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 and one ) increased in both grafts under low K, which could enable plants to cope with K deficiency. In addition, two ethylene response factors (ERFs), and , 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 binding and CIPK (CBL-interacting protein kinases), one up-regulated and several up-regulated 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.