Tepal Senescence Research Articles

DNA PROCESSING DURING TEPAL SENESCENCE OF SANDERSONIA AURANTIACA

DNA PROCESSING DURING TEPAL SENESCENCE OF SANDERSONIA AURANTIACA

Amino acid metabolism in senescing Sandersonia aurantiaca flowers: cloning and characterization of asparagine synthetase and glutamine synthetase cDNAs

We have studied nitrogen metabolism during senescence of the ethylene-insensitive flower Sandersonia aurantiaca Hook. We report here on the isolation and characterization of two cDNAs from senescing sandersonia tepals. SAND1 encodes asparagine synthetase (AS; EC 6.3.5.4) and SAND3 encodes glutamine synthetase (GS; EC 6.3.1.2). The accumulation of SAND3 mRNAs was consistently high throughout flower development and senescence and was not associated with senescence per se. SAND1 mRNAs started to accumulate in mature flowers just prior to visual signs of senescence. Sucrose feeding of individual flowers delayed tepal senescence and altered amino acid metabolism. The levels of Asn, Gln and Glu were higher and Asp levels were lower in the sucrose-fed flowers. The senescence-associated peak in Gln was delayed in sucrose-fed flowers compared to controls that were held in water. Sucrose feeding had no effect on the initiation of SAND1 transcription at the onset of flower senescence; however, the later decline in SAND1 mRNA abundance was delayed in sucrose-fed flowers. Elimination of GS activity and the subsequent reduction in Gln levels caused by phosphinothricin treat-ment was associated with a delay of SAND1 mRNA accumulation in senescing flowers.

Analysis of the expression of two thiolprotease genes from daylily (Hemerocallis spp.) during flower senescence.

A cDNA clone encoding a daylily (Hemerocallis spp.) thiolprotease (SEN11), whose expression is strongly upregulated in flower tepal senescence, has been isolated. The amino acid sequence, deduced from the nucleotide sequence, showed highest similarity to plant thiolproteases of Vigna mungo, Phaseolus vulgaris and Hemerocallis (SEN102), and contains a putative ER retention signal that has been described in Vigna mungo. SEN102 and SEN11 transcripts were not detectable in flower buds at the opening stage, but two peaks of transcripts were seen after 9 h and 19 h, in both petals and sepals, when wilting symptoms were apparent. The pattern of protease activity migrating on a 26.3 kDa protein was similar to the SEN102 and SEN11 transcript profiles. These two genes were also expressed in stamens and leaves, but their transcripts were undetectable in carpels and rhizomes. The expression of SEN102 was lower in the senescent leaf than in the green leaf. The pattern of expression of these genes suggests their involvement in the protein hydrolysis occurring in tepals at the late senescence stage, whereas in leaves they could be involved in the constitutive protein turnover machinery. Exogenous gibberellic acid application to cut flowers increased transcripts of both genes.

MEMBRANE DEGRADATION AND PHOSPHOLIPASE D ACTIVITY DURING DEVELOPMENTAL CELL DEATH IN IRIS FLOWERS

Iris x hollandica (cv. Blue Magic) flowers show the first symptoms of senescence within 4 days after the onset of flower opening. Wilting of the tepals starts at the distal edges, rapidly proceeds to the base, and is followed by total discolouration of the tepal. Senescence of Iris flowers is not induced or promoted by ethylene, the hormone often involved in senescence processes in plants. Whether other signals trigger cell death in Iris flowers is still unknown. The senescence process of Iris tepals is characterized by specific changes on the cellular level prior to the visible symptoms. Mesophyll cells show the presence of organelle desintegration, except for mitochondria, and their subsequent autophagic lysis in vacuoles. The vacuoles, in turn, deteriorate when they have emptied most of the cytoplasm and digested its constituents. The total protein and phospholipid content of the tepals decreases dramatically. At a later stage the plasma membrane loses its integrity, resulting in leakage of cellular contents and loss of turgor, which is accompanied by tepal wilting. The decrease in protein levels seems mainly the result of increased protein degradation by protease activity. Membrane degradation and decreased phospholipid contents could be caused by a concerted action of enzymes with lipase activity. Phospholipase D (PLD) may initiate phospholipid degradation in plants, as it has high abundance and because of the lipid degradation products found. Since PLD activity can be regulated on a transcriptional and post-translational level, an in vivo assay for monitoring PLD activity was developed. An early increase in PLD activity was observed, which correlated well with a decrease in phospholipid content. The results, therefore, strongly indicate that PLD is involved in membrane degradation associated with tepal senescence.

Water relations and senescence of cut Iris flowers: effects of cycloheximide

Abstract When placed in water directly after harvest, flowering stems of Iris x hollandica Tub., cultivar ‘Blue Magic’, showed inrolling of the tepal edges after about five days and tepal wilting after about six days. These senescence symptoms were not due to an occlusion in the stems as both the water balance and the water potential did not decrease substantially when the stem ends were held in water. Cycloheximide at 50 μM, included in the water after flower opening, delayed inrolling by about two and wilting by about four days. Cycloheximide resulted in rapid stomatal closure and a decrease in the rates of transpiration and water uptake, but its effect on senescence was not related with improvement of the water balance of the flowers. The results show that the delay of tepal senescence by cycloheximide is located in the tepal cells, and not in the overall water relations. Irreversible water loss from the tepals is apparently controlled by de novo synthesis of proteins. The results also indicate that flower senescence in ethylene-insensitive species, such as Iris , responds similarly to cycloheximide as carnation, an ethylene-sensitive flower.