Initial evaluation of sugarcane as a production platform for p-hydroxybenzoic acid

Sugarcane (Saccharum hybrids) was evaluated as a production platform for p-hydroxybenzoic acid using two different bacterial proteins (a chloroplast-targeted version of Escherichia coli chorismate pyruvate-lyase and 4-hydroxycinnamoyl-CoA hydratase/lyase from Pseudomonas fluorescens) that both provide a one-enzyme pathway from a naturally occurring plant intermediate. The substrates for these enzymes are chorismate (a shikimate pathway intermediate that is synthesized in plastids) and 4-hydroxycinnamoyl-CoA (a cytosolic phenylpropanoid intermediate). Although both proteins have previously been shown to elevate p-hydroxybenzoic acid levels in plants, they have never been evaluated concurrently in the same laboratory. Nor are there any reports on their efficacy in stem tissue. After surveying two large populations of transgenic plants, it was concluded that the hydratase/lyase is the superior catalyst for leaf and stem tissue, and further studies focused on this pathway. p-Hydroxybenzoic acid was quantitatively converted to glucose conjugates by endogenous uridine diphosphate (UDP)-glucosyltransferases and presumably stored in the vacuole. The largest amounts detected in leaf and stem tissue were 7.3% and 1.5% dry weight (DW), respectively, yet there were no discernible phenotypic abnormalities. However, as a result of diverting carbon away from the phenylpropanoid pathway, there was a severe reduction in leaf chlorogenic acid, subtle changes in lignin composition, as revealed by phloroglucinol staining, and an apparent compensatory up-regulation of phenylalanine ammonia-lyase. Although product accumulation in the leaves at the highest level of gene expression obtained in the present study was clearly substrate-limited, additional experiments are necessary before this conclusion can be extended to the stalk.