Relationship between hexokinase and cytokinin in the regulation of leaf senescence and seed germination

Authors


  • Editor
    T. Elzenga

David Granot, Institute of Plant Sciences, Agricultural Research Organization, The Volcani Center, PO Box 6, Bet Dagan 50250, Israel.
E-mail: granot@agri.gov.il

Abstract

Arabidopsis hexokinase (AtHXK1), an enzyme that catalyses hexose phosphorylation, accelerates leaf senescence, whereas the plant hormone cytokinin inhibits senescence. Previous work in our laboratory has shown that isopentenyl transferase (IPT), a key gene in the biosynthesis of cytokinin, expressed under promoters of the senescence-associated genes SAG12 or SAG13 (PSAG12::IPT and PSAG13::IPT, respectively), inhibits leaf senescence in tomato plants. To study the relationship between hexokinase and cytokinin in the regulation of leaf senescence, we created and analysed double-transgenic tomato plants expressing both AtHXK1 and either PSAG12::IPT or PSAG13::IPT. We found that expression of IPT in the double-transgenic plants could not prevent the accelerated senescence induced by over-expression of AtHXK1. Since cytokinin inhibits senescence via an apoplastic invertase that produces extracellular hexoses, whereas AtHXK1 is an intracellular mitochondria-associated hexokinase, our results suggest that intracellular sugar sensing via AtHXK1 is dominant over extracellular sugar sensing with regard to leaf senescence. Interestingly, the heterologous SAG12 and SAG13 promoters are also expressed in germinating tomato seed, around the radicle penetration zone, suggesting that seed germination involves a senescence process that is probably necessary for radicle emergence. Indeed, seed expressing PSAG12::IPT and PSAG13::IPT exhibited delayed radicle emergence, possibly due to delayed endosperm senescence.

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