Table 2. Typical applications of biosensors in lignocellulose valorization.

Applications	Products	Biosensors	Mechanisms	Effectors	Achievements	Ref.
Monitoring	Xylose	XylR	In the absence of xylose, XylR does not bind to the operator xylO, repressing reporter gene expression. When xylose is present, XylR binds xylose, enabling its binding to xylO and activating transcription of reporter.	Xylose	Screening the XylR mutants, the detecting range was increased by nearly 10-fold	33
	Salicylic acid	MarR	MarR protein binds on PmarO to inhibit promoter expression and SA binds with MarR protein to release the repression.	Salicylic acid	Two promoter variants I12AII4T and I12AII14T that exhibited improved responsive strengths and shifted dynamic ranges were obtained	58
	Levulinic acid	LvaR	The LvaR regulator senses LA as a cognate ligand	Levulinic acid	Screening optimal chassis, like P. putida KT2440, the maximal fluorescence increase was 12.3-fold	50
Screening enzymes or genes	Lignin transformation products	EmrR	EmrR is necessary and sufficient for the compound-dependent activation of the emrRAB operon.	Phenol	Identified genes encoding six functional classes mediating lignin transformation phenotypes	49
	Vanillin	MuYqhC	In the presence of an aldehyde, the transcriptional regulator YqhC activates the promoter of the yqhD gene, which drives the expression of a reporter gene (RFP), resulting in red fluorescence.	Vanillin	Identified a COMT variant, Mu176, that displayed a 7-fold increase in the conversion rate	43
Dynamic regulation	Protocatechuic acid	Transcriptional repressor PadR	An auto-regulatory system was established, by coexpressing key rate-limiting enzymes, 4-hydroxybenzoate hydroxylase, vanillate-O-demethylase, and transporter HcnK under the biosensor element	p-Coumaric acid (p-CA)	A titer of 12.7 g L−1 of protocatechuic acid	59
	Vanillic acid	Transcriptional repressor PP3359	Two-layer genetic controller that interconnects two independent genetic sensors to decouple growth and production	Feruloyl-CoA	Reduced metabolic stress by two-fold, while also increasing the growth rate and productivity by two-fold and five-fold, produced vanillic acid at titers up to 880 mg/L	60
	Itaconic acid	Nitrogen starvation-detecting biosensor	Limiting the expression of the apparently toxic CadA protein to the production phase via dynamic regulation	Nitrogen starvation	Maximum yield of 56% (mol mol−1) and titer of 1.3 g L−1 from p-coumarate, and 1.4 g L−1 titer from monomeric aromatic compounds produced from alkali-treated lignin	61
	Catechol	Vanillin-inducible promoter ADH7	This system is composed of a catechol biosynthesis pathway coexpressed with an active aromatic transporter CouP under induction by a vanillin self-inducible promoter, ADH7, to effectively convert the lignin-derived aromatics into value-added chemicals	Vanillin	Improved the catechol yields about 30% and 40% under promoter pTrc and ADH7, respectively.	42
	Polyhydroxybutyrate	PcaQ	Control the functional genes expression in response to heterologous lignin-derived aromatics	Various lignin-derived monomers	Polyhydroxybutyrate titer of 2.38 g L−1 from the alkaline pretreatment liquor	62