Table 2.

Examples of molecular studies in model plants and crops under elevated/low CO₂ conditions along with other stress responses.

Stress condition	Crop	Molecular tool used	Findings	References
eCO2 along with Mg or elevated O3	Arabidopsis	Transcriptome/ Small RNA-Seq	1) Altered gene expression of the genes involved in regulating flowering time 2) Delayed flowering at eCO2 is associated with sustained expression of the floral repressor gene, FLOWERING LOCUS C (FLC), in an eCO2‐adapted genotype. 3) Carbon accumulation, defense mechanism redox control, transport, signaling, transport and chromatin remodeling. 4) Alter microRNA expression in Arabidopsis growth and development and miR156/157 and miR172 regulated transcriptional network for early flowering. 5) eCO2 decreased the expression of genes related to cell redox homeostasis, cadmium response and lipid localization, but enhanced signal transduction, protein phosphorylation, NBS-LRR disease resistance proteins and subsequently programmed cell death (FADB, ATFAH2, WAX2, FATTY ACID DESATURASE 2, FATTY ACID DESATURASE 7, CYTIDINEDIPHOSPHATE DIACYLGLYCEROL SYNTHASE 5 and QUIRKY) in low-Mg shoots. 6) eCO2 enhanced the response of lipid localization (mainly LTP transfer protein/protease inhibitor), endomembrane system, heme binding and cell wall modification in high-Mg roots.	[108] [109] [110] [111]
aCO2 of 400 μmol/mol and eCO2 of 700 μmol/mol concentrations + pea aphid interaction	Medicago	Plant iTRAQ proteomic analysis + gene silencing (VIGS)	Susceptible plants: eCO2 (↑) PTI defenses including the MAPK signaling pathway (↑), Ca2+signaling pathways (↑), SA signaling pathway (↑) and JA signaling pathway (↓) Resistant plants: silencing of HSP90 in Jester plants impaired ETI signaling and the JA signaling pathway (↓) and nullified the plant‐mediated negative effects of eCO2 on aphid performance	[112]
eCO2 of 370 μmol/mol and eCO2 of 550 μmol/mol and ozone and Japanese beetles	Soybean	Microarray	Leaf-specific transcripts were greater, comprising of Jasmonic acid defense regulatory mechanism (↑), isoprenoids and flavonoids metabolism (↑) related pathways under eCO2, elevated O3 and eCO2 + elevated O3 than in aCO2, mimicking the scenario of altered atmospheric component in changing climate	[113]
Low CO2 treatment, the stomata were first stabilized in 400 ppm CO2 balanced opening buffer for 15 min and then exposed continuously to 0 ppm CO2 balanced opening buffer for a period of 60 min	Rapeseed	Metabolic profiling	A total of 411 metabolites and 1397 proteins of various pathways are activated at low CO2 affecting guard cell stomatal closure and stomatal opening under high CO2. Diversion of JA biosynthesis to traumatic acid biosynthesis, the role of melatonin and phytohormone crosstalk, redox regulation and the functions of fatty acid metabolism and Ras-related proteins got affected.	[114]
aCO2 of 400 μmol/mol and eCO2 of 3000 μmol/mol concentrations	Carrot	qRT-PCR	The transcript profiles of 12 genes related to AsA biosynthesis and recycling were altered in response to eCO2 genes, included phosphoglucose isomerase (DcPGI), phosphomannose isomerase (DcPMI), GDP-D-manmose pyrophosphorylase (DcGMP), GDP-D-mannose-3′,5′-epimerase (DcGME), GDP-L-galactose phosphorylase (DcGGP), L-galactose-1-P phosphatase (DcGPP), myo-inositol oxygenase (DcMIOX), ascorbate oxidase (DcAO), ascorbic acid peroxidase (DcAPX), monodehydroascorbate reductase (DcMDHAR), dehydroascorbate reductase (DcDHAR) and glutathione reductase (DcGR). A total of six genes (DcPGI, DcPMI, DcGMP, DcGME, DcGGP and DcGPP) were identified in the L-galactose pathway. DcMIOX were involved in the myo-inositol and D-galacturonic acid pathways, respectively.	[115]
eCO2 of 550 μmol/mol in a FACE – 6-year exposure	Populus	cDNA Microarray + qRT-PCR	Pathways for secondary metabolism and glycolysis were significantly up-regulated by eCO2 during senescence, in particular, those related to anthocyanin biosynthesis. Expressed sequence tags (ESTs) representing the two most significantly up-regulated transcripts in eCO2, LDOX (leucoanthocyanidin dioxgenase) and DFR (dihydroflavonol reductase) gave eCO2⁄aCO2 expression ratios of 39.6 and 19.3, respectively.	[116]
eCO2 of 400 μmol/mol, 800 μmol/mol combined with heat stress e.g. at ambient temperature (30/25 °C day/night) and 45/40 °C day/night respectively	Bermuda grass	Metabolic profiling	Important metabolic pathways during which proteins and metabolites were up-regulated, including light reaction and TCA cycle, amino acid metabolism as well as the GABA shunt.	[117]