title: The role of Tre6P and SnRK1 in maize early kernel development and events leading to stress-induced kernel abortion.
abstract: Drought stress during flowering is a major contributor to yield loss in maize. Genetic and biotechnological improvement in yield sustainability requires an understanding of the mechanisms underpinning yield loss. Sucrose starvation has been proposed as the cause for kernel abortion; however, potential targets for genetic improvement have not been identified. Field and greenhouse drought studies with maize are expensive and it can be difficult to reproduce results; therefore, an in vitro kernel culture method is presented as a proxy for drought stress occurring at the time of flowering in maize (3 days after pollination). This method is used to focus on the effects of drought on kernel metabolism, and the role of trehalose 6-phosphate (Tre6P) and the sucrose non-fermenting-1-related kinase (SnRK1) as potential regulators of this response.
  
  
  
  A precipitous drop in Tre6P is observed during the first two hours after removing the kernels from the plant, and the resulting changes in transcript abundance are indicative of an activation of SnRK1, and an immediate shift from anabolism to catabolism. Once Tre6P levels are depleted to below 1 nmol∙g-1FW in the kernel, SnRK1 remained active throughout the 96 h experiment, regardless of the presence or absence of sucrose in the medium. Recovery on sucrose enriched medium results in the restoration of sucrose synthesis and glycolysis. Biosynthetic processes including the citric acid cycle and protein and starch synthesis are inhibited by excision, and do not recover even after the re-addition of sucrose. It is also observed that excision induces the transcription of the sugar transporters SUT1 and SWEET1, the sucrose hydrolyzing enzymes CELL WALL INVERTASE 2 (INCW2) and SUCROSE SYNTHASE 1 (SUSY1), the class II TREHALOSE PHOSPHATE SYNTHASES (TPS), TREHALASE (TRE), and TREHALOSE PHOSPHATE PHOSPHATASE (ZmTPPA.3), previously shown to enhance drought tolerance (Nuccio et al., Nat Biotechnol (October 2014):1-13, 2015).
  
  
  
  The impact of kernel excision from the ear triggers a cascade of events starting with the precipitous drop in Tre6P levels. It is proposed that the removal of Tre6P suppression of SnRK1 activity results in transcription of putative SnRK1 target genes, and the metabolic transition from biosynthesis to catabolism. This highlights the importance of Tre6P in the metabolic response to starvation. We also present evidence that sugars can mediate the activation of SnRK1. The precipitous drop in Tre6P corresponds to a large increase in transcription of ZmTPPA.3, indicating that this specific enzyme may be responsible for the de-phosphorylation of Tre6P. The high levels of Tre6P in the immature embryo are likely important for preventing kernel abortion.
authors.affiliation: EAG Laboratories, 4780 Discovery Drive, Columbia, MO, 65201, USA.
authors.forename: Samuel W
authors.initials: SW
authors.lastname: Bledsoe
authors.affiliation: Hawkesbury Institute for the Environment, Western Sydney University, Locked Bag 1797, Penrith, NSW, 2751, Australia.
authors.forename: Clémence
authors.initials: C
authors.lastname: Henry
authors.affiliation: Rothamsted Research, West Common, Harpenden, Hertfordshire, AL5 2JQ, UK.
authors.forename: Cara A
authors.initials: CA
authors.lastname: Griffiths
authors.affiliation: Rothamsted Research, West Common, Harpenden, Hertfordshire, AL5 2JQ, UK.
authors.forename: Matthew J
authors.initials: MJ
authors.lastname: Paul
authors.affiliation: Max Planck Institut fϋr Moleckulare Pflanzenphysiologie, Potsdam (OT) Golm, Germany.
authors.forename: Regina
authors.initials: R
authors.lastname: Feil
authors.affiliation: Max Planck Institut fϋr Moleckulare Pflanzenphysiologie, Potsdam (OT) Golm, Germany.
authors.forename: John E
authors.initials: JE
authors.lastname: Lunn
authors.affiliation: Max Planck Institut fϋr Moleckulare Pflanzenphysiologie, Potsdam (OT) Golm, Germany.
authors.forename: Mark
authors.initials: M
authors.lastname: Stitt
authors.affiliation: Department of Agronomy and Horticulture, University of Nebraska-Lincoln, 377I Plant Science, Lincoln, NE, 68583-0915, USA. mlagrimini2@unl.edu.
authors.forename: L Mark
authors.initials: LM
authors.lastname: Lagrimini
mesh_terms: D018506:Gene Expression Regulation, Plant / Q000187:drug effects; D055442:Metabolome / Q000187:drug effects; D008954:Models, Biological; D010940:Plant Proteins / Q000378:metabolism*; D017346:Protein Serine-Threonine Kinases / Q000378:metabolism*; D012333:RNA, Messenger / Q000235:genetics / Q000378:metabolism; D060888:Real-Time Polymerase Chain Reaction; D012639:Seeds / Q000187:drug effects / Q000196:embryology* / Q000235:genetics; D013312:Stress, Physiological / Q000187:drug effects* / Q000235:genetics; D013395:Sucrose / Q000494:pharmacology; D013403:Sugar Phosphates / Q000494:pharmacology*; D014199:Trehalose / Q000031:analogs & derivatives* / Q000494:pharmacology; D003313:Zea mays / Q000187:drug effects / Q000196:embryology* / Q000235:genetics / Q000502:physiology*
chemical_list: D010940:Plant Proteins; D012333:RNA, Messenger; D013403:Sugar Phosphates; C082722:trehalose-6-phosphate; D013395:Sucrose; D014199:Trehalose; C071570:SNF1-related protein kinases; D017346:Protein Serine-Threonine Kinases
keywords: Maize; SnRK1; drought; kernel culture; starvation; sugar metabolism; trehalose-6-phosphate