A Ca(2+)-dependent cysteine protease is associated with anoxia-induced root tip death in maize.
Imposition of anoxia on maize (Zea mays cv. B73) seedlings for 48 h or longer led to the death of the root tip. The necrosis extended into the root axis during postanoxic treatment, leading to the mortality of 30-50% of the seedlings. Using zymography, protease profiles in the root tissues of anoxic seedlings were studied. O2 deprivation for 24 h or longer repressed pre-existing protease activities and induced a novel soluble enzyme in the roots. The anoxia-induced protease (AIP) activity was predominant in the root apex at 24 h of anoxia and, subsequently, became the most abundant soluble activity in the root axis as well. The induction of AIP and its in vitro renaturation were Ca(2+)-dependent. Inhibitor sensitivity studies indicated that AIP is a cysteine protease. In SDS-acrylamide gels, the enzyme activity migrated as a 23.5 kDa polypeptide. The anoxic induction of the activity was repressed by cycloheximide treatment, suggesting that new protein synthesis was required for the AIP appearance. Excision of the root tip (de-tipping) before anoxia led to a superior recovery of seedlings from stress injury. De-tipped seedlings showed lesser root damage and an increased production of lateral roots compared to intact seedlings. Furthermore, the superior anoxia tolerance of de-tipped seedlings was associated with a decreased AIP activity. Thus, the appearance of AIP activity in the root tip at 24 h of anoxia was spatially and temporally associated with the root tissue death. These studies further indicate that the root tip elimination early during anoxia may provide an adaptive advantage.
Department of Crop Sciences, University of Illinois, Urbana 61801-4730, USA. subbaiah@uiuc.edu
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D000693:Anaerobiosis; D002118:Calcium / Q000378:metabolism; D003546:Cysteine Endopeptidases / Q000096:biosynthesis* / Q000378:metabolism; D004591:Electrophoresis, Polyacrylamide Gel; D018519:Meristem / Q000378:metabolism / Q000502:physiology; D009336:Necrosis; D010100:Oxygen / Q000378:metabolism*; D018517:Plant Roots / Q000378:metabolism / Q000502:physiology*; D003313:Zea mays / Q000378:metabolism / Q000502:physiology*
D003546:Cysteine Endopeptidases; D010100:Oxygen; D002118:Calcium