Effect Of Auxin Participating Tobacco Seedling Growth In Response To Water Stress

Recently float breeding is the mainly industrialized tobacco seedling production in China.In order to enhance survival rate of tobacco seedlings after transplanting,about 7-day water break is always adopted before tobacco seedling transplanting.However,we found that different tobacco genotypes have contrasting response to 7-d water break(water stress),and especially discrepant response of root morphology to water stress.The mechanisms how and why tobacco roots respond differently to water stress remains unclear.Because auxin plays a pivotal role to regulate plant root growth when plant facing environmental factors.How does auxin participate root formation and elongation in tobacco seedlings when plant facing water stress?In this study,plant growth including initiation and elongation of 1st and 2nd order lateral root was analyzed in five tobacco cultivars under water stress,in which application of 2.5%polyethylene glycol(PEG)mimicked moderate water deficiency.Auxin distribution in tobacco plants and application of exogenous auxin(NAA)and polar auxin transport inhibitor(NPA)was conducted to illustrate whether auxin participates growth of 1st and 2nd order lateral root under water deficiency.The results were listed as below.1.Formation and elongation of the 1st and 2nd order lateral roots of five tobacco cultivars were studied under application of 2.5%(PEG),which was regarded as moderate drought condition.Compared with control treatment,formation of first-and second-order lateral root was enhanced by PEG treatment in two drought-tolerant tobacco cultivars rather than in three drought-sensitive cultivars.Although net photosynthetic rate was decreased in drought-tolerant tobacco cultivars,less extent than in drought-sensitive cultivars.And PEG-decreased stomatal conductance,transpiration and leaf temperature were recorded only in three drought-sensitive cultivars as compared with control treatment.After number of first-order LRs under PEG treatment was trimmed to the similar level as that under control treatment during the experimental stage,surprisingly,shoot dry weight,net photosynthetic rate,stomatal conductance,and transpiration rate in drought-tolerant cultivar(Nongda202)were decreased to a comparable extent as comparison with those in three drought-susceptible tobacco cultivars under PEG treatment.This suggested that increment of LR formation was the main reason for two drought-tolerant tobacco cultivars to keep plant growth under drought stress relative to the control treatments.2.Time-course change of plant growth and 1st and 2nd order lateral roots was recorded in cv.Nongda202(drought-tolerant tobacco)under PEG application.Nongda202 kept similar biomass of shoot and root under 7-day PEG treatment as compared to control treatment;however,after 9-day PEG treatment,significant decrement of shoot and root biomass was observed in Nongda202 plant.And total root length and root volume had the similar changing tendency with plant growing with PEG treatment.Compared with control treatment,PEG treatment increased formation of 1st and 2nd lateral roots from3-day to 7-day PEG treatment;afterwards,the enhancement of initiation of lateral roots became slower and then higher 1st lateral root number and 2nd lateral root density was recorded in Nongda202 under control treatment.These further confirmed increased formation of lateral roots in drought-tolerant cultivars at 7-day water stress was the main reason that plant kept similar biomass.3.To illustrate whether auxin participates formation of lateral root in drought-tolerant tobacco plants,auxin distribution was analyzed by HPLC method and by constructing transgenic tobacco plants with DR5::GUS.Compared with control treatment,auxin concentration was higher in topmost expanded leaves and roots of Nongda202 from 3-day to 5-day PEG treatment.Afterwards,auxin concentration decreased under PEG treatment,which paralleled formation of lateral roots in Nongda202.Furthermore,expression of DR5::GUS confirmed the above results.At 5-day PEG treatment,[~3H]IAA polar transport was increased by PEG treatment;qPCR results showed that auxin synthetic genes YUCCA3/4/6 in topmost leaves and auxin polar transporters PIN3/3b/4/9 expressed more abandently under PEG treatment than under control treatment.4.The above results found that auxin concentration paralleled lateral root formation in drought-tolerant tobacco plants.Exogenous application of NAA and NPA would further confirmed auxin modulating lateral root formation in response to water deficiency.Application of NAA in addition to control treatment enhanced formation of 1st and 2nd lateral roots to the similar level as that under PEG treatment;conversely,application of NPA in addition to PEG treatment decreased formation of 1st and 2nd lateral roots to the similar level as that under control treatment.Furthermore,expression level of DR5::GUS under NAA application in addition to control treatment amounted to that under PEG treatment;conversely,expression level of DR5::GUS under NPA application in addition to PEG treatment amounted to that under control treatment.In conclusion,different tobacco cultivars had contrasting response to water stress,mainly showing contrasting response of 1st and 2nd lateral root formation.Enhanced of formation lateral roots in tobacco plants mainly contributed to induced auxin synthesis and polar auxin transport from the shoot to root.