Mechanisms Of Sucrose Supply From Aboveground Parts Of Rice To Feeding Sites Of Meloidogyne Graminicola

Sucrose is produced by photosynthesis in plants.Source-to-sink transport of sucrose contains the three major steps of phloem loading,long-distance transport in phloem and phloem unloading,which is mediated by plasmodesmata（PDs）and sugar transporters that include SUTs family and Sugars Will Eventually be Exported Transporters（SWEETs）Clade III genes.Previous studies showed that plant pathogens can hijack the sugar transports in plants to obtain sugars.Similarly,sedentary parasitical nematodes can also hijack the sugar transports in their hosts to the sugars for their growth and development,whereas there are limited recognitions of the mechanism of how the nematodes control the sugar transports.Meloidogyne graminicola（Mg）is an important soil-borne pathogen,causing great yield-loss on rice.There is a trend of Mg being rapidly spread in our country and causing more serious damage to rice.Until now,damage caused by Mg on rice has been reported in the main rice-production area of China,such as Hubei,Hunan,Anhui,Guangzhou as well,leading to the realistic threat to the safe production of rice.Here,several transgenic rice lines were developed and used,and the mechanism of sucrose supply from aboveground to roots and Mg-caused giant cells（GCs）were studied through CFDA（Carboxyfluoresceine-diacetate）phloem loading,GC-MS analysis of sucrose,transcriptome analysis and so on.The main results are as follows:1.PDs,instead of sucrose transporters,play key roles in sucrose supply from rice root phloem to Mg-caused GCs.Result from q RT-PCR indicated that Os SUT1,Os SUT2,Os SUT4 were down-regulated or unchanged at 3 dpi,7 dpi,14 dpi of Mg,whereas the expression levels of Os SUT3,Os SUT5 and Os SWEETs Clade III genes（Os SWEET11 to Os SWEET15）were extremely low.Then the Promoter::GUS transgenic rice lines of Os SUT1 to Os SUT5 were developed and used,and the analyses of Mg inoculation and GUS staining were performed.Results showed that no promoter activities of Os SUTs were detected in Mg-caused galls at each time point tested.These data elucidate that Os SUTs family genes and Os SWEETs Clade III genes are not the major players in the process of sucrose supply to Mg-caused GCs.Meanwhile,data from CFDA phloem loading showed that Mg-caused GCs were symplastically connected to the phloem of rice roots.In addition,Os GSL2-and Os GNS5-overexpressing rice lines were developed that synthesizes and degrades callose,respectively.Using Os GSL2-and Os GNS5-overexpressing rice lines as plant materials,and Mg inoculation and GC-MS analysis of sucrose were carried out.Results indicated that increasing callose deposition in Mg-caused galls led to the reduction of Mg parasitism ability on rice,where compared with rice wild-type the number of juveniles in root tissues were significantly reduced from 52 Juveniles per root system to 10 Juveniles per root system at 7 dpi of Mg,GC area from 28,212μm²to 22,877μm² and sucrose content in Mg-caused galls from 608μg g^-1 FW to 440μg g^-1 FW.These data implied that sucrose supply from rice root phloem to GCs caused by Mg were mediated by PDs.According to the results,it was elucidated that sucrose supply from rice root phloem to Mg-caused GCs were mediated by PDs,but not by Os SUTs or Os SWEETs Clade III genes.2.Mg enhances sucrose supply to rice roots.Based on the result of CFDA phloem loading analysis,it was showed that more green fluorescence of CFDA was detected in Mg-caused galls and root tissues carrying the galls compared with root tissues without galls,implying that Mg regulates PDs-mediated sucrose transport from aboveground to rice roots.Moreover,the change of sucrose content in roots after Mg infection were characterized by GC-MS.Results indicated that compared with non-inoculated rice roots Mg infection led to the increases of sucrose contents in Mg-caused galls,roots carrying the galls and root without carrying the galls with the sucrose content being increased from 33.5μg g^-1 FW to 592μg g^-1 FW、636μg g^-1 FW and 768μg g^-1 FW,respectively.These results elucidate that Mg positively regulates sucrose transport from rice aboveground to its roots.3.Rice basal nodes show a potential role in the mechanism of Mg positively regulating sucrose supply to rice roots.According to the result of transcriptome analysis of rice root basal nodes,there were 912,627,123 and 20 genes were specifically up-regulated in rice root basal nodes at 1 dpi,3 dpi,7 dpi and 15 dpi of Mg,respectively.Data from Gene Ontology（GO）annotation and Kyoto Encyclopedia of Genes and Genomes（KEGG）analysis of these genes indicated that they founded in resistance responses and cell wall modifications at early stage（1 dpi&3 dpi）,and involved in material transports at later stage（7 dpi&15 dpi）,implying that rice basal nodes as a hub for nutrient distribution might take part in the interaction mechanism between rice and Mg.Furthermore,gene expression patterns of genes being response for callose synthesis and degradation,Os SWEETs,aquaporin and other transporter genes were characterized.Results showed that the expression of them were regulated by Mg infection.Among them,several callose-degradation genes（1 dpi&3 dpi）,a callose-synthesis gene（1 dpi,3 dpi,7 dpi）and a Os SWEET（7 dpi）were induced by Mg at different time points.Based on these results,it was deduced that rice basal nodes might involving in the positive regulation of sucrose supply to rice roots by Mg.In conclusion,the roles of symplast and apoplast pathways in sucrose supply from rice root phloem to Mg-caused GCs were assessed,and found that PDs but not sucrose transporters play pivotal role in this process.Meanwhile,Mg was evidenced to be able to positively regulate sucrose transport from rice aboveground to roots,and the potential role of rice basal nodes in the mechanism was analyzed through transcriptome analysis.Our study promotes the recognition of the mechanism of how plant parasitical nematodes hijack the sugar transport of their hosts.In the future,we could further investigate the role of rice basal nodes in the mechanism of Mg enhancing sucrose supply to roots.