction of regulatory proteins with specific DNA sequences or secondary messengers. Protein kinases, calcium-calmodulin, phosphoinositide metabolism, cyclic AMP, pH changes, redox reactions at the cell surface, membrane fatty acids and protein methylation have all been implicated in secondary messenger pathways in plants. For example, once auxin binds to a receptor, phospholipase C (PLC) is induced to hydrolyze the membrane lipid phosphotidylinositol (PIP2) which releases inositol-1,4,5-triphosphate (IP3) and diacylglycerol (DAG) (Figure 10a). IP3 and DAG can act as secondary messengers. The released IP3 moves to the vacuolar membrane, the tonoplast, and binds to a receptor in that membrane (Figure 10b). The IP3 receptor then activates a calcium (Ca++) transporter that pumps Ca++ out of the vacuole, where it is stored, into the cytosol. Increased Ca++in the cytosol activates protein kinases that then activate other enzymes which catalyze the various reactions causing the metabolic changes associated with auxin or auxinic herbicide (Figure 10c). Diacylglycerol (DAG) can also activate protein kinases that induce metabolic changes (Figure 10d). The animation ’Auxin and Auxinic Herbicides Mechanisms of Action’ illustrates this example of the signal transduction pathway.
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Fig. 10a: Auxin binding at the receptor causes PIP2 hydrolysis by phospholipase C (PLC) (Image credit: Tracy Sterling, Deana Namuth, Jeremy Steele, and Smitha Kasinadhuni)
Fig. 10b: The released IP3 turns on Ca++ efflux from the vacuole (Image credit: Tracy Sterling, Deana Namuth, Jeremy Steele, and Smitha Kasinadhuni)
Fig. 10c: Calcium induces processes which cause metabolic changes such as protein kinase activation of various metabolic processes (Image credit: Tracy Sterling, Deana Namuth, Jeremy Steele, and Smitha Kasinadhuni)
Fig. 10d: The lipid, diacylglycerol (DAG), released during PIP2 hydrolysis, can also transduce these metabolic changes (Image credit: Tracy Sterling, Deana Namuth, Jeremy Steele, and Smitha Kasinadhuni)