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Critical consideration on the relationship between auxin transport and calcium transients in gravity perception of Arabidopsis seedlings

Masatsugu Toyota, Takuya Furuichi, Hitoshi Tatsumi and Masahiro Sokabe

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Plants regulate their growth and morphogenesis in response to gravity field, known as gravitropism. In the early process of gravitropism, changes in the gravity vector (gravistimulation) are transduced into certain intracellular signals, termed gravity perception. The plant hormone auxin is not only a crucial factor to represent gravitropism but also a potential signaling molecule for gravity perception. Another strong candidate for the signaling molecule is calcium ion of which cytoplasmic concentration ([Ca2+]c) is known to increase in response to gravistimulation. However, relationship between these two factors, say which is in the first place, has been controversial. This issue is addressed here mainly based on recent progress including our latest studies. Gravistimulation by turning plants 180o induced a two-peaked [Ca2+]c-increase lasting for several minutes in Arabidopsis seedlings expressing apoaequorin; only the second peak was sensitive to the gravistimulation. Peak amplitudes of the [Ca²⁺]_c-increase were attenuated by the 10 µM auxin transport inhibitor (TIBA) and vesicle trafficking inhibitor (BFA), whereas the onset time and rate of rise of the second peak were not significantly altered. This result indicates that polar auxin transport is not involved in the initial phase of the second [Ca²⁺]_c-increase. It is likely that the gravi-induced [Ca²⁺]_c-increase constitutes an upstream event of the auxin transport, but may positively be modulated by auxin since its peak amplitude is attenuated by the inhibition of auxin transport.

Authors

Masatsugu Toyota

Department of Physiology, Nagoya University Graduate School of Medicine, Japan

Takuya Furuichi

Department of Physiology, Nagoya University Graduate School of Medicine, Japan; Molecular Plant Physiology, University of Erlangen, Germany

Hitoshi Tatsumi

Nagoya University Graduate School of Medicine

Masahiro Sokabe

Nagoya University Graduate School of Medicine; Japan Science and Technology Agency; National Institute for Physiological Sciences

Purchase article for $19

Subscribe to this journal for $79/year