Abstract
In the petals of some species of flowers, programmed cell death (PCD) begins earlier in mesophyll cells than in epidermal cells. However, PCD progression in each cell type has not been characterized in detail. We separately constructed a time course of biochemical signs and expression patterns of PCD-associated genes in epidermal and mesophyll cells in Lilium cv. Yelloween petals. Before visible signs of senescence could be observed, we found signs of PCD, including DNA degradation and decreased protein content in mesophyll cells only. In these cells, the total proteinase activity increased on the day after anthesis. Within 3 days after anthesis, the protein content decreased by 61.8%, and 22.8% of mesophyll cells was lost. A second peak of proteinase activity was observed on day 6, and the number of mesophyll cells decreased again from days 4 to 7. These biochemical and morphological results suggest that PCD progressed in steps during flower life in the mesophyll cells. PCD began in epidermal cells on day 5, in temporal synchrony with the time course of visible senescence. In the mesophyll cells, the KDEL-tailed cysteine proteinase (LoCYP) and S1/P1 nuclease (LoNUC) genes were upregulated before petal wilting, earlier than in epidermal cells. In contrast, relative to that in the mesophyll cells, the expression of the SAG12 cysteine proteinase homolog (LoSAG12) drastically increased in epidermal cells in the final stage of senescence. These results suggest that multiple PCD-associated genes differentially contribute to the time lag of PCD progression between epidermal and mesophyll cells of lily petals.
Highlights
Petal senescence is usually classified as a process of programmed cell death (PCD) in which cellular constituents are degraded and nutrients are remobilized from senescent tissues [1,2]
The early onset of PCD only occurred in some mesophyll cells, and 22.8% of the mesophyll cells were lost from day 0 to day 3 after anthesis
Expression of KDEL-type cysteine proteinase (LoCYP) and S1/P1 nuclease (LoNUC) genes may be involved in the early onset of PCD in mesophyll cells
Summary
Petal senescence is usually classified as a process of programmed cell death (PCD) in which cellular constituents are degraded and nutrients are remobilized from senescent tissues [1,2]. Expression of multiple genes encoding, such as nucleases, lipases, and proteinases, are involved in the process of PCD [2]. Programmed Cell Death in Epidermal and Mesophyll Cells majority of proteinase activity during petal senescence is because of cysteine-type proteinases [4,5]. Multiple genes of cysteine proteinases demonstrated different temporal expression patterns through development and aging [6]. Six of nine cysteine proteinase genes were found to be upregulated in the natural aging process, whereas three genes were highly expressed before visible symptoms of senescence were observed in petals and were downregulated in the senescent stage [6]
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