A Study On Physiological And Transcriptional Mechanisms Of Poplars In Responses To Heat And Drought

Predictions suggest that climate change will increase global average temperature by 2.4–6.4 ? during this century,with increases in the frequency,length and severity of droughts.Recent researches suggest that heat-and drought-induced forest mortality events are widespread around the world.Heat and drought are expected to be major abiotic stresses for most plants including tree species in the near future.Poplar is a fast-growing woody tree species and sensitive to abiotic stresses.As global climate change imposes a great challenge for the survival of poplars,it is essential for us to obtain a better understanding of the physiological and molecular mechanisms during poplar acclimation to heat and drought.This study investigated the physiological responsive mechanisms on energy metabolism,hormone and oxidative metabolism in two poplar genotypes treated with heat and/ or drought stresses.Subsequently,the transcriptomic regulation mechanisms of the two poplar genotypes were analyzed using RNA-sequence technology.Finally,the poplar mortality mechanisms under severe drought were exploited by the analyses of nonstructural carbohydrates(NSC)and failure of hydraulic conductivity.The main results obtained are as follows:Cutting seedlings of two poplar genotypes(Populus simonii(Ps)and P.alba × P.tremula var.glandulosa (Pg)) with contrasting drought tolerance were treated with heat(ambient and high temperature)and drought(80% or 40% field capacity and no watering(denoted as 0%))for 8 days.Heat resulted in reduced the photosynthetic rates,leaf water potentials and relative water contents and induced foliar ?13C and foliar ?18O in poplars.Heat also exacerbated the decreases of photosynthetic rates and water contents caused by drought and the increases of foliar ?13C,foliar ?18O,ABA and H2O2 concentrations under drought.Ps showed stronger heat and drought tolerance than Pg as Ps preferred to reduce stomatal conductance and energy metabolism to accumulate more carbonhydrates and higher water contents under drought.RNA-sequencing analysis was used to explore the transcriptomic regulation mechanisms of physiological responses in roots and leaves of the two poplar genotypes under heat and drought.HSFs(HSFA4A,HSFA2)and HSPs(HSP70,HSP81.3)were significantly down/upexpressed to repair the damaged proteins caused by heat stress,while MAPK signal pathways were induced by oxidative stress caused by heat.The oxidative stress signaling was transducted by transcription factors,such as WRKY and HSFs,to the targeting genes and induced the expression levels of heat tolerant genes,such as CAT1,CAT2 and APX2.Drought stimulated the ABA-dependent and ABA-independent signal pathways in roots and leaves of two poplar genotypes and the stress signals were transducted by MYB,NAC,b ZIP and AP2/ERF and the expression levels of drought tolerant genes,such as SUS4,CAT1,GSH2 and DREB2 A,were activated.Drought aggravated HSF and oxidative signal pathways induced by heat stress.Pg was more sensitive to heat and/or drought stresses as more genes and greater changes in expression levels of HSFs,HSPs and genes involved in ABA-dependent signal transduction pathway in comparison with Ps.This results correspond well to the physiological responses of both poplar genotypes.To explore the xylem embolism vulnerability of different poplar genotypes,vulnerability curves of seven poplar genotypes were determined.Among these poplar genotypes,P.× euramericana(Pe)displayed the lowest xylem embolism vulnerability while P.alba × P.tremula var.glandulosa(Pg)showed the highest xylem embolism vulnerability.Subsequently,both contrasting poplar genotypes(Pe and Pg)were selected and used to explore the mortality mechanisms under shade and/or severe drought conditions.Carbon starvation and hydraulic failure were induced by shading(100,32 and 12 % light)and severe drought(without watering),respectively.The photosynthetic characteristics,NSC and stem hydraulic conductivity were determined to explore the relationships between bud survival rates and NSC or PLC(percentage loss of conductivity).NSC concentrations in the two poplar genotypes were reduced during shading,but only small changes in PLC occurred.Bud survival rates and NSC concentrations were significantly positively linear correlated.PLC was increased to about 100 % during severe drought,but a relatively high level of NSC remained under such a condition,leading to a negative relationship between bud survival rates and PLC.The higher initial NSC concentrations and lower xylem embolism vulnerability of Pe led to a higher plant survival rates than those of Pg.It is indicated that hydraulic failure combined with carbon starvation caused plant mortality during drought stress and the survival time of the plant depends on the dominant one of the two mechanisms: if hydraulic failure is dominant,plant survival time will decrease and if carbon starvation is dominant,plant survival time will be longer in comparison with that under hydraulic failure.Xylem embolism vulnerability and initial NSC concentrations in poplar genotypes also affected the tree survival time during drought.These data indicate that the above two mechanisms co-exist and interact with each other,leading to poplar mortality under severe drought stress.