## 5 Comparison and Prospects of Plant and Microbial Chassis  
Assuming a standard of g/L as a high-yield chassis, a systematic analysis of cases for the production of plant-derived bioactive compounds in both plant and microbial chassis (Tables 1 and 2) reveals the following:  
1. In instances wherein plant chassis have succeeded and microbial chassis have not, they include the production of the diterpene paclitaxel, polyphenols such as anthocyanin, rosmarinic acid, and the isoquinoline alkaloid berberine. 2. Microbial chassis have succeeded while plant chassis achieve lower yields for various monoterpenes, the sesquiterpene srtemisinic acid, the simple diterpene sclareol, and diterpene precursors like taxadiene and miltiradiene, the simple triterpene amyrin, linear tetraterpenes lycopene and \(\beta\)-carotene with little downstream tailoring enzyme modifications, polyphenic upstream products like quercetin and (+)-catechin, and the isoquinoline alkaloid precursor (S)-reticuline. * Triterpenes like ginseng and polyphenolic compounds such as resveratrol can be produced at high yields in both plant and microbial chassis. It is evident that microbial chassis are more efficient at producing structurally simple compounds or intermediates, while they face challenges in achieving high yields for complex bioactive compounds due to issues like interference with endogenous metabolism and low enzyme activity. For instance, even though the crucial isoquinoline alkaloid precursor (S)-reticuline can be produced at high yields in yeast (4.6 g/L) [203], it can only yield nM levels of downstream product morphine, even using (R,S)-norlaudanosoline as an externally added substrate [228]. Similarly, terpenoid indole alkaloids with complex synthesis pathways, such as vinblastine, face issues like promiscuous reactions, interactions between hosts and foreign substances, as well as challenges in expressing active enzymes. Plant chassis, especially those using natural hosts, excel in overcoming these challenges, often requiring simple fermentation condition optimizations to achieve high yields for certain bioactive components. However, due to the strict metabolic regulation and complex feedback mechanisms of plants, they may struggle to achieve high yields for even structurally simple products (Table 3). From a strategic perspective, both plant and microbial platforms employ similar approaches, involving pathway construction or enhancement, overexpression of key enzyme genes, reduction in or elimination of competitive metabolic flux, enzyme engineering, and transport engineering (Figure 2). However, plants have complex, mostly unexplored metabolic pathways, lower genetic transformation efficiency, and a limited promoter library, making them less successful in various strategy aspects compared to microbes, such as dynamic control and precise expression regulation. Also, strategies for eliminating bottleneck steps have shown more excellent results in microbial platforms. Specifically, if the product yield significantly increases upon the addition of a specific intermediate, it indicates that the enzyme catalyzing the synthesis of that intermediate may have insufficient activity or the intermediate is being used by the endogenous pathway [144]. If no intermediates are available for use, one can consider using LC-MS to measure the abundance of metabolites throughout the entire pathway [203]. \begin{table}