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I am interested in molecular responses of plants to
abiotic stress, i.e., stress associated with the physical environment.
Current research is focused on hypoxic stress and abscisic acid (ABA),
a plant hormone associated with a number of stress responses.
 Our
hypoxic stress research began with the discovery that hemoglobins, normally
associated with mammals, were ubiquitous in plants. These hemoglobins are
unique in that they bind oxygen very tightly, resulting in their being
oxygenated at very low oxygen concentrations. One of these hemoglobin genes
was activated in plant tissues exposed to hypoxic conditions. In the course
of our research to relate hemoglobin expression with metabolism during
hypoxia, we found that nitric oxide (NO) production increased substantially
in hypoxic plant tissue. The amount of measurable NO varied inversely with
the hemoglobin levels in the hypoxic tissue. This lead us to experimentally
establish the existence of a hemoglobin/NO cycle (see figure) in hypoxic
plant tissue. The pathway cycles nitrate through nitrite, NO and back to
nitrate with consumption of NAD(P). The NO to nitrate conversion uses oxygenated
hemoglobin forming oxidized hemoglobin (methemoglobin), which is reduced back
to hemoglobin by NAD(P)H. Since the redox state in the cell environment can
become highly reductive during hypoxia, the cycle should result in the cell
being having a higher oxidative state when the cycle is operative. This is
observed experimentally in cells with varying expression of hemoglobin. Cells
expressing hemoglobin during hypoxia maintain their energy status better than
cells not expressing the gene. We think we have an explanation for this. We
have found that root mitochondria can synthesize ATP under strictly anaerobic
conditions using nitrite and NAD(P)H. NO is one of the products of the
reaction. We believe that the hemoglobin serves two functions during this
process: maintaining anaerobic conditions in the cell by its ability to
scavenge oxygen at very low oxygen concentrations, and; reacting with NO, to
remove a potentially toxic product, regenerating nitrate for the next round
of the cycle. Our work continues to relate these events to other processes in
the hypoxic cell.
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Recent Publications
Igamberdiev AU, Hill RD (2009) Plant mitochondrial function during
anaerobiosis. Ann Bot 103: 259-268.
Stoimenova
M, Igamberdiev AU, Gupta KJ, Hill
RD (2007) Nitrite-driven anaerobic ATP synthesis
in barley and rice root mitochondria. Planta 226: 465-474[pdf]
Igamberdiev
AU, Baron KN, Hill RD
(2007) Nitric Oxide as an Alternative Electron Carrier During Oxygen
Deprivation. In L Lamattina, JC Polacco, eds, Nitric Oxide in Plant Growth,
Development and Stress Physiology Vol 6. Springer Verlag, Berlin,
pp 255-268 [pdf]
Dordas C, Hill RD (2006) Production of nitric oxide
and reactions with plant hemoglobins under hypoxic stress. In Y Blume, D
Durzan, P Smertenko, eds, Cell Biology and Instrumentation: UV Radiation,
Nitric Oxide and Cell Death in Plants Vol 371. IOS Press, Amsterdam,
pp 218-226
Igamberdiev
AU, Stoimenova M, Seregélyes C, Hill RD (2006) Class-1 hemoglobin and
antioxidant metabolism in alfalfa roots. Planta 233: 1041-1046 [pdf]
Igamberdiev
AU, Bykova NV, Hill RD (2006) Nitric oxide scavenging by
barley hemoglobin is facilitated by a monodehydroascorbate reductase-mediated
ascorbate reduction of methemoglobin. Planta 233: 1033-1040 [pdf]
Nie X, Durnin
DC, Igamberdiev AU, Hill RD (2006) Cytosolic calcium is
involved in the regulation of barley hemoglobin gene expression. Planta 223:
542-549 [pdf]
Guy PA, Duff SMG, Nie X, Hill
RD, Durnin D, Sowa A (2005) Nonsymbiotic plant
hemoglobins to maintain cell energy status. US Patent No. 6936749
Igamberdiev AU, Baron K, Manac'h-Little N, Stoimenova M, Hill RD (2005)
The haemoglobin/nitric oxide cycle: Involvement in flooding stress and effects
on hormone signalling. Ann Bot 96: 557-564 [pdf]
Manac'h-Little N., Igamberdiev AU, Hill RD (2005) Hemoglobin
expression affects ethylene production in maize cell cultures. Plant Physiol
Biochem 43: 485-489 [pdf]
Igamberdiev AU and Hill RD
(2004) Nitrate, NO and hemoglobin in plant adaptation to hypoxia: An
alternative to classic fermentation pathways. J Exp Bot 55: 2473-2483 [pdf]
Seregélyes C, Igamberdiev AU, Maassen A, Hennig A, Dudits D,
Hill RD (2004) NO-degradation by alfalfa class 1 hemoglobin (Mhb1): a
possible link to PR-1a gene expression in Mhb1-overproducing tobacco plants.
FEBS Letters 571: 61-66 [pdf]
Igamberdiev AU, Bykova NV,
Ens W, Hill RD
(2004) Dihydrolipoamide dehydrogenase from porcine heart catalyzes NADH-dependent
scavenging of nitric oxide. FEBS Letters 568: 146-150 [pdf]
Dordas C, Hasinoff BB, Rivoal J, Hill RD
(2004) Class 1 hemoglobins, nitrate and NO levels in hypoxic maize cell
suspension cultures. Planta 219: 66-72 [pdf]
Igamberdiev AU, Seregélyes C, Manac'h N, Hill RD (2004) NADH-dependent
metabolism of nitric oxide in alfalfa root cultures expressing barley
hemoglobin. Planta 219: 95-102 [pdf]
Dordas C, Hasinoff BB, Igamberdiev AU, Manac’h N, Rivoal J, Hill RD (2003) Expression of
a stress-induced hemoglobin affects NO levels produced by alfalfa root cultures
under hypoxic stress. Plant Journal 35: 763-770 [pdf]
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