Effect Of Low Temperature Stress On Morphological Structure And Physiological Characteristics Of Rice Seedlings

Rice(Oryza sativa L.)is one of the most important food crops,providing food for more than 3.5 billion people(about more than half of the world’s population).Although rice can be grown in different climatic regions(temperate,tropical and subtropical),it is more susceptible to low temperature stress than other crops because of its origin in tropical and subtropical regions.In particular,low temperature stress at the seedling stage can cause rice to lose its green color,wilt or even necrosis,thus reducing yield.Therefore,it is necessary to study the mechanism of cold tolerance in rice seedlings to provide a theoretical basis for crop production and stress tolerance breeding.In this study,we used Nipponbare,Wuyungeng 21,Xiangzaoxian 6,and Zhongzao 39 as materials,and subjected to low temperature stress at 12/8°C(day/night)for 4 days at the three-leaf stage,followed by recovery of growth at room temperature for 6 days.The growth and morphological structure indexes and physiological indexes of rice roots and leaves were measured.The Nipponbare and XZX6 were also selected for transcriptome and metabolome analysis.The response mechanism of rice seedlings under low temperature stress was comprehensively discussed from phenotype-structure-physiology-molecule.(1)Low temperature stress reduced the plant height,root length,leaf area and leaf dry matter weight by 27.62%,20.58% and 37.17% of the four varieties,and led to the destruction of chloroplast structure and the disorder of metabolism.However,the root dry matter weight of Japonica rice did not change significantly compared with that of indica rice.The decrease of aboveground water content showed that water transport in rice was inhibited under low temperature stress.For leaves,the structural changes such as the increase of vascular bundle spacing,the decrease of leaf thickness and leaf density caused by low temperature and the decrease of water potential together lead to the decrease of leaf hydraulic conductance,thus reducing the water loss under low temperature stress.The difference of water content between Japonica Rice and indica rice shows that their water absorption capacity is affected differently at low temperature.This is mainly reflected in the fact that the root morphology of Indica Rice(such as root volume,root surface area,root diameter,root tip number,branch number,cross number)and the xylem duct area in the root are reduced by low temperature,which weakens its ability to absorb water,while japonica rice can better maintain the growth of root under low temperature stress.(2)Low temperature led to oxidative stress in rice,which was manifested in the decrease of cat and POD activities in leaves and the significant increase of APX activities in roots.In order to protect the membrane structure from the destruction of oxides,the osmotic regulating substances such as soluble protein,soluble sugar and proline in leaves increased significantly.In the root,the osmotic adjustment substances such as soluble protein,soluble sugar and proline in indica rice also increased significantly under the influence of low temperature,while japonica rice was less affected by low temperature.In addition,low temperature also significantly increased the ABA content in japonica rice leaves,but had no significant effect on indica rice.(3)Both transcriptomic and metabolomic PCA analyses showed that the transcriptomic and metabolomic differences in rice seedlings under low temperature stress were mainly concentrated between the treated and control groups after low temperature treatment and between different varieties.In addition,DEGs in roots and leaves were much smaller after recovery from normothermia than after treatment,but unlike the transcriptome,the number of DEMs in roots and leaves did not change much after treatment and recovery.Transcriptome enrichment analysis,on the other hand,showed that differential genes in roots and leaves were mainly enriched in glycolysis/gluconeogenesis,starch and sucrose metabolism,carbon sequestration by photosynthetic organisms,amino acid biosynthesis,carbon metabolism,and fructose and mannose metabolism pathways under low temperature stress,and that other enriched pathways differed in roots and leaves.(4)The results of the combined transcriptome and metabolome analysis showed that transcripts and metabolites involved in the metabolism of carbohydrates(fructose,glucose,inositol,and cottonseed),amino acids(e.g.,threonine,valine,isoleucine,glutamate,and proline),and TCA cycles(e.g.,pyruvate citrate,α-ketoglutarate,succinate,and malate)were significantly regulated under low temperature stress,a result that suggests they play an important role in the response of rice seedlings to low temperature stress.In this study,the response mechanism of rice to low temperature stress was investigated from several perspectives.The photosynthetic mechanism of rice was disrupted and growth was significantly inhibited under low temperature stress.The effects of low temperature on the leaves of different rice varieties were more consistent,but Nipponbare and WYG21 were able to maintain the root system indicators under low temperature and thus maintain the water supply under low temperature stress to reduce the damage symptoms of rice.In addition,transcriptomic and metabolomic studies revealed that carbohydrate,amino acid and TCA cycle metabolism have important roles under low temperature stress.