Date Published: February 22, 2019
Publisher: Public Library of Science
Author(s): Kamalraj Subban, Ramesh Subramani, Vishnu Priya Madambakkam Srinivasan, Muthumary Johnpaul, Jayabaskaran Chelliah, Vijai Gupta.
Salicylic acid (SA) is an effective elicitor to increase taxol production in Pestalotiopsis microspora. Addition of SA at the concentration of 300 μM yielded taxol 625.47 μg L-1, 45- fold higher than that of the control. Elicitation of the role of SA in the fungal taxol biosynthetic pathway revealed that SA enhanced reactive oxygen species and lipid peroxidation of unsaturated fatty acids of P. microspora mycelia. This oxidative process stimulates isoprene biosynthetic pathway by triggering expression of the geranylgeranyl pyrophosphate synthase gene leading to improved biosynthesis of taxol in P. microspora.
Paclitaxel (taxol) is a potent anticancer drug, with a unique mechanism of action . It is isolated from Taxus plant species and diverse endophytic fungi . Taxus plant species are unable to meet the growing pharmaceutical demand, as the growth of Taxus species is relatively slow and their propagation is unsuccessful after prolonged seed dormancy. Hence, several alternative methods have been attempted to obtain taxol such as total chemical synthesis [3,4], semi-synthesis  and plant tissue cell culture . These methods have proved inefficient due to various causes including large number of reaction steps, complex extraction methods, expensive procedures, long incubation period, low biomass, meagre yield and genetic instability. For these reasons, microbial sources of taxol production present an attractive alternative because they are simple, productive and inexpensive . Endophytic fungi produce relatively lower quantities of taxol compared to plants, however yields can be improved by employing biotechnology techniques of high cell density cultivation and biomass parameter optimization and scaling up process . On the other hand, biosynthesis of taxol in endophytic fungi has been rarely reported . An endophytic fungus Stemphylium sedicola SBU-16 showed the gene of taxadiene synthase (TXS) and produced taxol and its intermediate 10-deacetylbaccatin (10-DAB) . Some endophytic fungi possess taxol biosynthetic genes such as geranylgeranyl pyrophosphate synthase (GGPPS), taxadiene synthase (TXS), 10-deacetylbaccatin III-10-β-O-acetyltransferase (DBAT) and C-13 phenylpropanoid side chain-CoA acetyl transferase (BAPT), known to be present in plants . The fungal taxol biosynthetic genes such as 3-hydroxyl-3-methylglutaryl-CoA (HMG CoA) reductase, taxane 5-alpha hydroxylase (T5αH), taxane 13-alpha-hydroxylase (T13αH) and taxane 2α-O-benzoyltransferase (TBT) were recognized in Cladosporium cladosporioides MD2 by transcriptome analysis . The genes encoding GGPP synthase is of particular interest because it codes for the branch point prenyltransferase, which is involved in the formation of diterpenoid moiety, a precursor for taxol biosynthesis .
The highest yield of taxol from microbial origin is reported in this study. Furthermore, this study reports for the first time on the role of SA on the biosynthetic pathway of taxol biosynthesis in fungi. The supplementation of SA increased antioxidative activities of the CAT, SOD and PX enzymes and decreased the lipid content in the culture of P. microspora. Production of peroxides in P. microspora stimulates oxidative stress that induce regulatory proteins to activate HMGR protein. The HMGR protein cascade eventually triggers GGPPS for enhanced taxol biosynthesis in P. microspora.