Abstract
Many studies have shown that exogenous abscisic acid (ABA) promotes leaf abscission and senescence. However, owing to a lack of genetic evidence, ABA function in plant senescence has not been clearly defined. Here, two-leaf early-senescence mutants (eas) that were screened by chlorophyll fluorescence imaging and named eas1-1 and eas1-2 showed high photosynthetic capacity in the early stage of plant growth compared with the wild type. Gene mapping showed that eas1-1 and eas1-2 are two novel ABA2 allelic mutants. Under unstressed conditions, the eas1 mutations caused plant dwarf, early germination, larger stomatal apertures, and early leaf senescence compared with those of the wild type. Flow cytometry assays showed that the cell apoptosis rate in eas1 mutant leaves was higher than that of the wild type after day 30. A significant increase in the transcript levels of several senescence-associated genes, especially SAG12, was observed in eas1 mutant plants in the early stage of plant growth. More importantly, ABA-activated calcium channel activity in plasma membrane and induced the increase of cytoplasmic calcium concentration in guard cells are suppressed due to the mutation of EAS1. In contrast, the eas1 mutants lost chlorophyll and ion leakage significant faster than in the wild type under treatment with calcium channel blocker. Hence, our results indicate that endogenous ABA level is an important factor controlling the onset of leaf senescence through Ca2+ signaling.
Highlights
Leaf senescence, involving photosynthesis cessation, degradation of macromolecules, and increase of reactive oxygen species (ROS), as well as contributing to the mobilization of nutrients from old leaves to growing or storage tissues, is regulated by various external and internal factors
Recent studies showed that an Arabidopsis NAC-LIKE, ACTIVATED BY AP3/propidium iodide (PI) (NAP) transcription factor promotes chlorophyll degradation by enhancing transcription of ABSCISIC ALDEHYDE OXIDASE3 (AAO3), which leads to increased levels of the senescence-inducing hormone abscisic acid (ABA) (Yang et al, 2014)
We have established a leaf senescence screening system based on chlorophyll fluorescence and successfully isolated eas1mutantsby chlorophyll fluorescence imaging
Summary
Leaf senescence, involving photosynthesis cessation, degradation of macromolecules, and increase of reactive oxygen species (ROS), as well as contributing to the mobilization of nutrients from old leaves to growing or storage tissues, is regulated by various external and internal factors. It has long been considered that ABA accelerates leaf senescence because exogenously applied ABA was shown to promote leaf senescence (Gepstein and Thimann, 1980; Pourtau et al, 2004; Raab et al, 2009; Lee et al, 2011) and endogenous ABA levels have been found to be increased during leaf senescence in many plants (Gepstein and Thimann, 1980; Leon-Kloosterziel et al, 1996; Cheng et al, 2002; He et al, 2005; Breeze et al, 2011; Yang et al, 2014) Both the upregulation of genes associated with ABA signaling and a dramatic increase in endogenous ABA levels can be observed in many plants during leaf senescence (Tan et al, 2003). Leaf senescence was accelerated in transgenic plants overexpressing RPK1 and ABA-induced senescence was delayed in rpk mutant plants, suggesting that RPK1 has a role in promoting leaf senescence
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