Study On The Development Process Of Strobilus Organs And The Mechanism Of Gibberellin-induced Flowering Of Hybrid Larch

The supply of improved varieties of larch is limited by the instability of seed orchard yield.Seed yield has been improved to some extent through fertilization,hormone treatment,and tree management,but the situation of low yield and unstable seed setting has not been fundamentally resolved.Therefore,it is necessary to study the flowering mechanism of larch in order to improve the yield and stability of improved varieties.In this study,using the second generation seed orchard of hybrid larch as the material,the flowering biological characteristics,floral organ development process,and flowering induction mechanism of hybrid larch were studied by means of flowering observation,morphological anatomy,hormone determination,transcriptional sequencing,and exogenous hormone treatment,combined with morphological,cellular,physiological,and gene expression characteristics during the entire process from reproductive bud formation to pollination,Laying a theoretical foundation for high and stable yield in seed orchards.（1）The results of morphological observation and flower quantity investigation showed that the male and female cones of hybrid larch were mainly distributed in the middle and lower layers of the tree crown,the male cones were mainly planted on 2-5 year old branches,and the female cones were mainly planted on 3-5 year old branches.The flowering period is from about April 10th to about April 30th each year,and the pollination period is around April 25th.During pollination,the female gametophyte is in the free nucleus stage.（2）Through transcriptional sequencing,37 MIKC type MADS box genes were identified in male and female cones and vegetative twigs,23 of which were closely related to gender.LkoMADS1 and LkoMADS11 were identified as characteristic genes for female cone development,and LkoMADS21 was identified as characteristic genes for male cone development.（3）Combining phenotypic and molecular characteristics,the flowering induction period of hybrid larch was determined,and the development of new shoots of hybrid larch was divided into six stages:from early May to late June,the development period of new shoots was determined;The bud scale development period is from early July to late July;The end of July is the flowering induction period;The period from early August to late September is sexual organ development;The first ten days of October is the preparation period for overwintering;The flowering period is from early April to early May of the next year;Then a new cycle begins at the top of the nutrient short branch that opened in the same year.（4）Differential transcriptome analysis of different fertility materials during the flowering induction period showed that the AP2/EREBP gene family was significantly enriched during the flowering induction period,indicating that the family was involved in the flowering induction of hybrid larch 23 candidate genes for AP2/EREBP larch flowering induction were identified.The measurement of endogenous hormones during this period showed that gibberellin was the main regulatory hormone for flower induction in hybrid larch;Injection through the stem can increase the content of gibberellin in the new buds on the branches.Injecting 3ml of 500mg/L GA₃ solution has the best effect on promoting flowering and fruiting,and the closer the treatment time is to the flowering induction period,the better the effect is.Gibberellin is the main regulatory hormone in the flowering induction period of hybrid larch,and the gibberellin content in the shoots can be increased by stem injection.The GA₃ solution 3ml injected with 500mg/L had the best effect on promoting flowering and seeding,and the closer the treatment time was to the flowering induction period,the better the effect.（5）Combining multiple studies and response tests of related genes to gibberellin,it was predicted that the AP2 family and MADS family were involved in regulating the flowering pattern of hybrid larch.It was believed that exogenous gibberellin treatment could promote the flowering of hybrid larch by bypassing the inhibition of SVP-like on gibberellin and SOC1-like.