9
Hormone Regulation of Root Nodule
Formation in Lotus
Akihiro Suzuki
Abstract
Several phytohormones have been reported to positively or negatively
regulate the formation of nitrogen-fixing nodules inLotus japonicusand
other legumes. Auxin is important for root nodulation and required for
cortical cell division. Because auxin accumulation was observed in the root
of theL. japonicusmutantspontaneous nodule formation 2(snf2), which
has a gain-of-function mutation in a putative cytokinin receptor, it appears
that auxin acts downstream of cytokinin signaling. Activation of cytokinin
signaling is involved in the induction of root nodule formation. Ethylene,
gibberellin (GA), and abscisic acid (ABA) inhibit the cortical cell divisions
induced by cytokinin. ABA regulates nitrogenfixation activity through the
control of nitric oxide levels. Though jasmonic acid (JA) is known as a
negative regulator of nodulation, recent data suggest that it functions as a
positive regulator over a certain range of concentrations. The increase in
salicylic acid (SA) levels normally triggered as a defense response does not
occur upon infection with compatible symbionts.LjCCD7-silencedL.
japonicusplants, which were expected to have reduced concentrations of
strigolactone, produced fewer nodules than the controls, suggesting that
strigolactone promotes nodule formation inL. japonicus.9.1 Introduction
The root nodule, an organ where atmospheric
nitrogen isfixed, is formed by rhizobial infec-
tion. Because phytohormones are signal
molecules involved in plant development, mor-
phogenesis, and responses to the environment, it
is expected that most of them are directly or
indirectly involved in plant interaction with rhi-
zobia and in root nodule organogenesis. The
roles of phytohormones in root nodule formation
are gradually being discovered as molecular
genetics information from model legumes accu-
mulates. In this chapter, the phytohormones that
appear to play roles in root nodule formation are
reviewed.A. Suzuki (&)
Department of Environmental Sciences, Faculty of
Agriculture, Saga University, 1 Honjyo-machi, Saga
840-8502, Japan
e-mail: [email protected]
S. Tabata and J. Stougaard (eds.),The Lotus japonicus Genome, Compendium of Plant Genomes,
DOI 10.1007/978-3-662-44270-8_9,©Springer-Verlag Berlin Heidelberg 2014
85