Zea mays Brittle1-1 (ZmBT1-1) is an essential component of the starch biosynthetic machinery in maize endosperms, enabling ADPglucose transport from cytosol to amyloplast in exchange for AMP or ADP. Although ZmBT1-1 has been long considered to be an amyloplast-specific marker, evidence has been provided that ZmBT1-1 is dually localized to plastids and mitochondria (Bahaji et al., 2011b). The mitochondrial localization of ZmBT1-1 suggested that this protein may have as-yet unidentified function(s). To understand the mitochondrial ZmBT1-1 function(s), we produced and characterized transgenic Zmbt1-1 plants expressing ZmBT1-1 delivered specifically to mitochondria. Metabolic and differential proteomic analyses showed down-regulation of sucrose synthase (SuSy)-mediated channeling of sucrose into starch metabolism, and up-regulation of the conversion of sucrose breakdown products generated by cell wall invertase (CWI) into ethanol and alanine, in Zmbt1-1 endosperms compared to wild-type. Electron microscopic analyses of Zmbt1-1 endosperm cells showed gross alterations in the mitochondrial ultrastructure. Notably, the protein expression pattern, metabolic profile, and aberrant mitochondrial ultrastructure of Zmbt1-1 endosperms were rescued by delivering ZmBT1-1 specifically to mitochondria. Results presented here provide evidence that the reduced starch content in Zmbt1-1 endosperms is at least partly due to (i) mitochondrial dysfunction, (ii) enhanced CWI-mediated channeling of sucrose into ethanol and alanine metabolism, and (iii) reduced SuSy-mediated channeling of sucrose into starch metabolism due to the lack of mitochondrial ZmBT1-1. Our results also strongly indicate that (a) mitochondrial ZmBT1-1 is an important determinant of the metabolic fate of sucrose entering the endosperm cells, and (b) plastidic ZmBT1-1 is not the sole ADPglucose transporter in maize endosperm amyloplasts. The possible involvement of mitochondrial ZmBT1-1 in exchange between intramitochondrial AMP and cytosolic ADP is discussed.

College of Agronomy and Plant Protection Qingdao Agricultural University Qingdao China

COMAV Institute for Conservation and Improvement of Valencian Agrodiversity Universitat Politècnica de València Valencia Spain

Department of Cell Biology Faculty of Science Centre of the Region Haná for Biotechnological and Agricultural Research Palacky University Olomouc Czechia

Instituto de Agrobiotecnología Consejo Superior de Investigaciones Científicas Gobierno de Navarra Navarra Spain

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