Abstract
Storage roots are the main sink for photo-assimilate accumulation and reflect cassava yield and productivity. Regulation of sugar partitioning from leaves to storage roots has not been elucidated. Cell wall invertases are involved in the hydrolysis of sugar during phloem unloading of vascular plants to control plant development and sink strength but have rarely been studied in root crops like cassava. MeCWINV3 encodes a typical cell wall invertase in cassava and is mainly expressed in vascular bundles. The gene is highly expressed in leaves, especially mature leaves, in response to diurnal rhythm. When MeCWINV3 was overexpressed in cassava, sugar export from leaves to storage roots was largely inhibited and sucrose hydrolysis in leaves was accelerated, leading to increased transient starch accumulation by blocking starch degradation and reduced overall plant growth. The progress of leaf senescence was promoted in the MeCWINV3 over-expressed cassava plants with increased expression of senescence-related genes. Storage root development was also delayed because of dramatically reduced sugar allocation from leaves. As a result, the transcriptional expression of starch biosynthetic genes such as small subunit ADP-glucose pyrophosphorylase, granule-bound starch synthase I, and starch branching enzyme I was reduced in accordance with insufficient sugar supply in the storage roots of the transgenic plants. These results show that MeCWINV3 regulates sugar allocation from source to sink and maintains sugar balance in cassava, thus affecting yield of cassava storage roots.
Highlights
Cassava (Manihot esculenta Crantz) is one of the most important food crops producing starch as a source of dietary calories and for industrial applications, especially in the tropics (De Souza et al, 2017)
Amino acid sequence alignment showed that MeCWINV3 shared at least 71.26% identity with CWINs from A. thaliana, Bambusa oldhamii, Malus domestica, and Oryza sativa (Supplementary Figure S1A)
The phylogenetic tree constructed using all cassava and Arabidopsis cell wall INV (CWINV) genes showed that MeCWINV3 is closely related to AtcwINV1, AtcwINV3, and AtcwINV5, which belong to group III of CWINs in Arabidopsis (Yao et al, 2014; Supplementary Figure S1B)
Summary
Cassava (Manihot esculenta Crantz) is one of the most important food crops producing starch as a source of dietary calories and for industrial applications, especially in the tropics (De Souza et al, 2017). Since the substrates and products of this kind of enzyme are both nutrients and signal molecules, CWINVs participate in many aspects of plant development and growth (Roitsch and Gonzalez, 2004) These include sucrose and starch accumulation in carrot roots (Tang et al, 1999) and tomato fruit (Zhang N. et al, 2015); pathogenesis in tomato (Schaarschmidt et al, 2006; Kocal et al, 2008; Bonfig et al, 2010), tobacco (Essmann et al, 2008), and rice (Sun et al, 2014); and seed development in tomato (Jin et al, 2009), cotton (Wang and Ruan, 2012; Wang et al, 2013), maize, sorghum, and rice (Chourey et al, 2010; Jain et al, 2010; Li et al, 2013; French et al, 2014). The function and regulation of CWINV have been broadly studied in plant sink tissues (seed, fruit), the pivotal role of CWINV on sugar loading in source leaves has not been systematically studied and its effects on storage root development are unknown
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