4.2 | A combination of cuticular wax and related genes might promote the excellent drought resistance of Pse. libanotica
Cuticular waxes mostly comprise very long chain fatty acids (VLCFAs) and their derivatives, including alkanes, wax esters, branched alkanes, primary alcohols, alkenes, secondary alcohols, aldehydes ketones, and unsaturated fatty alcohols, as well as cyclic compounds including terpenoids and metabolites such as sterols and flavonoids. In this research, 14 genes involved in cuticular wax biosynthesis were identified in leaves of Pse. libanotica during 28 days drought treatment. The Acc was reported to catalyze the first step and increased significantly the overall rate of fatty acid biosynthesis up to C16:0, C16:1 and C18:1 (Baud et al., 2003). The long-chain fatty acids are further hydrolyzed by Fatb, the FATB thoresterase encode a group of enzymes with more heterogeneous substrate specificity, but generally showing high activities towards saturated acyl-ACPs which is the major determinant of the chain length and level of saturated fatty acid found on most plant tissues (Bonaventure et al., 2003). InPse. libanotica , Acc and Fatb , determined thede novo fatty acid biosynthesis, are significantly upregulated during 21 days water deficit which might indicate its excellent drought resistance.
Based on differences in VLCFA length and degree of unsaturation, 21 KCS genes in Arabidopsis showed substrate specificity and organ- or tissue-specific expression patterns during fatty acid elongation (Joubès et al., 2008; Zhao et al., 2019). It is proved that the Kcs1 is the important gene and has broad substrate specificity for saturated and mono-unsaturated C16–C24 acyl CoAs formation (Trenkamp et al., 2004). The Kcs20 and Kcs2 /DAISY are involved in the two-carbon elongation to C22 VLCFA, and required for cuticular wax and root suberin biosynthesis and expressed in stem epidermal peels (Lee et al., 2009). The Kcs6 is essential and crucial for the production of epicuticular and pollen coat lipids that are longer than C28 (Fiebig et al., 2000; Haslam and Kunst, 2021). In this study, two Kcs5transcripts which is essential for C26–C30 formation were upregulated. However, genes responsible for the C18 to C26 is missing. Theevm.TU.CTG175.54 displayed less DNA and protein sequences similarity with published KCS gene families. Thus, we speculate thatevm.TU.CTG175.54 might be a new gene responsible for C18 to C26 biosynthesis during VLCFA elongation in Pse. libanotica .
The fatty acyl-CoA reductase (FAR) gene family is expressed in the epidermis of aerial tissues and in roots, which is responsible for the primary alcohol formatting pathway. In Pse. libanotica , theFar1 was downregulated, while the Far4 was upregulated. It has been reported that Far4 catalyzes the reduction of VLC-acyl-CoAs to primary alcohols, presumably via an unreleased aldehyde intermediate which was produced through alkane forming pathway (Kosma et al., 2012). The Cer1 , Cer3 , Cyp96a15 /Mah1related to alkane forming pathway were significantly upregulated inPse. libanotica . Cer1 interacts with the wax-associated protein Cer3 and endoplasmic reticulum–localized Cytb5sresulted in VLC alkane synthesis (Bernard et al., 2012). The alkane was further hydrolyzed by Cyp96a15 /Mah1 , which leading from alkanes to secondary alcohols and corresponding ketones (Greer et al, 2007). To sum up, we assume that the aldehydes, alkanes, secondary alcohol and ketones accumulated in the alkane forming pathway are the main cuticular wax component in Pse. libanotica leaves, thereby increasing wax deposition and restricting water loss.