Regulation of Glandular Size and Phytoalexin Biosynthesis by a Negative Feedback Loop in Cotton.

Plants have evolved diverse defense mechanisms encompassing physical and chemical barriers. Cotton pigment glands are known for containing various defense metabolites, but the precise regulation of gland size to modulate defense compound levels remains enigmatic. Here, it is discovered that the VQ domain-containing protein JAVL negatively regulates pigment gland size and the biosynthesis of defense compounds, while the MYC2-like transcription factor GoPGF has the opposite effect. Notably, GoPGF directly activates the expression of JAVL, whereas JAVL suppresses GoPGF transcription, establishing a negative feedback loop that maintains the expression homeostasis between GoPGF and JAVL. Furthermore, it is observed that JAVL negatively regulates jasmonate levels by inhibiting the expression of jasmonate biosynthetic genes and interacting with GoPGF to attenuate its activation effects, thereby maintaining homeostatic regulation of jasmonate levels. The increased expression ratio of GoPGF to JAVL leads to enlarged pigment glands and elevated jasmonates and defense compounds, enhancing insect and pathogen resistance in cotton. These findings unveil a new mechanism for regulating gland size and secondary metabolites biosynthesis, providing innovative strategies for strengthening plant defense.

Single-cell RNA sequencing reveals a hierarchical transcriptional regulatory network of terpenoid biosynthesis in cotton secretory glandular cells.

Plants can synthesize a wide range of terpenoids in response to various environmental cues. However, the specific regulatory mechanisms governing terpenoid biosynthesis at the cellular level remain largely elusive. In this study, we employed single-cell RNA sequencing to comprehensively characterize the transcriptome profile of cotton leaves and established a hierarchical transcriptional network regulating cell-specific terpenoid production. We observed substantial expression levels of genes associated with the biosynthesis of both volatile terpenes (such as ß-caryophyllene and ß-myrcene) and non-volatile gossypol-type terpenoids in secretory glandular cells. Moreover, two novel transcription factors, namely GoHSFA4a and GoNAC42, are identified to function downstream of the Gossypium PIGMENT GLAND FORMATION genes. Both transcription factors could directly regulate the expression of terpenoid biosynthetic genes in secretory glandular cells in response to developmental and environmental stimuli. For convenient retrieval of the single-cell RNA sequencing data generated in this study, we developed a user-friendly web server . Our findings not only offer valuable insights into the precise regulation of terpenoid biosynthesis genes in cotton leaves but also provide potential targets for cotton breeding endeavors.

Dirigent gene editing of gossypol enantiomers for toxicity-depleted cotton seeds.

Axial chirality of biaryls can generate varied bioactivities. Gossypol is a binaphthyl compound made by cotton plants. Of its two axially chiral isomers, (-)-gossypol is the bioactive form in mammals and has antispermatogenic activity, and its accumulation in cotton seeds poses health concerns. Here we identified two extracellular dirigent proteins (DIRs) from Gossypium hirsutum, GhDIR5 and GhDIR6, which impart the hemigossypol oxidative coupling into (-)- and (+)-gossypol, respectively. To reduce cotton seed toxicity, we disrupted GhDIR5 by genome editing, which eliminated (-)-gossypol but had no effects on other phytoalexins, including (+)-gossypol, that provide pest resistance. Reciprocal mutagenesis identified three residues responsible for enantioselectivity. The (-)-gossypol-forming DIRs emerged later than their enantiocomplementary counterparts, from tandem gene duplications that occurred shortly after the cotton genus diverged. Our study offers insight into how plants control enantiomeric ratios and how to selectively modify the chemical spectra of cotton plants and thereby improve crop quality.

RES transformation for biosynthesis and detoxification.

The reactive electrophilic species (RES), typically the molecules bearing α,ß-unsaturated carbonyl group, are widespread in living organisms and notoriously known for their damaging effects. Many of the mycotoxins released from phytopathogenic fungi are RES and their contamination to cereals threatens food safety worldwide. However, due to their high reactivity, RES are also used by host organisms to synthesize specific metabolites. The evolutionary conserved glyoxalase (GLX) system scavenges the cytotoxic α-oxoaldehydes that bear RES groups, which cause host disorders and diseases. In cotton, a specialized enzyme derived from glyoxalase I (GLXI) through gene duplications and named as specialized GLXI (SPG), acts as a distinct type of aromatase in the gossypol pathway to transform the RES intermediates into the phenolic products. In this review, we briefly introduce the research progress in understanding the RES, especially the RES-type mycotoxins, the GLX system and SPG, and discuss their application potential in detoxification and synthetic biology.

Aromatization of natural products by a specialized detoxification enzyme.

In plants, lineage-specific metabolites can be created by activities derived from the catalytic promiscuity of ancestral proteins, although examples of recruiting detoxification systems to biosynthetic pathways are scarce. The ubiquitous glyoxalase (GLX) system scavenges the cytotoxic methylglyoxal, in which GLXI isomerizes the α-hydroxy carbonyl in the methylglyoxal-glutathione adduct for subsequent hydrolysis. We show that GLXIs across kingdoms are more promiscuous than recognized previously and can act as aromatases without cofactors. In cotton, a specialized GLXI variant, SPG, has lost its GSH-binding sites and organelle-targeting signal, and evolved to aromatize cyclic sesquiterpenes bearing α-hydroxyketones to synthesize defense compounds in the cytosol. Notably, SPG is able to transform acetylated deoxynivalenol, the prevalent mycotoxin contaminating cereals and foods. We propose that detoxification enzymes are a valuable source of new catalytic functions and SPG, a standalone enzyme catalyzing complex reactions, has potential for toxin degradation, crop engineering and design of novel aromatics.