Caffeic acid O-methyltransferases (COMTs) play an essential role in lignin synthesis procession, especially in the plant’s phenylalanine metabolic pathway. The content of COMT genes in cotton and the relationship between their expression patterns have not been studied clearly in cotton. In this study, we have identified 190 COMT genes in cotton, which were classified into three groups (I, II and III), and mapped on the cotton chromosomes. In addition, we found that 135 of the 190 COMT genes result from dispersed duplication (DSD) and whole-genome duplication (WGD), indicating that DSD and WGD were the main forces driving COMT gene expansion. The Ka/Ks analysis showed that GhCOMT43 and GhCOMT41 evolved from GaCOMT27 and GrCOMT14 through positive selection. The results of qRT-PCR showed that GhCOMT13, GhCOMT28, GhCOMT39 and GhCOMT55 were related to lignin content during the cotton fiber development. GhCOMT28, GhCOMT39, GhCOMT55, GhCOMT56 and GhCOMT57 responded to Verticillium Wilt (VW) and maybe related to VW resistance through lignin synthesis. Conclusively, this study found that GhCOMTs were highly expressed in the secondary wall thickening stage and VW. These results provide a clue for studying the functions of GhCOMTs in the development of cotton fiber and VW resistance and could lay a foundation for breeding cotton cultivates with higher quantity and high resistance to VW.

Cotton is a non-edible fiber crop with considerable potential for the remediation of copper-polluted soil. However, the Cu toxicity tolerance mechanism in cotton remains largely obscure. To address the issue, we first identified two cotton lines contrasting in response to Cu toxicity by examining 12 morphological and physiological attributes of 43 origin scattered cotton genotypes under Cu excess. Then both lines were subjected to a comprehensive comparative study, aiming to unravel the cotton Cu tolerance mechanism through integrated morphological, physio-biochemical, Cu uptake and distribution, and related molecular expression analyses. Based on the phenotypic values and corresponding tolerance indexes of 12 parameters, A2304 and A1415 were identified as Cu-tolerant and -sensitive, respectively. Compared to A1415, A2304 exhibited significantly higher antioxidant enzyme activities and non-enzymatic antioxidant levels, producing fewer amounts of reactive oxygen species and a lower level of malonyldialdehyde. On Cu excess, A2304 accumulated lower concentrations of Cu ions in various plant parts and subcellular components, and fewer Cu ions were presented in active chemical forms. However, the total Cu uptake amount per plant did not differ between both lines due to larger plant biomass with A2304. In contrast to A1415, Cu stress activated or increased the expressions of Cu homeostasis regulator (GhSPL7) and genes responsible for Cu delivery (GhCCS, GhCOX17), chelation (GhMT2), and compartmentation into vacuoles (GhHMA5), while inactivating or decreasing the expressions of genes accounting for Cu uptake (GhCOPT1) and Cu exporting from vacuoles (GhCOPT5) in the root cell with A2304. Additionally, A2304 may impede the root cell wall from binding Cu ions by enhancing the pectin methylesterification degree by up-regulating GhPMEI3 and GhPMEI9 encoding pectin methylesterase inhibitor and stabilizing the cell wall organization by down-regulating GhPLY8 and GhPLY20 encoding pectate lyases. To cope with Cu toxicity, the Cu-tolerant genotype activates its antioxidative defense system, immobilizing chemically active Cu ions, and lowering the Cu uptake, bioavailability and immigration within cells by regulating the expressions of genes related to Cu uptake, transport, delivery and cell wall metabolism. This comprehensive comparison study provides insights into breeding Cu-tolerant cotton cultivars that can be utilized for the phytoremediation of Cu-contaminated soils.

Short fruiting branch cotton (SFBC) has a compact plant architecture suitable for dense planting. Plant population density (PPD) and topping are important agronomic practices to achieve high yielding by optimizing cotton plant structure. However, their individual and interactive effects on SFBC growth and yield are poorly understood. This study aimed to explore cotton growth and yield responses to various combinations of PPD and topping time (TT) and the underlying physio-ecological mechanism. Four combinations were included in a two-year field experiment (2023–2024) involving two PPD levels (5.3 plants m⁻², low density LD; 8 plants m⁻², high density HD) and two TT levels (early topping for leaving ten sympodials per plant ET; late topping for leaving fifteen sympodials per plant LT), and compared in terms of biomass accumulation, photosynthetically active radiation capture, and leaf senescence during entire reproductive growth period. Compared to the other three combinations, the combination of HD and LT (HDLT) achieved a higher lint yield due to a greater biological yield, which was predominantly attributed to the higher average rate during the rapid biomass increasing period. Owing to delayed leaf senescence caused by the HD and the LT, the HDLT performed better in leaf senescence-related attributes at the late growth stage. Moreover, these improved attributes also contributed to a higher radiation interception rate and photosynthetic efficiency at the late growth stage. Taken together, combining high density with later topping tends to increase the lint yield of SFBC through increasing dry matter accumulation, delaying leaf senescence, and enhancing canopy radiation interception rate at the late growth.