Abstract
Chlorophyll (Chl) degradation occurs during leaf senescence, embryo degreening, bud breaking, and fruit ripening. The Chl catabolic pathway has been intensively studied and nearly all the enzymes involved are identified and characterized; however, the molecular regulatory mechanisms of this pathway are largely unknown. In this study, we performed yeast one-hybrid screening using a transcription factor cDNA library to search for factors controlling the expression of Chl catabolic genes. We identified ANAC046 as a common regulator that directly binds to the promoter regions of NON-YELLOW COLORING1, STAY-GREEN1 (SGR1), SGR2, and PHEOPHORBIDE a OXYGENASE. Transgenic plants overexpressing ANAC046 exhibited an early-senescence phenotype and a lower Chl content in comparison with the wild-type plants, whereas loss-of-function mutants exhibited a delayed-senescence phenotype and a higher Chl content. Microarray analysis of ANAC046 transgenic plants showed that not only Chl catabolic genes but also senescence-associated genes were positively regulated by ANAC046. We conclude that ANAC046 is a positive regulator of Arabidopsis leaf senescence and exerts its effect by controlling the expression of Chl catabolic genes and senescence-associated genes.
Highlights
IntroductionThe most visually apparent phenomenon during leaf senescence, is caused by chlorophyll (Chl) degradation
Loss of green color, the most visually apparent phenomenon during leaf senescence, is caused by chlorophyll (Chl) degradation
To identify the transcription factors (TFs) that directly control Chl degradation, we first constructed yeast strains carrying the HIS3 reporter gene driven by the promoter regions of the following Chl catabolic genes: NON-YELLOW COLORING 1 (NYC1), NOL, hydroxymethyl-chlorophyll a reductase (HCAR), PPH, pheophorbide a oxygenase (PaO), RCC reductase (RCCR), CYP89A9, MES16, SGR1, and SGR2
Summary
The most visually apparent phenomenon during leaf senescence, is caused by chlorophyll (Chl) degradation. This process facilitates redistribution of nutrients from senescent leaves to reproductive organs and increases reproductive success[1]. Many Chl catabolic genes, including NYC1, PPH, PaO, MES16, and SGR1 are coordinately expressed and their transcript levels increase in senescing leaves[4]. Several NAC TFs, including AtNAP/ANAC02921, ORESARA1/ANAC09220–23, ORE1 SISTER1/ANAC059 (ORS1)[24], and AtNAC01625, positively regulate leaf senescence. Because the TF cDNA library is composed of 10% or less number of all Arabidopsis genes, the rate of false positives is much lower than in traditional screening Another advantage of this approach is that we can directly employ the promoter region as bait. Our analysis of mutant and transgenic ANAC046 plants showed that ANAC046, ORE1, and ORS1 share common functions and have distinct roles in leaf senescence
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