Abscisic acid substantially inhibits senescence of cucumber plants (Cucumis sativus) grown under low nitrogen conditions

https://doi.org/10.1016/j.jplph.2012.02.001Get rights and content

Abstract

Low nitrogen (N) availability such as that found in both dry land and tropical regions limits plant growth and development. The relationship between the level of abscisic acid (ABA) in a plant and its growth under low-N conditions was investigated. The level of ABA in cucumber (Cucumis sativus) plants under low-N conditions was significantly higher at 10 and 20 d after transplantation compared with that under sufficient-N conditions. Chlorophyll was preserved in the aerial parts of cucumber plants grown under low-N conditions in the presence of ABA, while there was no significant difference between control plants and ABA-applied plants under sufficient-N conditions. ABA suppressed the reduction of chlorophyll biosynthesis under low-N conditions but not under sufficient-N conditions. On the other hand, ABA decreased the expression of the chlorophyll degradation gene in older cucumber plants grown under both conditions. In addition, transcript and protein levels of a gene encoding a chlorophyll a/b binding protein were positively correlated with ABA concentration under low-N conditions. The chloroplasts in control plants were round, and the stack of thylakoid membranes was reduced compared with that of plants treated with ABA 10−5 M. These results strongly suggest that ABA is accumulated in cucumber plants grown under low-N conditions and that accumulated ABA promotes chlorophyll biosynthesis and inhibits its degradation in those plants.

Introduction

The availability of nitrogen (N) in natural soils frequently limits plant growth and development. Nitrogen-deficient areas have been found throughout the world, e.g., the soil N content is decreased in both dry land (Fan and Zhang, 2000) and tropical regions (Batjes, 1996). The production of plant biomass depends on the level of N in the soil. The artificial application of N to plants can result in a large increase in plant biomass (Taylor et al., 2005). Photosynthesis is also affected by the concentration of N in leaves (Sinclair and Horie, 1989). Plants grow slowly and have leaves that are pale green under deficient-N conditions. It is also the case that N moves into younger tissues from older ones (Mae and Ohira, 1981), so the structures of chloroplasts are substantially degraded for the senescence by nitrogen deficiency. Leaf senescence plays an important role in the plant's N economy (Feller and Fischer, 1994), and N deficiency limits plant growth and chlorophyll accumulation (Walker et al., 2001).

Plant hormones regulate the growth and development of plants under various environmental conditions related to changes in their qualities and quantities. There are numerous reports on the close relationships between leaf senescence and endogenous levels of abscisic acid (ABA). ABA content increases during senescence (Gepstein and Thimann, 1980, Dong et al., 2008) or N deficiency (Zdunek and Lips, 2001, Schraut et al., 2005). Although there are also numerous reports concerning the relationship between the level of ABA and chlorophyll degradation in leaves, as described above, we propose a new role for ABA in the accumulation of chlorophyll in cucumber (Cucumis sativus) leaves grown under low-N conditions. We recently found that the endogenous levels of ABA increased in cucumber plants grown under low-N conditions, and exogenously applied ABA resulted in the inhibition of the senescence of leaves in these plants. In this study, we report the effects of ABA on the biosynthesis and degradation of chlorophyll, and senescence in cucumber plants grown under low-N conditions in the presence of ABA.

Section snippets

Plant material

After being dipped in 70% ethanol for 30 s, cucumber (Cucumis sativus L. cv. Hokusin) seeds were sterilized in a solution of 10% bleach (Kao, Japan) for 10 min, rinsed with sterile water three times, and placed on 1% agar medium without nutrients for 8–10 d. Seedlings were transferred to Technopots (SUMIRON, Japan) with low-N (deficient-N) medium containing 1% agar medium, consisting of 2.5 mM K-PO4 buffer (pH5.5), 2 mM MgSO4, 50 μM Fe-EDTA, 70 μM H3BO3, 14 μM MnCl2, 0.5 μM CuSO4, 1 μM ZnSO4, 0.2 μM Na2MoO

Chlorophyll accumulation in cucumber plants in the presence or absence of ABA

Fig. 1 shows cucumber plants grown in the presence or absence of ABA under sufficient-N or low-N conditions at 30 d after ABA treatment. There was little effect on the leaf color between the control and ABA-applied cucumber plants under sufficient-N conditions. The cucumber plants grown in the ABA-free medium turned yellow under low-N conditions, while the plants grown in the medium containing ABA remained green in a dose-dependent manner even under same conditions (Fig. 1). The levels of

Discussion

Several reports show that the endogenous levels of ABA in plants increased during senescence (Ray et al., 1983, Kumar et al., 2008). When ABA was applied to detached leaves, chlorophyll degradation was greatly promoted (Gepstein and Thimann, 1980, Annamalainathan et al., 1996, Ferrante et al., 2004); however, ABA is less effective with regard to plant senescence and chlorophyll degradation in intact plants. Chlorophyll contents were slightly different between the control and ABA-applied plants

Acknowledgement

This study was supported in part by the Joint Research Grant made by the Arid Land Research Center, Tottori University.

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