Journal
JOURNAL OF EXPERIMENTAL BOTANY
Volume 51, Issue 345, Pages 721-730Publisher
OXFORD UNIV PRESS
DOI: 10.1093/jexbot/51.345.721
Keywords
anoxia; root tip; cell death; cysteine protease
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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. O-2 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 Ca2+-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.
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