Ethylene plays an important role in the regulation of many growth and developmental processes of higher plants. In tomato, Le-ACS6, a member of the ACC synthase multigene family involved in system 1 ethylene biosynthesis during fruit ripening, is subject to negative feedback regulation by ethylene. To identify the cis-elements that are responsible for the negative feedback control, we established an in vitro transient assay system employing particle bombardment on mature-green tomato fruit pericarp to examine the expression of a luciferase (LUC) reporter gene driven by a 5′-serially deleted Le-ACS6 promoter. The results localized putative cis-elements required for negative ethylene-response between −347 and −266 upstream from the translational start site ATG. Several lines of stable transformation of the Le-ACS6 promoter and GUS reporter fusion gene containing internal deletion from −347 to −266 were generated. The expression pattern of the GUS reporter showed that removal of the nucleotides from −347 to −266 completely eliminated the response of the Le-ACS6 promoter to exogenous ethylene.

Leaf senescence that occurs in the last stage of leaf development is a genetically programmed process. It is very significant to elucidate the molecular mechanisms that control the initiation and progression of leaf senescence and the way the senescence signal is transduced. In a previous study on artificially induced soybean leaf senescence, we cloned a novel gene designated rlpk2 (Genbank Accession No. AY687391) that encodes a leucine-rich repeat (LRR) receptor like protein kinase. The expression level of rlpk2 gene was shown to be strongly up-regulated during both the natural leaf senescence process in this report and the artificially induced primary-leaf-senescence process in our previous work. The RNA interference (RNAi)-mediated knocking-down of rlpk2 dramatically retarded both the natural and nutrient deficiency-induced leaf senescence in transgenic soybean. The transgenic leaves showed more cell-aggregated surface structure and higher content of chlorophyll.

Ethylene plays an essential role in response to hypoxic stress in plants. In most plant species, 1-aminocyclopropane- 1-carboxylate synthase (ACS) is the key enzyme that regulates the production of ethylene. We examined the expression of ACS genes in Arabidopsis during hypoxia. Our data showed that the expression of 4 of the 12 Arabidopsis ACS genes, ACS2, ACS6, ACS7, and ACS9, is induced during hypoxia with three distinct patterns. The hypoxic induction of ACS9 is inhibited by aminooxy acetic acid, an inhibitor of ethylene biosynthesis. In addition, the hypoxic induction of ACS9 is also reduced in etr1-1 and ein2-1, two ethylene insensitive mutants in ethylene-signaling pathways, whereas the addition of 1-aminocyclopropane-1-carboxylic acid, a direct precursor of ethylene, does not induce ACS9 under normoxic conditions. These results indicate that ethylene is needed, but not sufficient, for the induction of ACS9 during hypoxia. This pattern of regulation is similar to that of ADH that encodes alcohol dehydrogenase, which we have reported previously. In contrast, the increased ethylene production during hypoxia has an inhibitory effect on ACS2 induction in roots, whereas ethylene has no effect on the hypoxic induction of ACS6 and ACS7. Based on these results, we propose that two signaling pathways are triggered during hypoxia. One pathway leads to the activation of ACS2, ACS6, and ACS7, whereas the other pathway leads to the activation of ADH and ACS9.

We report here the cloning and characterization of a soybean receptor-like kinase (RLK) gene, designated GmSARK (Glycine max senescence- associated receptor-like kinase), which is involved in regulating leaf senescence. The conceptual protein product of GmSARK contains typical domains of LRR receptor-like kinases: a cytoplasmic domain with all the 11 kinase subdomains, a transmembrane domain and an extracelullar domain containing 9 Leucine-Rich Repeat (LRR) units that may act as a receptor. The expression of GmSARK in soybean leaves was up-regulated in all the three tested senescence systems: senescing cotyledons, dark-induced primary leaf senescence and the natural leaf senescence process after florescence. Furthermore, the RNA interference (RNAi)-mediated knocking-down of GmSARK dramatically retarded soybean leaf senescence. A more complex thylakoid membrane system, higher foliar level of chlorophyll content and a very remarkable delay of senescenceinduced disintegration of chloroplast structure were observed in GmSARK-RNAi transgenic leaves. A homolog of maize lethal leaf-spot 1 gene, which has been suggested to encode a key enzyme catalyzing chlorophyll breakdown, was isolated and nominated Gmlls1. The expression level of Gmgtr1 gene, which encodes a key enzyme of chlorophyll synthesis, was also analyzed. It was found that Gmlls1 was up-regulated and Gmgtr1 was down-regulated during senescence in wild-type soybean leaves. However, both of the up-regulation of Gmlls1 and down-regulation of Gmgtr1 were retarded during senescence of GmSARK-RNAi transgenic leaves. In addition, over-expression of the GmSARK gene greatly accelerated the senescence progression of CaMV 35S:GmSARK transgenic plants. Taken together, these results strongly suggested the involvement of this LRR-RLK in regulation of soybean leaf senescence, maybe via regulating chloroplast development and chlorophyll accumulation. Multiple functions of GmSARK besides its regulation of leaf senescence were also discussed.

亚硒酸钠能提高转绿叶片的抗氧化作用，而有助于叶片内叶绿素的积累和其前体ALA的形成。但施用亚硒酸钠的浓度范围较窄，若使用适量的亚硒酸纳有利于调节植物的生长发育。

The plasma membrane frac-tion isolated from primary leaves of 14 d soybean shoot showed protein kinase (PK) and protein phosphatase (PP) ac- tivities (Fig. 1), and these activities declined dramatically when the leaves underwent senescence-inducing treat- ment (Table 1). 6-BA pretreatment, which showed retarding effect on soy- bean leaf senescence, could sustain the activities of both Pigs and PPs during the senescence-inducing treatment (Table 1). The results of autoradiogra- phy suggested that the main PKs whose activities declined during leaf senes- cence were calcium-dependent protein kinases (Fig. 3). The potent and specif- ic inhibitor of protein phosphatase 1 and 2A, okadaic acid, could fasten leaf senescence. These results mplied that retaining the balance of plasma mem- rane protein phosphorylation/dephos- phorylation was the key action of exoge- nous 6-BA in retarding leaf senescence.

外源IAA 处理可以显著增加小麦胚芽鞘细胞NAD激酶的催化活性，钙离子可以增强IAA的作用效果，而钙离子通道抑制剂LaCl3则起强烈的抑制作用，但在存在钙离子的条件下，这种抑制作用可以被钙离子载体A23187消除；钙调蛋白能够在离体条件下激活经过DEAE纤维素柱纯化的小麦胚芽鞘NAD激酶，经过IAA处理的胚芽鞘细胞中能够刺激NAD激酶活性的钙调蛋白含量明显增加，IAA的这一作用受LaCl3的抑制。上述结果表明Ca2+ /CaM复合物介导了生长素对小麦胚芽鞘细胞NAD激酶活性的促进作用。

Applying 10 pmol of okadaic acid (OA), a specific inhibitor of type 1 or type 2A serine/threonine protein phosphatases, to the orchid (Phalaenopsis species) stigma induced a dramatic increase in ethylene production and an accelerated senescence of the whole flower. Aminoethoxyvinylglycine or silver thiosulfate, inhibitors of ethylene biosynthesis or action, respectively, effectively inhibited the OA-induced ethylene production and retarded flower senescence, suggesting that the protein phosphatase inhibitor induced orchid flower senescence through an ethylene-mediated signaling pathway. OA treatment induced a differential expression pattern for the 1-aminocyclopropane-1-carboxylic acid synthase multigene family. Accumulation of Phal-ACS1 transcript in the stigma, labelum, and ovary induced by OA were higher than those induced by pollination as determined by "semiquantitative" reverse transcriptase-polymerase chain reaction. In contrast, the transcript levels of Phal-ACS2 and Phal-ACS3 induced by OA were much lower than those induced by pollination. Staurosporine, a protein kinase inhibitor, on the other hand, inhibited the OA-induced Phal-ACS1 expression in the stigma and delayed flower senescence. Our results suggest that a hyper-phosphorylation status of an unidentified protein(s) is involved in up-regulating the expression of Phal-ACS1 gene resulting in increased ethylene production and accelerated the senescence process of orchid flower.

Half-fronds of Spirodela polyrrhiza accumulated high level of endogenous serine when they had been cultivated under long-day conditions and on a cytokinin-free medium for 4 days, after which the irreversible senescence processwas triggered. When 1 M6-benzyl adenine (6-BA) was included in the medium, the accumulation of endogenous serine was significantly inhibited, and at the same time the biological activity of half-fronds was increased. Application of exogenous l-serine into medium at 1mM initiated senescence of half-fronds, as well as intact plants, under the conditions of long-day and darkness. Application of 6-BA into the medium with l-serine postponed the appearance of serine level peak and senescence syndrome, although the senescence process was not prevented. It was also observed that half-fronds cultivated in the darkness on a medium with only inorganic nutrients had a very low level of endogenous serine and maintained their biological activity for up to 2 months. These data shows that high level of endogenous l-serine can initiate senescence in Spirodela polyrriza and cytokinin can inhibit the accumulation of endogenous l-serine. The possible reason for the accumulation of endogenous serine under long-day conditions was investigated. The activity of serine glyoxylate aminotransferase (SGAT), which catalyzes the conversion of l-serine to hydroxypyruvate, was found to decrease gradually in half-fronds during cultivation under long-day conditions, but to be maintained by 6-BA application. Northern blot analysis was used to detect the changes in the level of transcript encoding SAGT, and results showed that the expression of this gene was up-regulated by cytokinin but down-regulated during senescence.

磷酸化/脱磷酸化机制是众多信号转导过程中的重要环节，很多信号物质被细胞受体识别后引发蛋白激酶和蛋白磷酸酶活性变化，通过磷酸化/脱磷酸化进一步调节多种酶活性而产生各种生理效应。在对生长素IAA的信号转导的研究中，发现IAA处理的小麦胚芽鞘质膜蛋白中蛋白激酶的活性和蛋白磷酸化程度都发生改变，并找到两种受到调节的蛋白激酶。钙离子通道抑制剂LaCl3阻断了IAA的这种作用，表明Ca2+参与了IAA的信号转导过程。