Submitted to: Plant Physiology
Publication Type: Peer reviewed journal
Publication Acceptance Date: 9/28/2000
Publication Date: N/A
Citation: Interpretive Summary: 1) Rationale: The goal here was to determine the role of a starch degrading enzyme during low oxygen conditions in maize during flooding. 2) Accomplishments: The results indicate that this enzyme is modified during flooding. This modified enzyme in turn modifies root tissue and appears to lead to the death of the root tip. This tissue draws a lot of energy from the rest of the seedling and is not necessary at this stage of the plant's growth. The process appears to be a mechanism to divert energy toward seedling survival during flooding stress. 3) Significance: The understanding of this enzyme's role in a plant's tolerance of low oxygen-stress conditions will allow for a greater understanding of how plants respond to this stress and may allow for effective methods to produce crop plants that are tolerant to flooding.
Technical Abstract: Root extracts made from maize (Zea mays L.) seedlings submerged for 2 h showed an increased labeling of a 90 kDa polypeptide, in a Ca2+-dependent manner. This protein was identified as sucrose synthase (SS) by immuno-precipitation and mutant analysis. Metabolic labeling with 32Pi indicated that the aerobic levels of SS phosphorylation were maintained or mildly increased up to 2 h of anoxia. In contrast, under prolonged anoxia, the protein was increasingly dephosphorylated and by 48 h, most of the protein existed in the unphosphorylated form. In seedlings submerged for 2 h or longer, a part of SS became associated with the microsomal fraction and this membrane localization of SS was confined only to the root apex. In O2 deprived seedlings, the SS redistribution preceded callose induction in the root tip and later its death. sh1 mutants lacked the anoxia-induced translocation of SS, indicating that it was the SS1 form of the enzyme that was redistributed during anoxia. Furthermore, sh1 mutants also showed less callose deposition and better survival than their normal siblings under prolonged anoxia. EGTA addition to the submergence buffer led to an increased dephosphorylation as well as membrane localization of SS, greater callose accumulation and necrosis, while Ca2+ addition decreased the proportion of membrane-bound SS and associated changes in the root tip. We propose that the dephosphorylation and membrane association of SS is an important early event in the death of the anoxic root tip.