Priming as a mechanism behind induced resistance against pathogens, insects and abiotic stress

Abstract

Upon treatment with a resistance-inducing agent, plants acquire an enhanced defensive capacity that results in a faster and/or stronger defence reaction at the moment the plant is exposed to biotic or abiotic stress. This phenomenon is commonly known as priming and has been associated with different forms of induced resistance. Priming accelerates and increases the plant’s ability to activate the defence that is best adapted to resist the stress situation encountered. Under conditions of disease pressure, primed plants exhibit a higher fitness than non-primed plants or defence-expressing plants. Hence, the benefits of priming outweigh its costs in environments where disease occurs. Although priming has been known to occur in plants for decades, most progress in the understanding of this phenomenon has been made over the past few years. Recent insights in the mechanisms behind systemic acquired resistance (SAR), beta-amino-butyric acid-induced resistance (BABA-IR), rhizobacteria-mediated induced systemic resistance (ISR), and volatile organic compound-induced resistance (VOC-IR) against insects have revealed various priming mechanisms that protect against different stresses. Whereas SAR and BABA-IR are associated with priming for salicylate (SA)-dependent defence that acts against biotrophic pathogens, ISR and VOC-IR seem to function through priming for jasmonate (JA)-dependent defence against pathogens and insects. Expression of BABA-IR and ISR against pathogenic fungi and oomycetes is also associated with an augmented formation of callose-rich papillae. This priming response depends on a largely unknown defence pathway, which involves abscisic acid (ABA) and phosphoinositide (PI) signalling, and is thought to target the cellular secretory pathway. Induction of the primed state may be mediated by an enhanced accumulation of signalling compounds, such as transcription factors (TFs) that remain inactive until the plant is exposed to stress. A Q-PCR-based transcription profiling of ~2.200 TF genes in Arabidopsis has revealed consistent changes in the expression of certain TF genes directly upon activation of ISR and BABA-IR. We are currently investigating the contribution of these transcription factors to the various priming responses.