# Abscisic Acid--Defensive Player in Flax Response to _Fusarium culmorum_ Infection
## 4 Discussion  
There are numerous pieces of data in the literature on the participation of PR proteins, salicylic acid and jasmonic acid, as well as antioxidative compounds in plant resistance to fungal infection [36; 37; 38]. The role of plant hormones in responses to environmental stresses has been proven, and the activation of the synthesis of these compounds has been well researched [39; 40]. However, some of them are better researched than others and in some cases such as ABA their role in abiotic stress is much better understood than biotic interaction. For years abiotic and biotic stress signaling were usually researched separately even though plants have to deal simultaneously with several stresses of differing nature at once [41]. It is clear that we are still far from the full picture of the mechanisms involved in plant response to stress conditions. In particular, the role of abscisic acid in plant-pathogen interactions remains unclear with contradictory evidence presented in the literature. However, recently ABA is being recognized as an important molecule modulating immune response [42]. Previously, the majority of work was carried out in  
Figure 6: Amounts of callose measured in 14 days old flax seedlings infected with _F. culmorum_ at 6 hpi, 12 hpi, 24 hpi, 36 hpi and 48 hpi. C—untreated control, Fc—_Fusarium culmorum_ treated. Results are presented as means of three biological repeats \(\pm\) sd. Statistically significant changes are marked with asterisks. the _Arabidopsis_ model system, and it becomes evident that other species may respond differently and plant-pathogen interactions need to be studied on case-by-case basis [43]. In our former study, we evidenced ABA involvement in flax resistance to _Fusarium oxysporum_ at early stages of infection [3]. It was manifested among others by elevated levels of _PMR4_ gene expression accompanied with higher levels of callose synthesized at 36 hpi and 48 hpi. In this study we show that the response of flax to _Fusarium culmorum_ infection exhibits some similarities with those studied previously. Most importantly, we observed increase in ABA levels in flax seedlings infected with _F. culmorum_ correlating well with transcript levels of ABA synthesis genes. It is estimated that at least 10% of protein-coding genes have ABA-responsive elements in their promoters [44], and a lot of induced genes overlap with those involved in pathogen response [45]. We also observed a similar profile of the expression of the terpenoid biosynthesis genes compared with that noted for _F. oxysporum_ infection. Activation of the core genes (_DXR, IDI, GPPS, HMGR, MVK_), mutual for both the cytoplasmic and plastidial parts of the pathway was observed, which was consistent with the data obtained for _F. oxysporum_. Similarly, phytosterol synthesis was inhibited, which translated to the activation of the plastidial branch of this pathway, from the earliest time-point studied (6 hpi). It may be connected to the need to protect the photosystems from free radicals produced after the first contact of the plant with the pathogen in so- called oxidative burst. In contrast to _F. oxysporum_ infection, where there were practically no changes in the terpenoid levels, we measured some increase in the carotenoids, especially at 12 hpi, but also at 24 hpi and 36 hpi. No direct correlation with the gene transcript levels was found. However, it may be due to the fact that activation of the key genes of this branch (_PSY, PDS, Z-ISO, ZCD_ and _CRTISO_) is critical, whereas others (_LCYB, LCYE_) may be inhibited as the pool of corresponding enzymes is sufficient to produce carotenoids [46]. Complex organisms, including plants, react to microorganism infection with rapid production of reactive oxygen species (ROS). ROS play a central role in the defense process of plants. The major source of cellular ROS are NADPH oxidases (NOX), which use NADPH as an electron donor in O\({}_{2}\) molecule reduction. NOXs were shown to play an essential role in plants' immunity.