# Abscisic Acid--Defensive Player in Flax Response to _Fusarium culmorum_ Infection
Some may act directly on pathogens, e.g., by disrupting cell membrane, such as thymol or carvacrol [17] by leading to the dysfunction of the fungal mitochondria by inhibiting efflux pumps, or by inhibiting the expression of gene clusters involved in mycotoxin production [18], some are signal or hormonal molecules (ABA, brassinosteroids), but one of their main functions is to act as antioxidants to prevent oxidation of chlorophylls (carotenoids, tocopherols) [19; 20]. Induced systemic resistance (ISR) and systemic acquired resistance (SAR) are two forms of induced resistance in plants. They are induced by infection or treatment that results in subsequent resistance against pathogen or parasite. SAR can be triggered by exposing the plant to virulent, avirulent, and nonpathogenic microbes and is connected with SA production and signaling [21]. In SAR, activation of defense genes including those encoding for PRs takes place before the upper leaf is reached by a pathogen. Increased flavonoid and anthocyanin synthesis occurs (activation of genes encoding chalcone synthase, chalcone isomerase, dihydroflavonol reductase, leucoanthocyanidin dioxygenase, numerous glucosyltransferases and others). No salicylic acid nor PR proteins participate in ISR, ethylene and jasmonate signaling is observed, though not always required, and priming rather than direct activation of defense is predominantly observed [22]. To some extent both plant responses are similar. As ROS act as a long-distance messenger in plants, genes involved in their synthesis and processing are activated in plants during resistance response [23]. _RbohD_ (NADPH/respiratory burst oxidase homolog protein D) gene activation encoding NADPH oxidase (_NOX_) was shown to be required for the initiation and self-propagation of a rapid cell to cell systemic signal of H\({}_{2}\)O\({}_{2}\) spreading through the extracellular spaces [24]. To keep the spread of free radicals in check, activation of low molecular weight antioxidants (such as phenylpropanoids) biosynthesis genes (phenylalanine ammonia lyase (_PAL_)) and genes encoding antioxidative enzymes (e.g., peroxidase gene, catalase gene) must be ensured. However, especially in the early stages, it is difficult to distinguish between SAR and ISR, both of which activate genes connected with the response to pathogen invasion and increase the accumulation of reactive oxygen species (ROS) and callose. It is known that different pathogenic species belonging to the same genus may trigger different responses in the target plant. Fungi of _Fusarium_ genus led to the development of ISR in early stages of infection, and later on, SAR is activated [25; 26; 27]. It is known that _F. oxysporum_ causes increased production of salicylic acid and methyl-salicylates, which indicates SAR activation in the infected flax. However, no information on _F. culmorum_ infection mechanism in flax is available. Based on our previous study we also established that ABA plays a crucial role in the early moments of infection of flax with _F. oxysporum_. Although the role of ABA in biogenic stress response is still ambiguous there is more and more information on its involvement in plants' response to pathogen infection [28; 29]. Plant resistance to pathogens was reported to be negatively affected by ABA. For instance, ABA suppressed immune responses of tomato infected with _Botrytis cinerea_[30] barley infected by _Magnaporthe oryzae_[31], or tobacco infected by _Ralstonia solanacearum_[28]. On the other hand, ABA increased resistance of _Arabidopsis_ to _Alternaria brassicicola_[32] and was involved in higher resistance of flax to _Fusarium oxysporum_[3]. The contrasting roles of ABA in plant defenses seem to be depending on the infection phase and pathogen lifestyle. Bearing this in mind studying plant responses to infection should be exerted for individual species and infection phases. It has been proposed that in general, the way ABA acts in response to infection depends on the pathogen type.