Overexpression Of SHR Transcription Factors Regulates The Outgrowth Of Axillary Buds In Poplar

Plant lateral branch growth is an important attribute in forestry production and application.It affects the quantity and quality of wood,and is regulated by environmental factors such as cold,drought and photoperiod signal.Taking the woody model plant poplar as the research object,this paper analyzes the role of SHORT-ROOT(SHR),an important member of GRAS transcription factor family[1],in the process of axillary bud maturation and developmentThrough homology comparison with Arabidopsis SHR gene(AtSHR),it was found that in INRA 717-IB4 hybrid poplar(Populus tremula × P.alba),named PtaSHR1,PtaSHR2A and PtaSHR2B respectively.qRT-PCR showed that the expression levels of PtaSHR1 and PtaSHR2(PtaSHR2A and PtaSHR2B)were higher in terminal buds and roots.However,with the gradual maturation of axillary buds from top to bottom along the stem and the activation of axillary bud germination by truncation,the expression and induction of PtSHR2 are more prominent than PtaSHR2,suggesting that SHR2 may be involved in the regulation of poplar axillary bud development.Then,PtSHR2 gene from Populus trichocarpa was isolated and transformed into INRA 717-IB4 hybrid poplar to construct overexpressing PtSHR2 poplar(PtSHR2OE).When PtSHR2OE poplar grows to 3 months,the terminal bud gradually enters dormancy.At the same time,the axillary bud below the axillary bud maturity point(BMP)has larger shape,more developed vascular structure,and can further germinate into lateral branches.By measuring the contents of auxin and cytokinin,it was found that the content of auxin in the terminal bud of PtSHR2OE poplar increased;In mature axillary buds below BMP site,auxin content decreased and cytokinin content increased,resulting in an increase in cytokinin/auxin ratio.At the same time,the change of this ratio is accompanied by the change of the expression level of specific marker genes involved in axillary bud formation and germination.Subsequently,a series of physiological experiments were carried out on PtSHR2OE poplars and wild-type poplars,such as defoliation,truncation and exogenous addition of hormones or hormone inhibitors.It was found that PtSHR2OE axillary buds were more sensitive to exogenous NAA and cytokinin inhibitor lovastatin,which could even completely inhibit PtSHR2OE axillary bud germination.On the contrary,exogenous addition of auxin polar transport inhibitor NPA had little effect on the germination of poplar axillary buds.In conclusion,PtSHR2OE poplar may interfere with the downward transport of auxin from the top bud along the stem,while the enhancement of auxin efflux transport in axillary buds below BMP site relieves the inhibition of cytokinin synthesis,which leads to the change of the ratio of cytokinin to auxin in axillary buds,thus promoting the germination of PtSHR2OE mature axillary buds.Finally,this study found and revealed the new physiological functions of SHR transcription factors in regulating plant axillary bud germination,which is of great significance for studying the adaptability of woody plants to the environment;At the same time,it is also confirmed that the mechanism of SHR is evolutionarily conservative in herbaceous and woody plants.