Rice Debranching Enzyme Isoamylase3 Facilitates Starch Metabolism and Affects Plastid Morphogenesis

Abstract

Debranching enzymes, which hydrolyze α-1 and 6-glucosidic linkages in α-polyglucans, play a dual role in the synthesis and degradation of starch in plants. A transposon-inserted rice mutant of isoamylase3 (isa3) contained an increased amount of starch in the leaf blade at the end of the night, indicating that ISA3 plays a role in the degradation of transitory starch during the night. An epitope-tagged ISA3 expressed in Escherichia coli exhibited hydrolytic activity on β-limit dextrin and amylopectin. We investigated whether ISA3 plays a role in amyloplast development and starch metabolism in the developing endosperm. ISA3–green fluorescent protein (GFP) fusion protein expressed under the control of the rice ISA3 promoter was targeted to the amyloplast stroma in the endosperm. Overexpression of ISA3 in the sugary1 mutant, which is deficient in ISA1 activity, did not convert water-soluble phytoglycogen to starch granules, indicating that ISA1 and ISA3 are not functionally redundant. Both overexpression and loss of function of ISA3 in the endosperm generated pleomorphic amyloplasts and starch granules. Furthermore, chloroplasts in the leaf blade of isa3 seedlings were large and pleomorphic. These results suggest that ISA3 facilitates starch metabolism and affects morphological characteristics of plastids in rice.

Deficiency of Starch Synthase IIIa and IVb Alters Starch Granule Morphology from Polyhedral to Spherical in Rice Endosperm

“…We also compared the levels of proteins involved in amyloplast division Kawagoe, 2009, 2010;Yun et al, 2011). The levels of FtsZ1 and FtsZ2-1 (components of the plastid division ring or Z-ring) increased only in ss3a ss4b #2013 (Supplemental Fig.…”

Starch Biosynthesis in the Developing Endosperms of Grasses and Cereals

“…In the ISA enzymes the function of the N-terminal domain may be to facilitate interactions between ISA1 and ISA2 (see below) [320]. The main function of both ISA3 and LDA is in the process of starch degradation and turnover [321][322][323][324], which is outside the scope of this review, and therefore these enzymes will not be discussed further.…”

The OsMYB30 Transcription Factor Suppresses Cold Tolerance by Interacting with a JAZ Protein and Suppressing β-Amylase Expression

“…A previous study reported that starch content and soluble sugars in leaves were responsive to various environmental stresses, but the complex starch synthesis and degradation processes were not completely revealed (Lu et al, 2005;Lu and Sharkey, 2006), and only a few studies on starch degradation were reported in rice (Kitajima et al, 2009;Yun et al, 2011;Hakata et al, 2012;Hirose et al, 2013). With the discovery of the maltose transporter MEX1, researchers considered that maltose was one of the predominant forms that was exported to the cytosol for further metabolism (Niittylä et al, 2004;Lu et al, 2005;Lu and Sharkey, 2006), and the key role in starch digestion was attributed to b-amylase (BMY; Orzechowski, 2008).…”