Differences In Boron Or Magnesium Uptake Between Different Citrus Rootstock-scion Combinations

Boron(B)and magnesium(Mg)deficiencies are frequently observed in citrus orchard.Also,different rootstocks show different tolerant to B deficiency or Mg deficiency.Previous studies showed that Carrizo citrange are more tolerant to B deficiency and Mg deficiency than trifoliate orange.However,in the research of citrus B nutrition,the effects of the two rootstocks on the transcription level of the scion are still unclear.In addition,in the research of citrus Mg nutrition,the studies to identify the different tolerance to Mg deficiency of citrus rootstocks are mainly under the conditions of direct Mg deficiency,but the performance against the ion antagonism caused Mg deficiency are poorly understood.Meanwhile,the Magnesium Transporter(MGT)gene family is still rarely reported in citrus.Information about the role of different members of MGT family in response for different Mg efficiency is still lacked in citrus.Therefore,the main results were as follows:1.Transcriptome changes associated with boron deficiency in leaves of two citrus rootstock-scion combinationsTo elucidate the transcript differences between two rootstock-scion combinations under the treatment of B efficiency,the navel orange scion was grafted onto trifoliate orange [To,Poncirus trifoliata(L.)Raf.] and Carrizo citrange [Cc,C.sinensis(L.)Osb.×P.trifoliata(L.)Raf.].Then,there were two experimental groups,namely,Ns grafted onto Cc(Ns/Cc)and Ns grafted onto To(Ns/To).Two grafting combinations were sand-cultured under sufficient or deficient B supply for 180 days.In the end,the leaves of each sample were used for RNA-Seq analysis and the physiological parameters' analysis.The results showed that:(1)Under the treatment of B deficiency,the reduction rate of B content,photosynthetic parameters,chlorophyll contentor carotenoid content,and the increase rate of soluble sugar and lignin were significantly higher in Ns/To leaves than in Ns/Cc leaves.(2)A total of 934 genes in Ns/To and 642 genes in Ns/Cc were significantly up-regulated by the treatment of B deficiency.On the other hand,198 and 282 genes were significantly down-regulated in Ns/Cc and Ns/To,respectively.Of them,transcripts of six genes involved in photosynthesis were significantly down-regulated in Ns/To while only slightly down-regulated in Ns/Cc;transripts of two chlorophyllase protein genes were significantly up-regulated inNs/To and Ns/Cc,respectively;the expression levels of two carotenoid synthesis genes were significantly decreased in Ns/To,whereas they were not significantly influenced in Ns/Cc.The expression levels of three glucose synthesis genes were significantly increased in Ns/To,whereas they were not significantly influenced in Ns/Cc.In Ns/To,the expression levels of six lignin synthesis-related genes were significantly up-regulated whereas in Ns/Cc,only the expression levels of three lignin synthesis-related genes were significantly up-regulated.The changes of gene expression were consistent with the trend of the physiological parameters' variation.Overall,these findings suggested that the influence of Carrizo citrange on the scion was better than that of trifoliate orange due to differently regulating these metabolic pathways under the long term of B-deficiency.2.Effect of direct Mg deficiency and ion antagonism caused Mg deficiency on the performance of two rootstock-scion combinationsTo elucidate the responses of two rootstock-scion combinations to the direct Mg deficiency and ion antagonism caused Mg deficiency,two grafting combinations were sand-cultured under the conditions of low Mg,K excess,low p H and Al toxicity for120 days.Moreover,the physiological parameters including Mg content,fresh weight and leaves chlorophyll,starch,soluble sugar and malondialdehyde(MDA)contents were also measured.The results showed that:(1)Under the treatment of direct Mg deficiency and K excess caused Mg deficiency,the Mg content in Ns/Cc had more in leaves while less in roots as compared to Ns/To.It suggested that Carrizo citrange had more tolerant to direct Mg deficiency and K excess caused Mg deficiency than trifoliate orange via transporting more Mg from root to scion.In addition,Ns/Cc leaves also had higher chlorophyll content and lower starch,soluble sugar and MDA contents than those in Ns/To.It showed that Carrizo citrange had better performance than trifoliate orange under the conditions of direct Mg deficiency or K excess.(2)Under the treatment of low p H and Al toxicity caused Mg deficiency,the entire Mg content in the leaves of Ns/To plant were significantly higher than that in Ns/Cc;the changes of other physiological parameters also indicated trifoliate orange had better influence on scion than Carrizo citrange.These results not only indicated trifoliate orange are more tolerant to the stresses of low p H and Al toxicity than Carrizocitrange,but also implied that trifoliate orange had stronger ability of Mg uptake than Carrizo citrange under these conditions.3.Expression analysis and functional characterization of MGT gene family in citrus(1)Identification of citrus MGT gene family: Eight PtrMGT genes were identified through genome-wide analyses and they are located to five chromosomes,respectively.It was found that the ORF length of PtrMGT genes varied from 1182 bp(PtrMGT6)to 1479 bp(PtrMGT3),encoding polypeptides of 394–493 amino acids with a predicted molecular mass ranging from 43.82 to 55.17 k Da and the theoretical p I ranging from 4.85 to 6.64.The phylogenetic distribution revealed that eight PtrMGT genes were divided into five subfamilies.All PtrMGT proteins contain two putative TM domains and a conserved GMN tripeptide motif.(2)Expression Analysis of MGT gene family in citrus: A quantitative RT-PCR analysis showed that eight PtrMGT genes were expressed in all of the detected tissues and they mainly expressed in the vegetative organs.Under the treatment of low Mg and excess K,more genes were up-regulated in Carrizo citrange roots,however,PtrMGT genes showed more drastic response to low p H and Al toxicity in trifoliate orange roots.It can partially explain the reason for the different Mg content of the two rootstocks under different Mg deficient conditions.(3)Functional characterization of PtrMGT5: PtrMGT5 localizes to the plasma membrane.PtrMGT5 overexpression considerably enhanced tolerance to absolute Mg deficiency and antagonistic Mg deficiency in transgenic Arabidopsis plants,which was accompanied by the increase of fresh weight and Mg content.PtrMGT5 function as a plasma membrane Mg transporter that involved in the Mg absorption and transport.On the other hand,PtrMGT5 promoter analysis showed that the region range of-1kb to-0.5kb from start codon was the core regulatory region.There were two WRKY and one MYB binding site in this region.Hence,the expression of these genes may be regulated by these two transcription factors.