| As one of the major limiting factors for agricultural crops,nitrogen has a profound influence on plant growth and development.Shaping efficient crop root architecture is an important way to nitrogen fertilizer reduction and benefit improvement.Research on the response of roots to low nitrogen is the basis for shaping efficient crop root architecture.In this study,K836(K1)and K837(K2)were used as test materials to carry out in situ root observation experiment(self-made root in-situ observation device).Then the cotton root senescence experiment was carried out with Guoxin 9 as the test material.Both experiments set two nitrogen concentrations:low nitrogen,0 mg N·kg-1 soil;normal nitrogen,138 mg·kg-1 soil.The effects of low nitrogen stress on cotton aboveground and lateral roots and root hair related traits were clarified,and the mechanism of low nitrogen stress delaying cotton root senescence was revealed through transcriptome analysis.The main results are as follows:1.Effects of different nitrogen treatments on shootThe shoot development was significantly inhibited by low nitrogen treatment.Compared with normal nitrogen treatment,the plant height of K1 and K2 under low nitrogen treatment decreased by 11.38%and 9.64%,and the stem diameter decreased by 12.50%and 7.83%,respectively.SPAD value and leaf area under low nitrogen treatment,K1 and K2 began at 50 days after emergence,and Guoxin 9 began at 40 days after emergence to be significantly lower than normal nitrogen treatment.Low nitrogen treatment promoted aboveground senescence.The activities of superoxide dismutase,peroxidase and catalase in leaves of low nitrogen treatment were significantly decreased by 54.38%,23.06%and 55.18%,respectively,compared with those of normal nitrogen treatment at 100 days after seedling emergence.Compared with normal nitrogen treatment,low nitrogen treatment significantly increased the contents of malondialdehyde and abscisic acid in leaves,which increased by 14.64%and 34.80%,respectively,at 100 days after emergence.2.Effects of different nitrogen treatments on dry matter weight and seed cotton yieldLow nitrogen treatment significantly reduced the dry matter accumulation of each organ and total dry matter of cotton plant.At 100 days after emergence,compared with normal nitrogen treatment,the weight of root,stem,leaf,bud/boll and total dry matter of Guoxin 9 decreased by 54.33%,65.55%,81.87%,93.89%and 78.54%,and the root dry matter distribution rate and root shoot ratio increased by 112.76%and 157.75%,respectively.At 110 days after emergence,compared with normal nitrogen treatment,the root,stem,leaf,bud/boll and total dry matter mass of K1 decreased by 21.86%,18.69%,30.38%,45.40%and 36.13%,and the root dry matter distribution rate and root shoot ratio increased by 22.78%and 33.33%,respectively.The root,stem,leaf,bud/boll and total dry matter mass of K2 decreased by 18.03%,17.56%,27.85%,42.77%and 33.81%,respectively,and the root dry matter allocation rate and root shoot ratio increased by 23.93%and 33.33%,respectively,compared with normal nitrogen treatment.The seed cotton yield of K1 and K2 under low nitrogen treatment was 22.18%and 20.72%lower than that under normal nitrogen treatment,respectively.3.Effects of different nitrogen treatments on lateral rootsDifferent nitrogen treatments had no significant effect on lateral root density.Root angle of low N treatment was much narrower than that of normal nitrogen treatment,reduced by 6.25° and 4.65° for K1 and K2,respectively.For K1 and K2,low nitrogen treatment markedly decreased lateral root diameters compared with those of normal nitrogen treatment,the first lateral root diameter decreased by 5.54%,and 6.59%,and the second lateral root diameter decreased by 6.38%,and 3.52%,respectively.For K1 and K2,compared with normal nitrogen treatment,low nitrogen treatment increased by 3.93 d,and 3.84 d for the first lateral root lifespan,and by 4.96 d,and 5.46 d for the second lateral root lifespan,respectively.4.Effect of different nitrogen treatments on root hairsLow nitrogen treatment significantly increased the length and density of root hairs,root hair length of K1 and K2 increased by 17.49%and 18.24%for the first lateral root,and increased by 14.15%and 22.55%for the second lateral root,and the root hair density increased by 7.71%and 5.26%for the first lateral root,increased by 12.43%and 10.28%for the second lateral root,respectively.Contrary to the results of root hair length and density,low nitrogen treatment significantly reduced the life of root hairs.Root hair lifespan of K1 and K2 was shortened by 1.19 d and 1.90 d for the first lateral root,and shortened by 1.78 d and 1.48 d for the second lateral root,respectively,compared with normal nitrogen treatment.5.Effects of different nitrogen treatments on dynamics of root developmentThe taproot diameter of K1 and K2 under low nitrogen treatment decreased by 12.1 1%and 9.85%,respectively,compared with that under normal nitrogen treatment.The diurnal variation of the taproot diameter can be well fitted by the logistic curve,and the coefficient of determination R2 was above 0.93.The maximum accumulation rate of taproot diameter under low nitrogen treatment was greater than that under normal nitrogen treatment,K1 and K2 increased by 4.09 μm·d-1 and 13.99 μm·d-1,respectively,and the rapid accumulation duration was shortened by 7.54 d and 10.92 d,respectively,compared with normal nitrogen treatment.The length and diameter of the lateral roots were about 11 d from birth to no longer change.Lateral root daily elongation rate decreased with the increase of the number of days after the appearance of it.On the 1st to 3rd,Daily elongation rate of lateral roots under low nitrogen treatment was significantly greater than that of normal nitrogen treatment at 1～3 days after root emergence.At 4～10 days after root emergence,the daily elongation of lateral roots under low nitrogen treatment was significantly lower than that of normal nitrogen treatment.At 11 days,there was no significant difference between two treatments.At 2 and 6～10 days after root emergence,the lateral root thickening rate of the low nitrogen treatment was significantly lower than that of the normal nitrogen treatment.Lateral root thickening rate of the low nitrogen treatment was significantly greater than that of normal nitrogen treatment at the 3～5 days after root emergence.6.Effects of different nitrogen treatments on senescence related enzyme activities and hormone content of rootLow nitrogen treatment significantly increased the superoxide dismutase activity of roots at 100 days after emergence,which was 31.54%higher than that of normal nitrogen treatment,and significantly increased catalase activity at 60,80,and 100 days after emergence,increased by 28.43%,108.70%,and 225.71%,compared with normal nitrogen treatment.The malondialdehyde content of low nitrogen treatment at 60,80,and 100 days after emergence was significantly lower than that of normal nitrogen treatment,which was 41.38%,55.17%,and 48.15%lower than that of normal nitrogen treatment,respectively.Compared with normal nitrogen treatment,low nitrogen treatment significantly reduced the content of auxin in roots.Different nitrogen treatments had little effect on the contents of gibberellin and zeatin nucleoside in roots.At 80 days after emergence,the content of methyl jasmonate under low nitrogen treatment was significantly lower than that under normal nitrogen treatment.At 80 and 100 days after emergence,the low nitrogen treatment significantly reduced the abscisic acid content by 47.32%and 48.82%,respectively,compared with the normal nitrogen treatment.7.Analysis of differentially expressed genes in lateral rootsCompared with the nitrogen treatment,a total of 14,607 differentially expressed genes(DEGs)were identified.There were 2,194(1,027 up-regulated/l,169 down-re gulated),7,271(2,714 up-regulated/4,557 down-regulated),6,740(1,833 up-regulated/4,907 down-regulated),4,778(1,680 up-regulated/3,098 down-regulated),and 5,228(1,735 up-regulated/3,493 down-regulated)DEGs at 20,40,60,80,and 100 DAE,r espectively.8.Analysis of related pathways of root li fespanIn this study,a total of 187 DEGs involved in reactive oxygen scavenging pathways were identified.Among them,the genes positively related to lifespan,such as HSP26-A,GSTU8,MIOX2,MIOX4,etc.were significantly up-regulated.A total of 90 DEGs involved in plant hormone pathways were identified.Abscisic acid synthesis genes NCED1,NCED6,and ABA receptor gene PYL12 were significantly down-regulated at 80 and 100 days after emergence.A total of 70 DEGs were found to be involved in the cell cycle,including 57 cyclin(CYC)genes,nine caspase genes,and four apoptosis genes,most of which were down-regulated.At 60,80 and 100 days after emergence,most of the MYB transcription factor family genes were down-regulated.9.Co-expression network analysis of weighted genes(WGCNA)A total of 16 different expression modules were obtained by means of WGCNA.Among them,module "MEbrown" was significantly positive correlation with phenotypic such as root length,diameter,surface area,and volume,and physiological traits such as soluble sugar,soluble protein,malondialdehyde,and abscisic acid content,while module"MEmagenta" was the opposite.We identified fieve key genes in the module"MEbrown":Ghir_A05G025560,Ghir_A11G025240,Ghir_A03G004210,Ghir_A02G001380,and Ghir_D02G002510,and three key genes in the module"MEmagenta":Ghir_D13G019870,Ghir_D10G012140,and Ghir_A02G010460.In summary,cotton can adapt to low nitrogen environment by decreasing lateral root diameter,lateral root angle and root hair lifespan,and increasing lateral root life,root hair length and density.The increase of lateral root lifespan(delaying senescence)was mainly related to the change of endogenous hormone content,reactive oxygen scavenging system,apoptosis and senescence related transcription factor gene expression under low nitrogen enviroment.These results revealed the response of lateral root and root hair morphology and lifespan with different root orders to low nitrogen,enriched the theory of cotton root research. |
