Summary: The growth and yield of many crops, including cotton, are affected by water deficit. Cotton has evolved drought specific as well as general morpho‐physiological, biochemical and molecular responses to drought stress, which are discussed in this review. The key physiological responses against drought stress in cotton, including stomata closing, root development, cellular adaptations, photosynthesis, abscisic acid (ABA) and jasmonic acid (JA) production and reactive oxygen species (ROS) scavenging, have been identified by researchers. Drought stress induces the expression of stress‐related transcription factors and genes, such as ROS scavenging, ABA or mitogen‐activated protein kinases (MAPK) signalling genes, which activate various drought‐related pathways to induce tolerance in the plant. It is crucial to elucidate and induce drought‐tolerant traits via quantitative trait loci (QTL) analysis, transgenic approaches and exogenous application of substances. The current review article highlights the natural as well as engineered drought tolerance strategies in cotton.

The cambium and procambium generate the majority of biomass in vascular plants. These meristems constitute a bifacial stem cell population from which xylem and phloem are specified on opposing sides by positional signals. The PHLOEM INTERCALATED WITH XYLEM (PXY) receptor kinase promotes vascular cell division and organization. However, how these functions are specified and integrated is unknown. Here, we mapped a putative PXY-mediated transcriptional regulatory network comprising 690 transcription factor-promoter interactions in Arabidopsis (Arabidopsis thaliana). Among these interactions was a feedforward loop containing transcription factors WUSCHEL HOMEOBOX RELATED14 (WOX14) and TARGET OF MONOPTEROS6 (TMO6), each of which regulates the expression of the gene encoding a third transcription factor, LATERAL ORGAN BOUNDARIES DOMAIN4 (LBD4). PXY signaling in turn regulates the WOX14, TMO6, and LBD4 feedforward loop to control vascular proliferation. Genetic interaction between LBD4 and PXY suggests that LBD4 marks the phloem-procambium boundary, thus defining the shape of the vascular bundle. These data collectively support a mechanism that influences the recruitment of cells into the phloem lineage, and they define the role of PXY signaling in this context in determining the arrangement of vascular tissue.

Transcription factors operate as important switches of transcription networks, and NAC (NAM, ATAF, and CUC) transcription factors are a plant-specific family involved in multiple biological processes. However, this gene family has not been systematically characterized in cotton. Here we identify a large number of genes with conservative NAC domains in four cotton species, with 147 found in Gossypium arboreum, 149 in G. raimondii, 267 in G. barbadense and 283 in G. hirsutum. Predicted membrane-bound NAC genes were also identified. Phylogenetic analysis showed that cotton NAC proteins clustered into seven subfamilies and homologous protein pairs showed similar characteristics. Evolutionary property analysis revealed that purifying selection of NAC genes occurred between diploid and polyploid cotton species, and variation analysis showed GhNAC genes may have been subjected to selection and domestication. NAC proteins showed extensive transactivation and this was dependent on the C-terminus. Some development and stress related cis-elements were enriched in the promoters of GhNAC genes. Comprehensive expression analysis indicated that 38 GhNAC genes were candidates for involvement in fiber development, and 120 in stress responses. Gene co-expression network analysis revealed relationships between fiber-associated NAC genes and secondary cell wall (SCW) biosynthesis genes. NAC genes were identified in diploid and tetraploid cotton, revealing new insights into their evolution, variation and homology relationships. Transcriptome analysis and co-expression network indicated roles for GhNAC genes in cotton fiber development and stress response, and NAC genes may prove useful in molecular breeding programmes.