Abstract
The formation of the apical hook in dicotyledonous seedlings is believed to be effected by gravity in the dark. However, this notion is mostly based on experiments with the hook formed on the hypocotyl, and no detailed studies are available with the developmental manners of the hook, particularly of the epicotyl hook. The present study aims at clarifying the dynamics of hook formation including the possible involvement of gravity. Time-course studies with normal Alaska pea (Pisum sativum L., cv. Alaska) and an agravitropic pea mutant, ageotropum, under the 1-g conditions and on a 3-D clinostat revealed that (1) the apical hook of the epicotyl forms by the development of the arc-shaped plumule of the embryo existing in the non-germinated seed. The process of formation consists of two stages: development and partial opening, which are controlled by some intrinsic property of the plumule, but not gravity. Approximately when the epicotyl emerges from the seed coat, the hook is established in both pea varieties. In Alaska the established hook is sustained or enhanced by gravity, resulting in a delay of hook opening compared with on a clinostat, which might give an incorrect idea that gravity causes hook formation. (2) During the hook development and opening processes the original plumular arc holds its orientation unchanged to be an established hook, which, therefore, is at the same side of the epicotyl axis as the cotyledons. This is true for both Alaska and ageotropum under 1-g conditions as well as on the clinostat, supporting finding (1). (3) Application of auxin polar transport inhibitors, hydroxyfluorenecarboxylic acid, naphthylphthalamic acid, and triiodobenzoic acid, suppressed the curvature of hook by equal extents in Alaska as well as ageotropum, suggesting that the hook development involves auxin polar transport probably asymmetrically distributed across the plumular axis by some intrinsic property of the plumule not directly related with gravity action.
Highlights
The apical hook is the arc-shaped transient structure formed in seed germination process on top of the hypocotyl or epicotyl of dicotyledonous seedlings
(2) During the hook development and opening processes the original plumular arc holds its orientation unchanged to be an established hook, which, is at the same side of the epicotyl axis as the cotyledons. This is true for both Alaska and ageotropum under 1-g conditions as well as on the clinostat, supporting finding (1). (3) Application of auxin polar transport inhibitors, hydroxyfluorenecarboxylic acid, naphthylphthalamic acid, and triiodobenzoic acid, suppressed the curvature of hook by equal extents in Alaska as well as ageotropum, suggesting that the hook development involves auxin polar transport probably asymmetrically distributed across the plumular axis by some intrinsic property of the plumule not directly related with gravity action
AGRAVITROPIC MUTANT AGEOTROPUM MIMICS ALASKA PEA SEEDLINGS GROWN UNDER MICROGRAVITY CONDITIONS Under the microgravity in space and a simulated microgravity on the 3-D clinostat in the dark, etiolated pea seedlings of normal cultivar Alaska represented abnormal growth and morphology, i.e., the epicotyl bearing the partially opened apical hook grew in the oblique direction deviated by about 40◦ away from the cotyledons and the root elongated in the oblique upward direction symmetric to the epicotyl
Summary
The apical hook is the arc-shaped transient structure formed in seed germination process on top of the hypocotyl or epicotyl of dicotyledonous seedlings. The anomalous shape occurs not at random but uniformly in the majority of seedlings tested, leading to the idea that it is regulated by some intrinsic property of the seedlings, which is manifested first when the action of gravity is removed. This concept was already proposed by Pfeffer (1904) as automorphosis (Eigenrichtung; reviewed by Stankovicet al., 1998) and served for explaining the establishment of intracellular polarity and determination of the growth direction in space (Volkmann et al, 1986) or on a clinostat (Hoson et al, 1992, 1996, 1997)
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