Abstract
Cultivated sunflower (Helianthus annuus L.), an important source of edible vegetable oil, shows rapid onset of senescence, which limits production by reducing photosynthetic capacity under specific growing conditions. Carbon for grain filling depends strongly on light interception by green leaf area, which diminishes during grain filling due to leaf senescence. Transcription factors (TFs) regulate the progression of leaf senescence in plants and have been well explored in model systems, but information for many agronomic crops remains limited. Here, we characterize the expression profiles of a set of putative senescence associated genes (SAGs) identified by a candidate gene approach and sunflower microarray expression studies. We examined a time course of sunflower leaves undergoing natural senescence and used quantitative PCR (qPCR) to measure the expression of 11 candidate genes representing the NAC, WRKY, MYB and NF-Y TF families. In addition, we measured physiological parameters such as chlorophyll, total soluble sugars and nitrogen content. The expression of Ha-NAC01, Ha-NAC03, Ha-NAC04, Ha-NAC05 and Ha-MYB01 TFs increased before the remobilization rate increased and therefore, before the appearance of the first physiological symptoms of senescence, whereas Ha-NAC02 expression decreased. In addition, we also examined the trifurcate feed-forward pathway (involving ORE1, miR164, and ETHYLENE INSENSITIVE 2) previously reported for Arabidopsis. We measured transcription of Ha-NAC01 (the sunflower homolog of ORE1) and Ha-EIN2, along with the levels of miR164, in two leaves from different stem positions, and identified differences in transcription between basal and upper leaves. Interestingly, Ha-NAC01 and Ha-EIN2 transcription profiles showed an earlier up-regulation in upper leaves of plants close to maturity, compared with basal leaves of plants at pre-anthesis stages. These results suggest that the H. annuus TFs characterized in this work could play important roles as potential triggers of leaf senescence and thus can be considered putative candidate genes for senescence in sunflower.
Highlights
As the last stage of leaf development, the genetically determined and highly ordered process of senescence involves characteristic changes in gene expression that result in decreased photosynthetic activity, active degradation of cellular structures, nutrient recycling, lipid peroxidation and, cell death [1,2]
We evaluated the transcription profiles of NAC, AP2/EREBP, WRKY, MYB and NF-Y family Transcription factors (TFs) as potential candidate genes, as well as miR164, in sunflower leaves at different developmental stages during natural senescence
We evaluated these transcripts in association with physiological parameters, quantifying transcript levels over time in senescing sunflower leaves, to monitor the natural progression of senescence in this crop
Summary
As the last stage of leaf development, the genetically determined and highly ordered process of senescence involves characteristic changes in gene expression that result in decreased photosynthetic activity, active degradation of cellular structures, nutrient recycling, lipid peroxidation and, cell death [1,2]. Multiple variables control the complex mechanism of senescence; these genetic and environmental variables have a strong effect on crop yield [3]. Annual plants, such as grain and oil crops, undergo visible senescence towards the end of the reproductive stage, accompanied by nutrient remobilization from leaves to developing seeds [4]. The development of the reproductive structure controls leaf senescence [5]. Induced senescence, caused by biotic or abiotic stress, can reduce crop yield. Leaf senescence has an economic impact, affecting the gap between potential and real yields
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