Induction of AGAMOUS gene expression plays a key role in ripening of tomato sepals in vitro

In vitro culture of VFNT Cherry tomato sepals (calyx) at 16–21 °C results in developmental changes that are similar to those that occur in fruit tissue [10]. Sepals become swollen, red, and succulent, produce ethylene, and have increased levels of polygalacturonase RNA. They also produce many flavor volatiles characteristic of ripe tomato fruit and undergo similar changes in sugar content [11]. We examined the expression of the tomato AGAMOUS gene, TAG1, in ripening, in vitro sepal cultures and other tissues from the plant and found that TAG1 RNA accumulates to higher levels than expected from data from other plants. Contrary to reports on the absence of AGAMOUS in sepals, TAG1 RNA levels in green sepals from greenhouse-grown plants is detectable, its concentration increasing with in vitro ripening to levels that were even higher than in red, ripe fruit. Sepals of fruit on transgenic tomato plants that expressed TAG1 ectopically were induced by low temperature to ripen in vivo, producing lycopene and undergoing cell wall softening as is characteristic of pericarpic tissue. We therefore propose that the induction of elevated TAG1 gene expression plays a key role in developmental changes that result in sepal ripening.     

A tomato HD-Zip homeobox protein, LeHB-1, plays an important role in floral organogenesis and ripening

Ethylene is required for climacteric fruit ripening. Inhibition of ethylene biosynthesis genes, 1-aminocyclopropane-1-carboxylate (ACC) synthase and ACC oxidase, prevents or delays ripening, but it is not known how these genes are modulated during normal development. LeHB-1, a previously uncharacterized tomato homeobox protein, was shown by gel retardation assay to interact with the promoter of LeACO1 , an ACC oxidase gene expressed during ripening. Inhibition of LeHB-1 mRNA accumulation in tomato fruit, using virus-induced gene silencing, greatly reduced LeACO1 mRNA levels, and inhibited ripening. Conversely, ectopic overexpression of LeHB-1 by viral delivery to developing flowers elsewhere on injected plants triggered altered floral organ morphology, including production of multiple flowers within one sepal whorl, fusion of sepals and petals, and conversion of sepals into carpel-like structures that grew into fruits and ripened. Our findings suggest that LeHB-1 is not only involved in the control of ripening but also plays a critical role in floral organogenesis.     

Genetic mapping of ripening and ethylene-related loci in tomato

 The regulation of tomato fruit development and ripening is influenced by a large number of loci as demonstrated by the number of existing non-allelic fruit development mutations and a multitude of genes showing ripening-related expression patterns. Furthermore, analysis of transgenic and naturally occurring tomato mutants confirms the pivotal role of the gaseous hormone ethylene in the regulation of climacteric ripening. Here we report RFLP mapping of 32 independent tomato loci corresponding to genes known or hypothesized to influence fruit ripening and/or ethylene response. Mapped ethylene-response sequences fall into the categories of genes involved in either hormone biosynthesis or perception, while additional ripening-related genes include those involved in cell-wall metabolism and pigment biosynthesis. The placement of ripening and ethylene-response loci on the tomato RFLP map will facilitate both the identification and exclusion of candidate gene sequences corresponding to identified single gene and quantitative trait loci contributing to fruit development and ethylene response. Received: 26 October 1998 / Accepted: 13 November 1998     

番茄果实中乙烯与多聚半乳糖醛酸酶的关系

乙烯与多聚半乳糖醛酸酶(PG)都是果实成熟过程中关键的调节因子.一方面,在有乙烯合成缺陷的转反义ACS番茄和乙烯感受缺陷的Nr突变体番茄果实中PG基因表达量都明显下降,PG酶活性明显降低;用外源乙烯(100 μL/L)处理绿熟期番茄果实使PG基因的表达明显增强,而1-甲基环丙烯(1-MCP,1 μL/L)处理转色期番茄果实明显抑制PG基因表达.另一方面,转反义PG基因番茄果实乙烯释放量在授粉后低于其野生型,番茄乙烯受体基因LeETR4和乙烯反应因子LeERF2基因表达量比野生种低.PG降解果胶的产物D-GA(100 mg/L)促进未熟期番茄果实中的乙烯生成和LeETR4、LeERF2基因的表达.     

PG在番茄果实成熟中的作用及二价金属离子与乙烯对PG活性的影响

对采后番茄果实的电镜观察表明:当果实成熟衰老时,叶绿体数量减少,多数基粒结构丧失;成熟果实胞壁中胶层水解成中空的电子透明区,初生壁的纤丝也发生一定程度的水解,相邻细胞分离;外源 PG(多聚半乳糖醛酸酶)提取物处理绿熟期果实组织,也可引起胞壁结构和叶绿体发生与正常衰老相同的变化。Ca~(2+)、Mg~(2+)、Co~(2+)二价金属离子处理果实,可明显降低番茄红素含量和 PG 活性,延缓果实软化。外源乙烯处理果实,可促进番茄红素的形成,提高 PG活性,并能解除钙对 PG 活性的抑制。本文也对 PG 在乙烯和 Ca~(2+)调节果实成熟中的作用进行了讨论。     

Antisense inhibition of the Nr gene restores normal ripening to the tomato Never-ripe mutant, consistent with the ethylene receptor-inhibition model

The hormone ethylene regulates many aspects of plant growth and development, including fruit ripening. In transgenic tomato (Lycopersicon esculentum) plants, antisense inhibition of ethylene biosynthetic genes results in inhibited or delayed ripening. The dominant tomato mutant, Never-ripe (Nr), is insensitive to ethylene and fruit fail to ripen. The Nr phenotype results from mutation of the ethylene receptor encoded by the NR gene, such that it can no longer bind the hormone. NR has homology to the Arabidopsis ethylene receptors. Studies on ethylene perception in Arabidopsis have demonstrated that receptors operate by a "receptor inhibition" mode of action, in which they actively repress ethylene responses in the absence of the hormone, and are inactive when bound to ethylene. In ripening tomato fruit, expression of NR is highly regulated, increasing in expression at the onset of ripening, coincident with increased ethylene production. This expression suggests a requirement for the NR gene product during the ripening process, and implies that ethylene signaling via the tomato NR receptor might not operate by receptor inhibition. We used antisense inhibition to investigate the role of NR in ripening tomato fruit and determine its mode of action. We demonstrate restoration of normal ripening in Nr fruit by inhibition of the mutant Nr gene, indicating that this receptor is not required for normal ripening, and confirming receptor inhibition as the mode of action of the NR protein.     

Cloning of 9-cis-epoxycarotenoid dioxygenase (NCED) gene and the role of ABA on fruit ripening

In order to understand more details about the role of abscisic acid (ABA) in fruit ripening and senescence, six 740 bp cDNAs (LeNCED1, LeNCED2, PpNCED1, VVNCED1, DKNCED1 and CMNCED1) which encode 9-cis-epoxycarotenoid dioxygenase (NCED) as a key enzyme in ABA biosynthesis, were cloned from fruits of tomato, peach, grape, persimmon and melon using an RT-PCR approach. A Blast homology search revealed a similarity of amino acid 85.76% between the NCEDs. A relationship between ABA and ethylene during ripening was also investigated. At the mature green stage, exogenous ABA treatment increased ABA content in flesh, and promoting ethylene synthesis and fruit ripening, while treatment with nordihydroguaiaretic acid (NDGA), inhibited them, delayed fruit ripening and softening. However, ABA inhibited the ethylene synthesis obviously while NDGA promoted them when treated the immature fruit with these chemicals. At the breaker, NDGA treatment cannot block ABA accumulation and ethylene synthesis. Based on the results obtained in this study, it was concluded that ABA plays different role in ethylene synthesis system in different stages of tomato fruit ripening.Key words: tomato, NCED gene, ABA, ethylene, fruit ripening, peach, grape, persimmon, melon     

Reduced ethylene synthesis by transgenic tomatoes expressing S-adenosylmethionine hydrolase

We have utilized a gene from bacteriophage T3 that encodes the enzyme S-adenosylmethionine hydrolase (SAMase) to generate transgenic tomato plants that produce fruit with a reduced capacity to synthesize ethylene. S-adenosylmethionine (SAM) is the metabolic precursor of 1-aminocyclopropane-1-carboxylic acid, the proximal precursor to ethylene. SAMase catalyzes the conversion of SAM to methylthioadenosine and homoserine. To restrict the presence of SAMase to ripening fruit, the promoter from the tomato E8 gene was used to regulate SAMase gene expression. Transgenic tomato plants containing the 1.1 kb E8 promoter bore fruit that expressed SAMase during the breaker and orange stage of fruit ripening and stopped expression after the fruit fully ripened. Plants containing the 2.3 kb E8 promoter expressed SAMase at higher levels during the post-breaker phases of fruit ripening and had a substantially reduced capacity to synthesize ethylene.