SignificanceCotton is an important crop, and terpenoids form the largest group of natural products. Gossypol and related sesquiterpene aldehydes in cotton function as phytoalexins against pathogens and pests but pose human health concerns, as cotton oil is still widely used as vegetable oil. We report the isolation and identification of four enzymes and the recharacterization of one previously reported P450. We are now close to the completion of the gossypol pathway, an important progress in agricultural and plant sciences, and the data are beneficial to improving food safety. Among the six compounds (intermediates) isolated following gene silencing, one affected plant disease resistance significantly. Thus, these “hidden natural products” harbor interesting biological activities worthy of exploration.

Cotton is not only the world's most important natural fiber crop, but it is also an ideal system in which to study genome evolution, polyploidization, and cell elongation. With the assembly of five different cotton genomes, a cotton-specific whole-genome duplication with an allopolyploidization process that combined the A- and D-genomes became evident. All existing A-genomes seemed to originate from the A0-genome as a common ancestor, and several transposable element bursts contributed to A-genome size expansion and speciation. The ethylene production pathway is shown to regulate fiber elongation. A tip-biased diffuse growth mode and several regulatory mechanisms, including plant hormones, transcription factors, and epigenetic modifications, are involved in fiber development. Finally, we describe the involvement of the gossypol biosynthetic pathway in the manipulation of herbivorous insects, the role of GoPGF in gland formation, and host-induced gene silencing for pest and disease control. These new genes, modules, and pathways will accelerate the genetic improvement of cotton. Expected final online publication date for the Annual Review of Plant Biology, Volume 72 is May 2021. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.

Terpene synthases (TPSs) are responsible for the extremely diversified and complex structure of terpenoids. Amorpha-4,11-diene synthase (ADS) has a high (90%) fidelity in generating the sesquiterpene precursor for the biosynthesis of artemisinin, an antimalarial drug, however, little is known about how active site residues of ADS are involved in carbocation rearrangement and cyclization reactions. Here, we identify seven residues that are key to most of the catalytic steps in ADS. By structural modeling and amino acid sequence alignments of ADS with two functionally relevant sesquiterpene synthases from , we performed site-directed mutagenesis and found that a single substitution, T296V, impaired the ring closure activity almost completely, and tetra-substitutions (L374Y/L404V/L405I/G439S) led to an enzyme generating 80% monocyclic bisabolyl-type sesquiterpenes, whereas a double mutant (T399L/T447G) showed compromised activity in regioselective deprotonation to yield 34.7 and 37.7% normal and aberrant deprotonation products, respectively. Notably, Thr296, Leu374, Gly439, Thr399, and Thr447, which play a major role in directing catalytic cascades, are located around conserved metal-binding motifs and function through impacting the folding of the substrate/intermediate, implying that residues surrounding the two motifs could be valuable targets for engineering TPS activity. Using this knowledge, we substantially increased amorpha-4,11-diene production in a near-additive manner by engineering Thr399 and Thr447 for product release. Our results provide new insight for the rational design of enzyme activity using synthetic biology.