Studies On Molecular Mechanism Of Terminal Bud Death In Paulownia

Paulownia is an important fast-growing woody timber species in China,with the advantages of fast growth and excellent material,but there are also some problems.In the natural environment,the terminal buds of Paulownia seedlings die in autumn,while the opposite lateral buds sprout to form branches the following spring,resulting in a large crown and a short stem.This severely affects the height,straightness,and volume of the trunk,thus directly affecting the utility and economic value of the wood.In recent years,in order to solve the problem of ’big crown and low stem’ of Paulownia,many scholars has conducted research on the development of terminal and lateral buds,environmental factors,internal and external morphology,endogenous hormones and programmed cell death,and has achieved certain results.It is concluded that the death of the terminal bud is not a programmed death,and is closely related to the activity of the shoot apical meristem.In this process,photoperiod and endogenous hormones have an important influence.The molecular regulation mechanism of terminal bud death is still unclear.Therefore,it is of great theoretical value and practical significance to investigate the regulatory mechanism of terminal bud death for the cultivation of high stem wood and the development of buds.In this paper,one-year-old seedlings of Paulownia fortunei(Seem.)Hemsl.were treated with different photoperiods so that their terminal buds were at different stages of development.Through the observation of internal and external morphology,determination of photosynthesis and endogenous hormones,and combined with transcriptome sequencing analysis,the key metabolic pathways and key differential genes regulating growth and terminal bud death were found,and the function and expression of key regulatory genes were analyzed to preliminarily clarify the molecular regulatory mechanism of terminal bud death.The main results are as follows:(1)Photoperiod plays an important role in regulating the growth and terminal bud development of Paulownia.In an artificial climate chamber at a constant temperature of 25℃,Short day(8 h light/16 h dark)treatment for 2 weeks is height growth period,treatment for 4 weeks is the height growth stop period,and treatment for 6 weeks is the death of terminal buds period;Long day(24 h light)treatment for 2 weeks,4 weels,6 weeks are the height growth period.(2)In long day treatment,the net photosynthetic rate decreased with increasing treatment time,but the cells of the shoot apical meristem remained active and GA3,IAA and ZR remained high and not significantly different(P>0.05).The circadian rhythm pathway plays an important regulatory role in this process.With the increase of long day treatment time,the down-regulated expression of LHY and CO and up-regulated expression of TOC1 prolonged the growth period time and delayed the formation of terminal buds,which enabled the continuous growth.(3)In short day treatment and in comparison with the same period of long day treatment,the process of height growth arrest to terminal bud death occurred.The net photosynthetic rate increased and was higher than in the same period of long day treatment,the shoot apical meristem could not form a layer of bud scales like the lateral dormancy buds,in which the circadian rhythm and plant hormone signal transduction pathway play an important regulatory role.In the circadian rhythm pathway,the LHY and CO were down-regulated in expression the same as in the long day treatment period,which was not favorable for dormant bud formation.In the plant hormone signal transduction pathway,the ABA response factor ABF was down-regulated at this time,while the GA3 content remained at a high level and was not significantly different from that of the same period in long day treatment(P>0.05),leads to the weakening of bud resistance and inhibition of growth,making dormancy unable to be established and maintained.The upregulated expression of ETH effector ERF1 promotes the shedding of young leaves enveloping the apical meristem.The up-regulated expression of TCH4 and CYCD3 in BR signal transduction pathway,the down-regulated expression of MYC2 in JA signal transduction pathway and the up-regulated expression of ARR-A in CTK signal transduction pathway all promote the division of apical meristem cells.Therefore,after the cessation of height growth in Paulownia,the expression of ABF,ERF1,TCH4,CYCD3,MYC2 and ARR-A on the signal transduction pathway of LHY and CO and plant hormones in the circadian rhythm led to the disruption of the internal molecular regulation balance of the shoot apical meristem,and its cell activity continued without forming dormant buds wrapped in bud scales and with strong resistance to death.