Two rhamnosyltransferases for <i>de novo</i> biosynthesis of polyphyllins in <i>Nicotiana benthamiana</i>

Polyphyllins, a prominent class of steroidal saponins in Paris species, owe their diverse bioactivities to their sugar unit configurations, though their glycosylation pathways remain poorly understood. Here, we identified and characterized two UDP-rhamnosyltransferases, PpUGT73YD1 and PpUGT738A2, using heterologous expression systems. These enzymes sequentially catalyze the conversion of polyphyllin V and VI into trisaccharide and tetrasaccharide derivatives, respectively. While PpUGT73YD1 accommodates both spiro and furo saponins, PpUGT738A2 specifically recognizes spiro saponins. Both enzymes exhibit strict specificity for UDP-l-Rha as a sugar donor. Structural modeling and site-directed mutagenesis of PpUGT73YD1 revealed that mutations at T149M and L283A shifted sugar donor preference toward UDP-d-Glc and UDP-d-Xyl. Furthermore, co-expression of PpUGT genes with upstream biosynthetic genes in Nicotiana benthamiana enabled de novo synthesis of polyphyllins III and II, achieving yields of 93.64 and 68.39 μg g^-1 dry weight in leaves, respectively. This study elucidates the roles of two key rhamnosyltransferases in polyphyllin biosynthesis and demonstrates their involvement in steroidal saponin production through an engineered plant chassis.