## 5 Comparison and Prospects of Plant and Microbial Chassis
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**Host** & **Advantages** & \multicolumn{2}{c}{**Disadvantages**} \\ \hline \multirow{8}{*}{_Plant_} & & & Complex endogenous metabolic pathways that interfere with the target pathway; \\  & & The natural host has prepared cofactors & & Strict metabolic regulation, including potential unknown regulators; \\  & & Organelles and higher-order membrane & & Transgene expressions are prone to silencing; \\  & & structures allow for the storage of intermediates & & Limited gene expression elements and unclear driving force of promoters; \\  & & and products; & & Slow growth rates; \\  & & Rich redox partners that are better matched with & & Demanding fermentation conditions and bioreactors and challenging to achieve high-density fermentation. \\  & & & & \\  & & & & \\  & & & & \\  & & Relatively clear endogenous metabolic pathways & & Lack of advanced membrane structures and organelles; \\  & & & & \\  & & & & \\  & & & & \\  & & & & \\  & & & & \\  & & & & \\ \end{tabular} \begin{tabular}{c} \(\bullet\) \\ \(\bullet\) \\ \(\bullet\) \\ \(\bullet\) \\ \(\bullet\) \\ \(\bullet\) \\ \(\bullet\) \\ \(\bullet\) \\ \(\bullet\) \\ \(\bullet\) \\ \(\bullet\) \\ \(\bullet\) \\ \(\bullet\) \\ \(\bullet\) \\ \(\bullet\) \\ \(\bullet\) \\ \(\bullet\) \\ \end{tabular}  \ \begin{tabular}{c} \(\bullet\) \\ \(\bullet\) \\ \(\bullet\) \\ \(\bullet\) \\ \(\bullet\) \\ \(\bullet\) \\ \(\bullet\) \\ \(\bullet\) \\ \(\bullet\) \\ \(\bullet\) \\ \(\bullet\) \\ \(\bullet\) \\ \(\bullet\) \\ \(\bullet\) \\ \(\bullet\) \\ \(\bullet\) \\ \(\bullet\) \\ \(\bullet\) \\ \(\bullet\) \\ \(\bullet\) \\ \end{tabular} \begin{tabular}{c} \(\bullet\) \\ \(\bullet\) \\ \(\bullet\) \\ \(\bullet\) \\ \(\bullet\) \\ \(\bullet\) \\ \(\bullet\) \\ \(\bullet\) \\ \(\bullet\) \\ \(\bullet\) \\ \(\bullet\) \\ \(\bullet\) \\ \(\bullet\) \\ \(\bullet\) \\ \(\bullet\) \\ \(\bullet\) \\ \(\bullet\) \\ \(\bullet\) \\ \(\bullet\) \\ \(\bullet\) \\ \(\bullet\) \\ \end{tabular}  \begin{  
Although microbial chassis have the advantages and achievements mentioned above, they also face their own challenges. Since endogenous metabolic pathways are interconnected, optimal regulation strategies cannot be found without globally controlling the distribution of metabolic flux. For example, in the synthesis of troponin alkaloids using yeast, it was surprisingly found that removing leucine auxotrophy by expressing 3-isopropylmalate dehydrogenase significantly increased the conversion of hyoscyamine (85% decrease) to scopolamine (more than 3-fold increase) [229]. Current analysis of global metabolic flux can only be performed through somewhat unsatisfactory model construction and simulation [230], or more promisingly, using artificial intelligence [231; 232]. However, they all rely on repeatable and reliable datasets, while standardized strains and operations are difficult to unify in the academic community. As mentioned above, transport engineering of secondary metabolites is a highly promising strategy. However, we currently have many gaps in the understanding of the functions and activity regulation mechanisms of plant secondary metabolite transporters and even microbial endogenous transporters. Therefore, this strategy has not yet reached its maximum potential. In summary, in the current research progress, both microbial and plant platforms have seen successful cases. Due to the clearer metabolic pathways of microbes and the ease of genetic transformation and scale-up cultivation, they have achieved a series of remarkable accomplishments, especially eukaryotic yeasts, in the synthesis of various structurally uncomplicated compounds. In the future, if microbial platforms can successfully address global metabolic regulation, maintenance of heterologous enzyme activity, and issues  
Figure 2: Schematic graph for strategies applied in plant secondary metabolites production in plant and microbial chassis. Substrate **a** undergoes a series of enzyme-catalyzed reactions to ultimately yield product **j** through intermediates **b**–**i**.