Studies On The Molecular Mechanism Of Leaf Shape Development And Flowering Time In Cotton

Cotton spp.),as an important oil crop and cash crop in the world,is an important source of natural fiber.The leaf of the source organ is the main site of photosynthesis,and the flower of the sink organ is closely connected with the development of cotton fiber.The yield and fiber quality of cotton are influenced by the photosynthetic efficiency of leaves in vegetative growth,the resistance to diseases and insect pests,and the reasonable flowering time in reproductive growth.In terms of vegetative growth,cotton plants have a variety of leaf shapes,including broad leaves and lobed leaves.GhOKRA,a LATE MERISTEM IDENTITY 1(LMI1)-like gene,controls the development of an okra leaf shape.Class I KNOTTED1-like homeobox(KNOXI)genes are involved in compound leaf development and are repressed by the ASYMMETRIC LEAVES 1(AS1)-AS2 complex.We cloned the cotton homologs,GhAS1,GhAS2 and GhKNLs(GhKNL2-GhKNL8),corresponding to A.thaliana AS1,AS2 and KNOX1 genes.Through virus-induced gene silencing technology,we found that either GhAS1 or GhAS2-silenced cotton plants showed a great change in leaf shape from okra leaves to trifoliolate dissected leaves.In the shoot tips of these plants,the expression of the cotton ortholog of KNOX1,GhKNOTTED1-LIKE2/3/4(GhKNL2/3/4),was increased.However,GhKNLs-silenced plants maintained the wild-type okra leaves.A novel dissected-like leaf in A.thaliana was further generated by crossing plants constitutively expressing GhOKRA with either as1-101 or as2-101 mutant plants.The dissected-like leaves showed two different leaf vein patterns.This report reveals that the LMI1-like and KNOX1 genes coordinately control leaf development,and different combinations of these genes produce diverse leaf shapes including broad leaves,lobed leaves and compound leaves.This is the first report on the artificial generation of compound leaves from simple leaves in cotton.In terms of reproductive growth,FLOWERING LOCUS T(FT)and its homologs are the key flowering regulators,encoding florigen in many plant species.These proteins are very conserved in function.They are synthesized in leaves and transported to shoot apical meristem(SAM)through phloem,and then promote the expression of a series of floral meristem identity genes,and ultimately regulate the development of flower buds.The GmFT2a of soybean is a typical representative of this kind of gene in dicotyledons.In short-day photoperiod,it has been proven to have the function of promoting flowering.We synthesized the GmFT2a cDNA sequence using cotton codon preference modification,named as artificial-GmFT2a(aGmFT2a).We developed two transgenic 35S::aGmFT2a lines showed early flower phenotype.By quantitative PCR(qPCR)detection,we found that high expression of FT homologs in cotton strongly promoted flowering,and the branch pattern changed from monopodial to sympodial.When the expression decreased to the same level as the control,the cotton returned to vegetative growth,and the branch pattern changed from sympodial back to monopodial.Furthermore,we found that the high expression of FT homologs could promote the up-regulated expression of the floral meristem identity genes,GhAP1,GhFUL and GhLFY,in the cotton terminal bud,so as to promote the development of flower bud.We proved that FT homologs are functionally conserved in cotton,play a role in promoting flowering,and could affect branching patterns.The cDNA sequence of GhFT1 gene cloned from different cotton accessions,Taiyuan3,TM-1,Shiduan5 and Yucatanense,were found to be completely same and conserved.The GhFT1 expression in leaves of the above four accessions was further quantified during the vegetative growth period under long day conditions.The longer flowering time of cotton,the lower GhFT1 expression level.The expression of GhFT1 in Yucatanense was almost zero.The Yucatanense,a photoperiod sensitive accession grafted on a day-neutral accession rootstocks of TM-1,successfully induced flower buds under long day conditions.This result proved that the reason why Yucatanense is unable to bloom under long day conditions is due to the low expression of GhFT1,which may be due to the photoperiod sensitive genotypes upstream of GhFT1 in the photoperiod pathway.