| An appropriate planting density contribute largely to the radial growth of trees and the yield of wood with high quality.Therefore,it is of great significance to understand the internal mechanism of planting density affecting radial growth for trees.However,it is not yet available.Here,several related experiments were conducted for a widely planted Eucalyptus urophylla ×E.grandis clone using plant physiology and molecular biology technologies.Firstly,conducted comparisons of growth traits,photosynthetic characteristics and transcriptomes from stems of plants between high(10 plants per square meters,0.3 m×0.3 m)and low(5 plants per square meters,0.6 m×0.6 m)planting densities.Secondly,analysed the effects of planting density on plant photosynthetic characteristics and on gene expression of xylem.Then,identified planting density-reponsive candidate genes.Lastly,a network model that how the planting density affects radial growth of E.urophylla × E.grandis was proposed.This work will advance the intensive study of the mechanism that planting densities affect wood formation.The main results were as follows:(1)At 3 month age,the low planting density(LPD)plants has 6.84 mm of average diameter,which was significantly higher(i.e.,26%)than the high planting density(HPD,i.e.,5.41 mm)plants.The LPD plants also showed significantly higher(i.e.,67%)monthly diameter increment than the HPD(i.e.,2.36 mm vs 1.41 mm)plants.Moreover,the crown width and the crown length increased by 56% and 25% in LPD plants,respectively.However,the tree height and the tree height increment were similar between the two planting density plants.(2)The biomass accumulation of roots,stems,branches,leaves and the whole plant increased with the decreasing of planting density.The biomass of the stem of 3-month-old E.urophylla × E.grandis in the LPD significantly increased by 76% while the root biomass ratio(RMR)and the root shoot ratio(R/C)were decreased by 17% and 21%,respectively.The LPD plants also had increased total leaf area of a single plant,net photosynthetic rate and photosynthetic enzymes activities(Rubisco,RCA,Su Sy,AGPase and SBPase).Each parameters increased by 102%,146%,44%,41%,25%,52%,58%,respectively.(3)Via a combination of analysis of the Pac Bio Iso-Seq and RNA-Seq transcriptomes,a total of 45,490 non-redundant full-length transcripts were obtained from the xylem and the phloem of 3-month-old E.urophylla × E.grandis in both planting density plants.Comparisons of the gene profiles of the xylem cells obtained a total of 443 differentially expressed transcripts between the LPD plants and the HPD plants,including 60 transcripts encoding regulatory factors.Among these 60 transcripts,28 were up-regulated and 32 were down-regulated in the LPD plants.The up-regulated regulatory factors were mostly related to growth and development while the down-regulated regulatory factors were related to stress response.A combination of analysis of the differentially expressed transcripts between phloem and xylem,q RT-PCR and bioinformatics analysis,several planting density-reponsive candidate genes related to xylem development were identified.They are genes related to cell division(i.e.,PXL2 and its interactional genes CUL1,T15D22.7),secondary wall regulation(i.e.,MYB46,C3H14 and their downstream genes Ces As,LACs),phloem and xylem development(i.e.,NAC86)and vascular tissue developmentnt(i.e.,PTL).They were expressed at higher levels in the xylem and in the LPD plants than in the HPD plants.We investigated the photosynthetic characteristics and stem gene expression patterns of E.urophylla × E.grandis in two planting densities(LPD and HPD).We found that an enhanced photosynthetic capacity,increased biomass allocation to the stem and differential gene expression profiles contributed to the larger radial growth in the LPD plants.Several planting density-reponsive candidate genes related to xylem development were identified.These results provided new insights into the internal mechanism of planting density affecting radial growth for trees. |
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