Abstract
Gravity is a critical environmental factor affecting the morphology and function of plants on Earth. Gravistimulation triggered by changes in the gravity vector induces an increase in the cytoplasmic free calcium ion concentration ([Ca2+]c) as an early process of gravity sensing; however, its role and molecular mechanism are still unclear. When seedlings of Arabidopsis thaliana expressing apoaequorin were rotated from the upright position to the upside-down position, a biphasic [Ca2+]c-increase composed of a fast-transient [Ca2+]c-increase followed by a slow [Ca2+]c-increase was observed. We find here a novel type [Ca2+]c-increase, designated a very slow [Ca2+]c-increase that is observed when the seedlings were rotated back to the upright position from the upside-down position. The very slow [Ca2+]c-increase was strongly attenuated in knockout seedlings defective in MCA1, a mechanosensitive Ca2+-permeable channel (MSCC), and was partially restored in MCA1-complemented seedlings. The mechanosensitive ion channel blocker, gadolinium, blocked the very slow [Ca2+]c-increase. This is the first report suggesting the possible involvement of MCA1 in an early event related to gravity sensing in Arabidopsis seedlings.
Highlights
Gravity is a critical environmental factor affecting the morphology and function of plants on Earth
The slow [ Ca2+]c-increase has been demonstrated as a gravistimulation-specific C a2+-response using μg conditions produced by parabolic flights (PF)[13]; the μg conditions allowed the rotation of seedlings without gravistimulation, the fast-transient [ Ca2+]c-increase was induced by rotation under the μg condition, and the slow [Ca2+]c-increase was initiated by transition from μg to ca. 1.5 g when the μg condition was terminated[12,14], confirming the idea that the fast-transient is principally induced by rotational stimulation, and the slow increase is genuinely induced by gravistimulation
Elongation growth of hypocotyls was suppressed under the hypergravity condition in wild-type Arabidopsis seedlings, whereas the extent of the suppression was reduced in mca1-knockout seedlings, but was augmented in MCA1-overexpressing seedlings[28,29]. These findings suggest that MCA1 is a plasma membrane C a2+-permeable MS cation channel that is potentially involved in gravity sensing and the subsequent morphological changes in Arabidopsis seedlings
Summary
Gravity is a critical environmental factor affecting the morphology and function of plants on Earth. Gravistimulation triggered by changes in the gravity vector induces an increase in the cytoplasmic free calcium ion concentration ([Ca2+]c) as an early process of gravity sensing; its role and molecular mechanism are still unclear. When seedlings of Arabidopsis thaliana expressing apoaequorin were rotated from the upright position to the upside-down position, a biphasic [Ca2+]c-increase composed of a fast-transient [Ca2+]c-increase followed by a slow [Ca2+]c-increase was observed. The mechanosensitive ion channel blocker, gadolinium, blocked the very slow [Ca2+]c-increase This is the first report suggesting the possible involvement of MCA1 in an early event related to gravity sensing in Arabidopsis seedlings. Abbreviations [Ca2+]c Cytoplasmic free calcium ion concentration MCA1 mid1-Complementing activity 1 PFs Parabolic flights MSCC Mechanosensitive Ca2+-permeable channel IICR InsP3-induced Ca2+-release InsP3 Inositol 1,4,5-trisphosphate MscS Mechanosensitive channel of small conductance MS Mechanosensitive. The slow [ Ca2+]c-increase has been demonstrated as a gravistimulation-specific C a2+-response using μg conditions produced by parabolic flights (PF)[13]; the μg conditions allowed the rotation of seedlings without gravistimulation, the fast-transient [ Ca2+]c-increase was induced by rotation under the μg condition, and the slow [Ca2+]c-increase was initiated by transition from μg to ca. 1.5 g when the μg condition was terminated[12,14], confirming the idea that the fast-transient is principally induced by rotational stimulation, and the slow increase is genuinely induced by gravistimulation
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