Abstract
Within the last decade, NAC transcription factors have been shown to play essential roles in senescence, which is the focus of this review. Transcriptome analyses associate approximately one third of Arabidopsis NAC genes and many crop NAC genes with senescence, thereby implicating NAC genes as important regulators of the senescence process. The consensus DNA binding site of the NAC domain is used to predict NAC target genes, and protein interaction sites can be predicted for the intrinsically disordered transcription regulatory domains of NAC proteins. The molecular characteristics of these domains determine the interactions in gene regulatory networks. Emerging local NAC-centered gene regulatory networks reveal complex molecular mechanisms of stress- and hormone-regulated senescence and basic physiological steps of the senescence process. For example, through molecular interactions involving the hormone abscisic acid, ArabidopsisNAP promotes chlorophyll degradation, a hallmark of senescence. Furthermore, studies of the functional rice ortholog, OsNAP, suggest that NAC genes can be targeted to obtain specific changes in lifespan control and nutrient remobilization in crop plants. This is also exemplified by the wheat NAM1 genes which promote senescence and increase grain zinc, iron, and protein content. Thus, NAC genes are promising targets for fine-tuning senescence for increased yield and quality.
Highlights
Plant senescence has been extensively reviewed by a number of authors, and the general understanding of senescence in the developmental process is well established
Overexpression of AtNAP [58], ORE1 [59,65], Oresara sister1 (ORS1) [60], ANAC016 [63], and activating factor1 (ATAF1) [64] resulted in precocious senescence, whereas blocking the function of these genes delayed senescence suggesting that they function as nonredundant positive regulators of senescence in Arabidopsis
Future experiments will show if ANAC046 and DEHYDRATION RESPONSIVE ELEMENT-BINDING PROTEIN2A (DREB2A) and other transcription factors compete for interactions with RCD1, and if such interactions play a functional role during senescence (Figure 2b)
Summary
Plant senescence has been extensively reviewed by a number of authors (see e.g., [1,2,3,4,5,6,7,8,9,10]), and the general understanding of senescence in the developmental process is well established. Ethylene is essential for many plant processes such as seed germination, seedling development and fruit ripening, but it has been shown to have a role in the regulation of the onset of senescence [30]. We review the current knowledge of no apical meristem (NAM), ATAF1/2, cup-shaped cotyledon (CUC2) (NAC) transcription factor association with and function in senescence by relating NAC gene expression and structure to specific molecular interactions and presenting emerging local senescence-associated gene regulatory networks (GRNs) with central NAC genes and proteins. This reveals significant physiological functions of NAC transcription factors of putative use in future breeding for improved food quality
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