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The Involvement of Flowering-time and Stress-induced Genes in Regulating Various Early-flowering Phenotypes in Arabidopsis thaliana
|摘要:||在擬南芥中，早開花的性狀可由早開花基因的突變，或環境逆境如乾旱的刺激中得到。為探究擬南芥的早開花突變種elf1, 2及3中，控制早開花性狀的機制為何，我們分析種子萌發後不同天數的植株中，晚開花基因（late-flowering genes）及開花起始基因（floral-initiation genes）的表現。結果顯示，在elf1及elf3萌發3天後，GI的表現量上升。接著，發現這兩突變種在萌發後第5及8天的植株中CO, SOC1及LFY的表現量增加。然而在elf1及elf3中，FT的誘發表現是不同的。在elf3突變株中，FT在萌發後3天可發現其清楚的表現，而在elf1突變種中的表現卻直到萌發11天後才升高。與elf1及elf3不同的是，GI及CO在elf2突變種中的表現沒有影響，而FT, SOC1和LFY在elf2突變種中的表現皆上升。在此三種早開花突變種中AP1的表現量都被誘發。然而elf2及elf3（萌發後第5天）比elf1突變種還早被誘發（萌發後第11天）。此不同也許反映出elf2及elf3突變種開花較elf1突變種早的事實。在此三種早開花突變種中，LD的表現量不受到影響。我們的結果顯示，ELF1可能在光週期開花途徑（photoperiod flowering pathway）中位於位於GI的上游，直接抑制調控GI或間接調控GI上游之基因。ELF3可能作用於抑制一路徑，而此途徑不受光影響的正調節GI的活性。ELF2可能在光週期路徑上直接負調控FT，進而調節AP1及LFY。ELF1, 2及3基因在自主性開花路徑（autonomous flowering pathway）中，可能負調控某LD的下游基因。此研究結果支持先前遺傳上的雙突變種分析，那就是早開花基因位於晚開花基因的上游以調控開花時間。
相反的，乾旱及擁擠之逆境造成的早開花植株中，開花誘導基因（floral-induction genes）或開花起始基因（floral-initiation genes）的表現皆無被誘導上升的情形出現。為了更進一步探討逆境與開花之間的相互關係，擬南芥中的Arabidopsis Basic Chitinase Like gene 1（AtBCL1）基因被轉植入擬南芥植株。大量表現反股(sense)AtBCL1促進擬南芥提早開花，而擬南芥中大量表現反股(antisense)之AtBCL1則延遲開花。此顯示AtBCL1應參與促進花的轉換（flower transition）。然而不管sense或antisense植物其晚開花基因及開花起始基因的表現量皆不受影響。有趣的是，在ft及fwa突變種中AtBCL1的表現明顯減少，而在其他晚或早開花突變種內，AtBCL1的表現並沒有受到影響。此外，AtBCL1大量表現救回gi突變種的晚開花性狀，但卻無法互補ld的晚開花突變性狀。我們的結果顯示， 在擬南芥中AtBCL1作用於光週期開花路徑中FT之下游，以調節開花時間。|
In Arabidopsis thaliana, early-flowering phenotype can be obtained by mutations in early-flowering genes, or by environmental stress such as drought. To explore the mechanisms controlling early-flowering phenotype in Arabidopsis elf1, 2 and 3 mutants, the expression of the late-flowering and floral-initiation genes at different days after germination was analyzed. The result indicated that the expression of GI was increased 3 days after germination in elf1 and elf3 mutants. Later, the increase of CO, SOC1 and LFY expression was observed in these two mutants 5 to 8 days after germination. The induction of FT expression was however different in elf1 and elf3 mutants. In elf3 mutants, the induction of FT expression was clearly observed 3 days after germination whereas the FT expression was unaffected in elf1 mutant up to 16 days after germination. Different from elf1 and elf3, the expression of GI and CO was unaffected in elf2 mutants. The expression of FT, SOC1 and LFY was up-regulated in elf2 mutants. The expression of AP1 was induced in all three early flowering mutants. However, its induction was much earlier in elf2 and elf3 (5 days after germination) than in elf1 (11 days after germination). These differences may reflect the fact that elf2 and elf3 were flowering slightly earlier than elf1 mutants. The expression of LD was unaffected in these three early-flowering mutants. Our results indicated that ELF1 might directly negatively regulate GI or unidentified upstream genes for GI in photoperiod flowering pathway. ELF3 may function to suppress an identified constitutive pathway that positively regulated GI activity independently from photoperiod. ELF2 may directly negatively regulate FT in regulating AP1 and LFY in photoperiod pathway. ELF1, 2 and 3 genes may negatively regulate an unidentified gene upstream for LD in autonomous flowering pathway. The results obtained in this study supported previous genetic double mutant analysis which indicated that late-flowering mutants are epistatic to early-flowering mutants and early-flowering genes function upstream of late-flowering genes. By contrast, none of the expression of floral-induction or floral-initiation genes was induced in plants grown under stress such as drought and denseness. To further explore the relationship between stress and flowering, transgenic Arabidopsis plants ectopically expressed an Arabidopsis Basic Chitinase Like gene 1 (AtBCL1) were generated. Ectopic expression of AtBCL1 in Arabidopsis promoted flowering whereas the flowering time was delayed in Arabidopsis ectopically expressing antisense AtBCL1. This indicated that AtBCL1 is involved in the promotion of flower transition. However, the expression of late-flowering genes and flower initiation genes was unaffected in either sense or antisense plants. Interestingly, the expression of AtBCL1 was significantly down-regulated in late-flowering mutants ft and fwa and was not influenced in other late- or early- flowing mutants. Furthermore, ectopic expression of AtBCL1 rescued late-flowering phenotype in gi mutant but not for ld mutants. Our results indicated that AtBCL1 regulated flower transition by acting as a downstream gene for FT in the photoperiod flowering pathways in Arabidopsis.
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