Final answer:
The compound that is not used in the localized response to pathogenic invasion is salicylic acid; instead, it plays a central role in systemic acquired resistance, coordinating plant-wide defense mechanisms.
Step-by-step explanation:
The compound that is not used in the localized response to pathogen invasion is salicylic acid (d). While nitric oxide, proteinase inhibitors, and phytoalexins are involved in immediate and localized defense responses, salicylic acid plays a pivotal role in the systemic acquired resistance (SAR) of plants, which is a longer-term and more widespread response to pathogen infection. Salicylic acid is a signaling molecule that helps to establish SAR, ensuring that the whole plant becomes more resistant to pathogen attack.
Though not employed in the initial direct localized response, salicylic acid is critical in coordinating a plant-wide defense mechanism. It enhances the expression of pathogenesis-related proteins and may potentiate defense signals. Salicylic acid is also involved in the oxidative burst and acts as a key molecule in local and systemic responses to pathogen challenges, as shown by Martinez et al. (2000) and Delaney et al. (1994).