ASR1 Mediates Glucose-Hormone Cross Talk by Affecting Sugar Trafficking in Tobacco Plants      

Abstract

Asr (for ABA, stress, ripening) genes are exclusively found in the genomes of higher plants, and the encoded proteins have been found localized both to the nucleus and cytoplasm. However, before the mechanisms underlying the activity of ASR proteins can be determined, the role of these proteins in planta should be deciphered. Results from this study suggest that ASR is positioned within the signaling cascade of interactions among glucose, abscisic acid, and gibberellins. Tobacco (Nicotiana tabacum) transgenic lines with reduced levels of ASR protein showed impaired glucose metabolism and altered abscisic acid and gibberellin levels. These changes were associated with dwarfism, reduced carbon dioxide assimilation, and accelerated leaf senescence as a consequence of a fine regulation exerted by ASR to the glucose metabolism. This regulation resulted in an impact on glucose signaling mediated by Hexokinase1 and Snf1-related kinase, which would subsequently have been responsible for photosynthesis, leaf senescence, and hormone level alterations. It thus can be postulated that ASR is not only involved in the control of hexose uptake in heterotrophic organs, as we have previously reported, but also in the control of carbon fixation by the leaves mediated by a similar mechanism.

ASR1 transcription factor and its role in metabolism

“…ASR1 levels are inversely related to glucose levels in tobacco and maize leaves of Asr1 transgenic plants. 29,41 Moreover, Ht1 mRNA levels are reduced in the Asr1-silenced tobacco leaves. 41 HT1 is a plasma membrane transporter with high affinity for glucose, and is involved in the retrieval of hexoses from the apoplast in the phloem region of vascular bundles of grape leaves, petioles and fruits.…”

“…29,41 Moreover, Ht1 mRNA levels are reduced in the Asr1-silenced tobacco leaves. 41 HT1 is a plasma membrane transporter with high affinity for glucose, and is involved in the retrieval of hexoses from the apoplast in the phloem region of vascular bundles of grape leaves, petioles and fruits. 38,42,43 These findings suggest that ASR1 induces the expression of Ht1 in tobacco leaves and that in this way it regulates the glucose uptake from the apoplast into the phloem companion cells.…”

“…These plants showed altered glucose levels correlating with the levels of a hexose transporter mRNA in tubers. 40 Later studies revealed that Asr1-silenced tobacco plants showed increased glucose levels and lower Ht1 mRNA levels in source leaves 41 and that the heterologous over-expression of plantain ASR1 in Arabidopsis increased total soluble sugar levels in leaves. 26 In addition, Asr1 overexpression in maize plants increased yield and reduced sugar and amino acid levels; which suggests that ASR1 also participates in plant growth through the regulation of this kind of metabolites.…”

Senescence, dormancy and tillering in perennial C4 grasses

“…Sugar-sensing is an important circuit in plant cells [13,55,56], and seasonal changes in the flux in both the amounts and type of sugars could be important triggers of cell development and arrest in rhizomes. These triggers could lead to internal signaling cascades based on well-defined mechanisms involving sucrose non-fermenting-1 related kinase (SnRK-1), hexokinase, invertase, glucose levels and their intersection with hormone signaling [55,[57][58][59][60][61]. Presumably a shift in the balance from starch biosynthesis during most of the period of active shoot growth toward starch degradation once shoot senescence is initiated will also impact hexose pools within the rhizomes.…”

The stay-green phenotype of TaNAM-RNAi wheat plants is associated with maintenance of chloroplast structure and high enzymatic antioxidant activity

“…Determination of carbohydrates in the ethanol extract and the treated pellet was performed through the phenol-sulfuric method (Dubois et al, 1956). Additionally, sucrose, glucose and fructose were measured by an enzymatic method in the ethanol extract (Dominguez et al 2013). …”