Parental Drought-Priming Enhances Tolerance To Drought Stress In Offspring Of Wheat And Its Mechanisms

Drought is the major abiotic stress which decreases plant water status,inhibits photosynthesis,induces oxidative stress,restricts growth and finally lead to the reduction of wheat yield.The frequency and severity of drought stress events are expecting to increase due to global climate change.Priming(pre-exposure to a moderate stress)could enhance tolerance to subsequent stresses,which is known as the term of "stress memory".In this study,drought priming was successively performed for three successive generations during grain filling to obtain the plants of T1(primed for one generation),T2(primed for two generations),T3(primed for three generations).The plants without drought priming were annotated as TO.And then the offspring plants were subjected to drought stress through hydroponic cultivation or pot experiments to investigate whether parental drought priming could enhance tolerance to drought in offspring.The winter wheat cultivar Ningmai 13 was used.Three experiments were included in this study:(1)Wheat seedlings grown hydroponically in Hoagland solution were subjected to 20%PEG osmotic stress at five leaf stage for 0.5,1,3,6,12,24,and 48 h,to analysis the osmotic adjustment mechanism in wheat cultivar Ningmai 13;(2)T0,T1,T2,T3 seedlings grown hydroponically in Hoagland solution were subjected to 20%PEG osmotic stress at five leaf stage to investigate whether drought priming could improve tolerance to drought during the seedling stage in offspring of wheat and its physiological mechanisms;(3)T0,T1,T2,T3 plants grown in pots under field conditions were subjected to drought stress at 10 days after anthesis to investigate whether drought priming could improve tolerance to drought during grain filling in offspring of wheat and its physiological mechanisms.The main results of this study are as follows:1.The osmotic adjustment mechanism in response to drought in wheat.Plant water potential and osmotic potential was significantly decreased by drought stress.The decrease in leaf osmotic potential was mainly resulted from more accumulation of proline and glycine betaine under osmotic stress,in line with the up-regulation of the activities and gene expression level of P5CS and BADH,indicating that accumulation of proline and glycine betaine plays an important role at the early stage of osmotic stress.In addition,signal transduction related gene DREB2 and NCED1 was strongly induced in the early stage of PEG treatment,then decreased,and maintained a high expression level in 12-48 h;P5CS and BADH expression was strongly induced by PEG treatment and maintained a high level from 3 h to 48 h.These results indicate that regulate genes responded more quickly and played important roles in signal transduction involving in osmotic adjustment.2.Drought priming could improve tolerance to drought during the seedling stage in offspring of wheat.Under PEG osmotic stress,compared with the non-primed(TOD)plants,the parentally primed(T1D,T2D,T3D)plants showed higher leaf water status,photosynthesis performance and antioxidant capacity,indicating that drought priming during parental generation could enhance tolerance to drought in offspring of wheat during the seedling stage.Primed plants showed more accumulation of proline and up-regulation of activity and gene expression level of P5CS,in line with the decrease of leaf osmotic potential and the maintenance of water potential,indicating that osmotic adjustment plays important roles in enhanced tolerance to drought by drought priming.As for root growth response,the parentally primed(T1D,T2D,T3D)plants showed higher dry matter weight,length and superficial area of root,which was resulted from higher IAA content,IAA/CTKs value,proline content and antioxidant capacity of roots in these plants than the non-primed plants.Under PEG osmotic stress,there was no significant difference in the measured indexes among T1D,T2D,and T3D.And drought priming had no significant effect on the morphology and physiological indexes of wheat plants grown under CK conditions.3.Drought priming could improve tolerance to drought during grain filling in offspring of wheat.Under drought stress,the parentally primed(T1D,T2D,T3D)plants,disregarding the number of generations,showed higher photosynthetic rate,antioxidant capacity as well as higher remobilization amount of pre-anthesis stored dry matter and stem fructan remobilization,so as to maintain higher grain yield than the non-primed plants(TOD),suggesting that drought priming induced the transgenerational stress tolerance to drought stress.Moreover,the parentally primed plants showed higher leaf water status,which may be resulted from the higher contents of proline and glycine betaine,as well as higher gene expression levels and activities of P5CS and BADH compared with the non-primed plants under drought stress.P5CS and BADH were suggested to be the potential "trainable genes" in transgenerational drought stress memory.Further research has been found that the up-regulation of P5CS and BADH gene expression in the primed plants under drought may be related to the DNA demethylation in promoter region.There was no significant difference among three generations under drought,and the drought priming in parental generations did not affect the grain yield of the offspring plants under control condition.In conclusion,drought priming in parental plants could induce transgenerational tolerance to drought in offspring,which was related to the higher osmotic adjustment ability in primed plants so as to decrease leaf osmotic potential and maintain water potential.Proline and glycine betain played critical roles in parental priming induced tolerance to drought stress.Moreover,the acquisition of drought tolerance may be related to DNA demethylation in promoter region of P5CS and BADH.There was no significant difference in the alleviation effects on drought stress induced by different generations of priming,implying that one generation' s priming is enough to improve the tolerance of offspring plants to drought stress and the further improvement may need priming for more generations.In addition,the parental drought priming had no significant effect on offspring in terms of physiological processes and grain yield under non-drought conditions.This infers a potential approach to cope with the unpredicted drought stress by parental abiotic stress priming without side effect if the drought stress does not occur.