Transcriptome Analysis Unravels Key Factors Involved in Response to Potassium Deficiency and Feedback Regulation of K(+) Uptake in Cotton Roots

To properly understand cotton responses to potassium (K(+)) deficiency and how its shoot feedback regulates K(+) uptake and root growth, we analyzed the changes in root transcriptome induced by low K(+) (0.03 mM K(+), lasting three days) in self-grafts of a K(+) inefficient cotton variety (CCRI41/CCRI41, scion/rootstock) and its reciprocal grafts with a K(+) efficient variety (SCRC22/CCRI41). Compared with CCRI41/CCRI41, the SCRC22 scion enhanced the K(+) uptake and root growth of CCRI41 rootstock. A total of 1968 and 2539 differently expressed genes (DEGs) were identified in the roots of CCRI41/CCRI41 and SCRC22/CCRI41 in response to K(+) deficiency, respectively. The overlapped and similarly (both up- or both down-) regulated DEGs in the two grafts were considered the basic response to K(+) deficiency in cotton roots, whereas the DEGs only found in SCRC22/CCRI41 (1954) and those oppositely (one up- and the other down-) regulated in the two grafts might be the key factors involved in the feedback regulation of K(+) uptake and root growth. The expression level of four putative K(+) transporter genes (three GhHAK5s and one GhKUP3) increased in both grafts under low K(+), which could enable plants to cope with K(+) deficiency. In addition, two ethylene response factors (ERFs), GhERF15 and GhESE3, both down-regulated in the roots of CCRI41/CCRI41 and SCRC22/CCRI41, may negatively regulate K(+) uptake in cotton roots due to higher net K(+) uptake rate in their virus-induced gene silencing (VIGS) plants. In terms of feedback regulation of K(+) uptake and root growth, several up-regulated DEGs related to Ca(2+) binding and CIPK (CBL-interacting protein kinases), one up-regulated GhKUP3 and several up-regulated GhNRT2.1s probably play important roles. In conclusion, these results provide a deeper insight into the molecular mechanisms involved in basic response to low K(+) stress in cotton roots and feedback regulation of K(+) uptake, and present several low K(+) tolerance-associated genes that need to be further identified and characterized.