Abstract
Leaf senescence, the last stage of leaf development, is a well-regulated and complex process for investigation. For simplification, dark-induced leaf senescence has frequently been used to mimic the natural senescence of leaves because many typical senescence symptoms, such as chlorophyll (Chl) and protein degradation, also occur under darkness. In this study, we compared the phenotypes of leaf senescence that occurred when detached leaves or intact plants were incubated in darkness to induce senescence. We found that the symptoms of non-programmed cell death (non-PCD) with remaining green coloration occurred more heavily in the senescent leaves of whole plants than in the detached leaves. The pheophorbide a (Pheide a) content was also shown to be much higher in senescent leaves when whole plants were incubated in darkness by analyses of leaf Chl and its metabolic intermediates. In addition, more serious non-PCD occurred and more Pheide a accumulated in senescent leaves during dark incubation if the soil used for plant growth contained more water. Under similar conditions, the non-PCD phenotype was alleviated and the accumulation of Pheide a was reduced by overexpressing 7-hydroxymethyl Chl a (HMChl a) reductase (HCAR). Taken together, we conclude that a high soil water content induced non-PCD by decreasing HCAR activity when whole plants were incubated in darkness to induce senescence; thus, the investigation of the fundamental aspects of biochemistry and the regulation of leaf senescence are affected by using dark-induced leaf senescence.
Highlights
Leaf senescence, the final stage in leaf development, is important for plants to recycle and reallocate valuable resources to actively growing organs [1,2]
It was shown that the senescence process of leaves with similar age was slower in intact plants than in detached leaves
A previous report showed that both hydroxymethyl Chl a (HMChl a) and Pheide a accumulated in the senescent leaves of WT and hcar after the dark incubation of intact plants [9]
Summary
The final stage in leaf development, is important for plants to recycle and reallocate valuable resources to actively growing organs [1,2]. Leaf senescence is a complex but finely regulated process during leaf development [3]. Leaf cells undergo a dramatic transition, including the disorganization of chloroplasts and the degradation of chlorophyll (Chl) and photosystems, resulting in the loss of green color. The breakdown of Chl is usually considered a biomarker of leaf senescence [4]. If Chl cannot be degraded when leaf senescence starts, the senescence process will be affected; senescence processes will become disordered [4,5]. Free Chl and its metabolic molecules are deleterious molecules that generate reactive oxygen species because of their light-absorbing properties [6]. Previous studies have shown that light-dependent and lightindependent non-programmed cell death (non-PCD)—shown by leaves dehydrated with
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