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Abstract
Plants respond to a large variety of environmental signals, including changes in the gravity vector (gravistimulation). In Arabidopsis (Arabidopsis thaliana) seedlings, gravistimulation is known to increase the cytoplasmic free calcium concentration ([Ca(2+)](c)). However, organs responsible for the [Ca(2+)](c) increase and the underlying cellular/molecular mechanisms remain to be solved. In this study, using Arabidopsis seedlings expressing apoaequorin, a Ca(2+)-sensitive luminescent protein in combination with an ultrasensitive photon counting camera, we clarified the organs where [Ca(2+)](c) increases in response to gravistimulation and characterized the physiological and pharmacological properties of the [Ca(2+)](c) increase. When the seedlings were gravistimulated by turning 180 degrees, they showed a transient biphasic [Ca(2+)](c) increase in their hypocotyls and petioles. The second peak of the [Ca(2+)](c) increase depended on the angle but not the speed of rotation, whereas the initial peak showed diametrically opposite characters. This suggests that the second [Ca(2+)](c) increase is specific for changes in the gravity vector. The potential mechanosensitive Ca(2+)-permeable channel (MSCC) inhibitors Gd(3+) and La(3+), the Ca(2+) chelator 1,2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid (BAPTA), and the endomembrane Ca(2+)-permeable channel inhibitor ruthenium red suppressed the second [Ca(2+)](c) increase, suggesting that it arises from Ca(2+) influx via putative MSCCs in the plasma membrane and Ca(2+) release from intracellular Ca(2+) stores. Moreover, the second [Ca(2+)](c) increase was attenuated by actin-disrupting drugs cytochalasin B and latrunculin B but not by microtubule-disrupting drugs oryzalin and nocodazole, implying that actin filaments are partially involved in the hypothetical activation of Ca(2+)-permeable channels. These results suggest that the second [Ca(2+)](c) increase via MSCCs is a gravity response in the hypocotyl and petiole of Arabidopsis seedlings.
Highlights
Plants respond to a large variety of environmental signals, including changes in the gravity vector
We developed an imaging system to provide a spatial resolution of aequorin luminescence during gravistimulation, which clarified the organs that responded to gravistimulation in Arabidopsis seedlings
Changes in [Ca21]c induced by various endo- and exogenous signals have been extensively investigated in plants (Gilroy et al, 1993; Sanders et al, 2002; Scrase-Field and Knight, 2003)
Summary
A plate of seedlings of Arabidopsis expressing apoaequorin was mounted under an ultrasensitive photon-counting camera (PCC) in a light-tight dark box (Fig. 1A) and subjected to gravistimulation. When a plate of seedlings was subjected to gravistimulation by turning 180° at the speed of 6 rpm, a biphasic [Ca21]c transient was observed (Fig. 3A, white and black arrowheads), which was consistent with the previous report (Plieth and Trewavas, 2002). To characterize the biphasic [Ca21]c transient in more detail, we examined the effect of rotational angle and speed on the peak amplitudes of the initial and second [Ca21]c transients. The amplitude of the initial [Ca21]c transient was dependent on the speed of gravistimulation (Fig. 4B) but not on its angle (Fig. 3B), suggesting that the initial [Ca21]c transient is induced by forces related to the rotational speed. Forces generated by the rotational motion may be effectively transduced into MSCCs in the plasma membrane via actin filaments
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