# Abscisic Acid--Defensive Player in Flax Response to _Fusarium culmorum_ Infection
## 3 Results
### Analysis of Transcript Levels of Genes Involved in Isoprenoid Biosynthesis  
Two general routes are involved in the biosynthesis of isoprenoids in plants. The mevalonate pathway (MP) is carried out in the cytosol, and the non-mevalonate pathway (NMP) in plastids. Generally, the MP is responsible for phytosterol biosynthesis, whereas the NMP is greatly branched giving rise to a plethora of different compounds, with chloroplasts, tropcherols, carotenoids, and some phytohormones (GA, ABA) among them. Both of the pathways start with the synthesis of two isomers, isopentenyl pyrophosphate (IPP), and dimethylallyl pyrophosphate (DMAPP), though different enzymatic machineries are utilized. We measured the transcript levels of the genes (called here the terpenoid backbone synthesis genes) involved in the biosynthesis of those isomers (Figure 3A). In contrast to deoxy-d-xylulose 5-phosphate synthase (\(DXS\)) gene expression, which was decreased in all time-points measured (even to 0.57-fold and 0.51-fold of the control at 24 hpi and 36 hpi, respectively), the transcript level of deoxy-d-xylulose 5-phosphate reductase (\(DXR\)) gene was increased in all time-points after infection. The activation of this gene reached 2.67- and 2.5-fold of the control at 12 hpi and 24 hpi, respectively, and maximally 3.23-fold of the control at 48 hpi. Similarly, the expression levels of isopentenyl pyrophosphate isomerase (_IDI_) and geranyl pyrophosphate synthase (_GPPS_) genes were increased. The activation of _IDI_ gene increased in each time-point, whereas the maximum expression (ca. 2.5-fold of the control) of _GPPS_ gene was measured at 24 hpi and 36 hpi. In the cytoplasmic route the expressions of 3-hydroxy-3-methylglutaryl-CoA synthase and reductase genes (_HMGS_ and _HMGR_, respectively) were not changed significantly relative to the control (0.75-fold of the control for _HMGS_ at 36 hpi). The expression of mevalonate kinase (\(MVK\)) was increased to 1.75-fold of the control at 36 hpi. The transcript level of farnesyl pyrophosphate synthase (_FPPS_) was decreased in all time-points with minimum expression at 0.23-fold of the control at 36 hpi. Genes of the cytoplasmic route of isoprenoid biosynthesis play a role in the production of phytosterols and brassinosteroids. After measuring the transcript levels of genes involved in these compounds' biosynthesis, we observed a general inactivation of this pathway (Figure 3B). The minimal expression was observed at 24 hpi and 36 hpi (except sterol methyltransferase 2 (\(SMT2\)) gene, for which the expression was 1.5-fold of the control at 24 hpi). In contrast, activation of _CYP710A_ gene encoding a cytochrome p450 protein responsible for desaturation of the double bond at the C-22 position in the side chain was observed in all time-points, with a maximum (9.1-fold of the control) at 48 hpi. Measuring the expression of genes involved intocopherol biosynthesis we noted a consequent activation of this branch of the pathway except geranyl-geranyl diphosphate reductase (\(GGR\)) gene, which, excluding the 6 hpi, was decreased in all remaining time-points (to 0.17-fold of the control at 36 hpi). Although the transcript levels of homogentisate phytyl transferase (\(VTE2\)), MPBQ methyltransferase(\(VTE3\)), tocopherol cyclase (\(VTE1\)) and \(\gamma\)-tocopherol methyltransferase (\(VTE4\)) were virtually all increased at an early time- points at 36 hpi all of them showed a sharp dropdown with minimal value of 0.26-fold of the control for \(VTE3\) (Figure 3C).