Transcriptomic and functional characterization reveals CsHAK5;3 as a key player in K+ homeostasis in grafted cucumbers under saline conditions

CmoNAC1 in pumpkin rootstocks improves salt tolerance of grafted cucumbers by binding to the promoters of CmoRBOHD1, CmoNCED6, CmoAKT1;2 and CmoHKT1;1 to regulate H2O2, ABA signaling and K+/Na+ homeostasis

“…Improving the K + /Na + ratio represents a vital strategy to enhance salt tolerance in grafted vegetable crops. In grafted cucumbers, pumpkin rootstocks upregulate CmoHKT1;1 expression to facilitate Na + transport [ 58 ], while CmoHAK5 in pumpkin rootstocks and CsHAK5;3 in cucumber scion leaves to increase K + absorption [ 20 , 46 ].…”

“…H 2 O 2 and ABA can promote stomatal closure in response to salt stress [ 13 , 14 ]. The regulatory mechanisms of ion transport mainly include: (1) increased SOS1 and SOS2 expression promoting Na + efflux [ 15 , 16 ]; (2) upregulated NHX1 expression causing the storage of excessive Na + in vacuoles [ 17 , 18 ]; (3) increased HKT1 expression leading to the inhibition of Na + translocation from the root to the shoot [ 19 ]; and (4) upregulated expression of K + transporter genes such as HAK5 and AKT1 , increasing K + absorption [ 20 , 21 ].…”

“…Pumpkin and cucumber are typical cucurbit crops with well-developed vascular tissues, which are ideal vectors for the study of long-distance signal transduction between rootstocks and scions [ 41 ]. Several primary mechanisms contribute to the enhancement of salt tolerance in vegetable crops through grafting: (1) synthesis of H 2 O 2 and ABA in rootstocks, followed by their transport to scions to promote stomatal closure [ 42 , 43 ]; (2) enhancing CsHAK5;3 expression in scion leaves to improve K + absorption [ 20 ]; (3) regulation of CmHKT1 localization to the plasma membrane through CmCNIH1, thereby restricting Na + transfer to scions [ 44 ]; (4) enhancement of the SOD and POD enzyme activity in scion leaves to mitigate oxidative damage [ 36 ]; and (5) stimulating the accumulation of osmotic regulators like proline to reduce water loss in scion leaves [ 45 ]. Our previous study showed that in grafted cucumbers, H 2 O 2 ( CmoRBOHD1 -mediated) and ABA in pumpkin rootstocks were upregulated and transported to scions, which promoted stomatal closure at 3 h of 75 mM NaCl treatment and thus enhanced salt tolerance [ 42 , 43 , 46 ].…”

Pumpkin CmoDREB2A enhances salt tolerance of grafted cucumber through interaction with CmoNAC1 to regulate H2O2 and ABA signaling and K+/Na+ homeostasis

“…The transcriptome data for pumpkin roots exposed to 75 mM NaCl for 24 hours can be accessed from the following bioproject in NCBI website: PRJNA952931 (transcriptome data for grafted cucumber and self-grafted pumpkin roots) [ 32 ], PRJNA437579 (transcriptome data for pumpkin root tip) [ 49 ].…”

“…Furthermore, both cucumber and pumpkin possess highly developed vascular tissues, making them suitable subjects for studying grafting signal transduction [ 26 , 27 ]. The key mechanisms through which grafting improves plant salt tolerance include: (i) upregulation of genes involved in H 2 O 2 and ABA synthesis in rootstock to facilitate the synthesis and transport of root signals to the shoot [ 28 , 29 ]; (ii) induction of mobile mRNA movement to the shoot [ 30 ]; (iii) restriction of Na + transport to the scion [ 31 ] and enhancement of K + absorption [ 32 ]; and (iv) increased activity of antioxidant enzymes and elevated proline content in the scion [ 33 , 34 ]. Our recent study revealed that in grafted cucumbers, CmoNAC1 from the pumpkin rootstock not only interacts with the CmoRBOHD1 and CmoNCED6 promoters, promoting H 2 O 2 and ABA synthesis, but also interacts with the CmoAKT1;2 and CmoHKT1;1 promoters to maintain K + /Na + homeostasis [ 21 ].…”