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Sucrose transporter genes for efficient remobilization of stem water soluble carbohydrate to grain in wheat under different environments

Al-Sheikh Ahmed, Sarah Meqdam Taha (2020) Sucrose transporter genes for efficient remobilization of stem water soluble carbohydrate to grain in wheat under different environments. PhD thesis, Murdoch University.

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Wheat is one of the largest crops in the world with annual production of around 600 million tonnes. About 24 million tonnes of wheat is produced in Australia under rain-fed conditions and almost 80% of those is exported to overseas. Australia is experiencing climate change with increased incidence and severity of droughts and declining winter rainfall in areas with Mediterranean climate. Stem water soluble carbohydrate (WSC) deposited in wheat stems represent an important carbon sources for grain filling, especially under terminal drought conditions. Sucrose is the major transportable form of carbon in plants and the sucrose transporter (SUT) gene family is important in facilitating phloem loading and unloading. This PhD explores SUT functions on stem WSC remobilization under two environmental constraints, drought and nitrogen (N) supply in two wheat cultivars, Westonia and Kauz. It also examines diurnal changes in gene expression and WSC remobilization.

Firstly, to determine the major functional SUT gene groups in the shoot of wheat during grain development, drought tolerant varieties, Westonia and Kauz, were investigated in field drought experiments. The homologous genes to OsSUT1-5 were identified in Westonia and Kauz, namely TaSUT1_4A, TaSUT1_4B, TaSUT1_4D; TaSUT2_5A, TaSUT2_5B, TaSUT2_5D; TaSUT3_1A, TaSUT3_1D; TaSUT4_6A, TaSUT4_6B, TaSUT4_6D; TaSUT5_2A, TaSUT5_2B, and TaSUT5_2D. TaSUT1-5 gene expression patterns in stem, leaf sheath, rachis, lemma and developing grain were explored from pre-anthesis to grain maturity. TaSUT1 was the major sucrose transporting group in all studied organs and the expression was particularly higher in grain. TaSUT3 was preferentially expressed in the lemma before anthesis thus, it is hypothesised that it may contribute to pollination and seed setting. TaSUT5 was weakly expressed in developing grain while it was not expressed in other tissues.

Secondly, the effects of drought on expression of SUT in wheat under well-watered and drought conditions during grain filling are unknown. Plants were harvested from pre-anthesis to grain maturity, and the stem and developing grain were used for analysing TaSUT gene expression. The upregulation of TaSUT1 in Westonia in the stem and grain suggest a crucial role for the remobilization of stem WSC to grain under drought. Also, TaSUT1 gene expression was significantly correlated with high total grain weight (TGW) in Westonia under drought stress. In Kauz, the significant correlations between TaSUT1 gene expression and TGW and kernel number per spike demonstrated the contribution of TaSUT1 to high grain yield in an irrigated environment. Thus, efficient stem WSC remobilization to grain under drought should enhance grain yield.

Thirdly, this thesis characterised the diurnal patterns of WSC and its main components together with the TaSUT1 gene expression in flag leaves and main stems during grain filling. On average, the total WSC and fructan levels in the stems were double those in flag leaves, which further indicates the carbon storage function for wheat stems. Diurnal patterns of WSC and sucrose appeared mainly in leaves across all developmental stages, while diurnal effects for glucose and fructose were apparent before 7 days after anthesis (DAA). Leaf fructan diurnal patterns were exhibited at heading and at 14 DAA in both varieties, and at 21 DAA in Kauz; while in the stem a diurnal effect was present only at anthesis in Kauz. The significant correlations between the levels of TaSUT1 expression and sucrose indicate that TaSUT1 gene expression may be moderated by the level of sucrose. Elevated levels of TaSUT1 expression and sucrose in Kauz may play a role in securing grain yield.

Fourthly, N fertilizer is widely used by farmers around the world. Therefore, the effects of N fertilizer on yield components, water soluble carbohydrates and TaSUT1 gene expression were examined during grain filling. A field experiment was carried out in 2016 with Westonia and Kauz, and two N treatments (25 and 125 kg N ha-1, low N and high N treatments, respectively) at Wongan Hills in Western Australia. At the high N level, stem WSC increased by about 23% in both cultivars when grain filling commenced. Also, the high N treatment increased sucrose levels by around one third in Kauz and twice in Westonia. Moreover, high N promoted TaSUT1 gene expression in wheat stems. High N increased TaSUT1 expression about 2 and 3 folds in Kauz and Westonia, respectively, at 28 DAA. The high remobilization of sucrose may contribute to the higher grain weight as the grain weight per spike was almost doubled in Kauz and 20 % higher in Westonia at high N.

In conclusion, this thesis identified TaSUT1 as the major TaSUT gene family for sucrose transportation in wheat. TaSUT1 expression was strongly moderated by the sucrose levels in well-watered and N treatment conditions. The genotypic variations in TaSUT1 gene expression between Westonia and Kauz showed the differentiations of sucrose remobilization efficiency under drought environments. The genotypic differences at the gene sequence level need further investigation for TaSUT1 gene marker generation.

Item Type: Thesis (PhD)
Murdoch Affiliation(s): College of Science, Health, Engineering and Education
United Nations SDGs: Goal 12: Responsible Consumption and Production
Supervisor(s): Dell, Bernard and Zhang, Jingjuan
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