Evaluation Of Potassium Efficiency In Different Watermelon Genotypes And Root Transcriptome Study Under Potassium Starvation

Potassium (K) is one of the essential nutrients for crops, and K+deficiency highly restricts crop yield and quality. However, potassium (K) deficiency is a prominent obstacle in protected horticultural industry of China. Watermelon [Citrullus lanatus (Thunb.) Matsum.&Nakai] is an economically important crop that often suffers from K+deficiency. Germplasm evaluation of K efficiency was conducted in watermelon; genotypes with contrasting K efficiency were identified; and comparative transcriptome profilings of potassium starvation responsiveness in roots of K-efficient and-inefficient genotypes were analyzed. This study presents the first global transcriptome analysis in watermelon roots and provides new insights into the molecular mechanisms underlying the tolerance of the K-efficient watermelon genotype to K+deficiency. The main results are as follows:1. Sixty-four watermelon genotypes were grown under conditions of ample (6mM) and limited (0.1mM) K supply to evaluate the genotypic variation for potassium efficiency in watermelon. Thirty-eight wild genotypes (C. lanatus subsp. lanatus) and26domesticated genotypes (C. lanatus subsp. vulgaris) were employed. The evaluation method for watermelon K efficieny in vegetative stage was established:(i) A clustering based on plant growth vigor under ample K supply was conducted,(ii) K efficiency was classified among genotypes with similar growth vigor based on a medium-efficiency interval, which is equivalent to the95%confidence interval of the mean relative shoot dry weight and relative shoot K concentration. Genotypic data above or below this interval were classified as either K-efficient or K-inefficient.(iii) K uptake and K use index were employed to confirm the K-efficient and-inefficient genotypes. The results showed that significant differences in shoot dry weight, shoot K concentration, K uptake, and K use index were observed among genotypes; four wild genotypes were identified as K-efficient and four domesticated genotypes were definitively considered K-inefficient.2. The K-efficient genotype "YongShi"(YS) and K-inefficient genotype "ZaoJia"(8424) were used to compare the different responsive patterns to K+starvation. Plant growth, mineral nutrient analysis (K+, Mg2+, Ca+concentration), gas exchange parameters, the relative expression of high affinity K+transporter gene Cla012760, and root K+uptake kinetic characters were studied. The results showed that YS is more tolerant to K+deficiency at120h after K+starvation. The more vigorous root system, the higher affinity to low rhizosphere K+concentration, and the higher expression level of Cla012760explained higher K uptake efficiency of YS; YS can substitute K+with Mg2+and Ca2+more effectively, which partly explained the higher K use efficiency of YS; and better gas exchange and plant growth were expressions of higher K use efficiency of YS.3. Roots of YS and8424seedlings with or without K+supply were harvested at6and120h after treatment, and their transcriptomes were analyzed by Illumina RNA sequencing. We found differences in the transcriptome between YS and8424can be basically classified into two categories:responsive differences (differences in transcriptomes between K+stressed and unstressed plants) and non-responsive differences (differences in transcriptomes between YS and8424under unstressed conditions). After6h of K+starvation,813(332up-regulated and481down-regulated) and2347(1161up-regulated and1186down-regulated) genes were differentially expressed in YS and in8424, respectively, compared with the control. At120HAT,248(58up-regulated and190down-regulated) differentially expressed genes in YS and248(176up-regulated and72down-regulated) in8424were regulated by K+. The two genotypes exhibited similar regulatory and responsive pathways at the early-stage of K+starvation but significantly different transcriptome response under long-term K+starvation4. Based on the analysis of transmembrane transportation-related genes responsive to K+starvation, different regulatory patterns of the K+acquisition system were found between short-and long-term K+starvations. The regulation of AKT1activity through the SYP121-KC1-AKT1complex is assumed as an early adaptive response to K+deficiency to prevent K+leakage, followed by the regulation of AKT1activity by the CBL-CIPK-AIP module to enhance K+uptake at long-term K+starvation. With respect to the HAK5transporter, RCI3-RAP2.11-HAK5pathway was found function only at an early stage, while the transmembrane proton gradient is possibly persistent in regulating its gene expression. The vacuole K channel TPK1gene was found induced by the short-term K+starvation, mediating the release of K+from the vacuole. At the late-stage of K+starvation, genes involved in jasmonic acid and reactive oxygen species production; Ca2+and receptor-like kinase signaling; lignin biosynthesis; and other stress-related genes were repressed in YS, whereas a large number of stress-related genes were induced in8424.5. The non-responsive differences represented that most genes related to mineral element transportation, ethylene and jasmonic acid metabolism, signal transduction, and stress-related transcriptional factors such as MYB, WRKY, bHLH and AP2/EREBP were highly expressed in YS. At the late-stage of K+starvation, among the YS-specific genes,74%of the down-regulated genes showed significantly higher basal expressions in YS than those in8424under normal growth conditions, indicating that even a down-regulation of these genes in YS could still result in a similar expression level with those in8424.6. Twenty-nine candidate genes related to the tolerance to potassium deficiency was selected for further studies. And the hypothetical model of molecular mechanism underlying the low-K+tolerance in YS is:(i) highly developed roots, high expression level of K+transporter genes, good affinity with low concentration of rhizosphere K+, and the effectiveness of substituting K+with Ca2+and Mg2+;(ii) the highly expressed defense-and stress-related genes in YS under optimal K+supply;(iii) the repressed defense and stress response during the long-term K+-starvation can save energy for better root growth in YS, which can facilitate K+uptake and increase K efficiency and tolerance to K+deficiency.