Q49. Which ONE of the following organelles controls gravitropism in the roots
of higher plants?
(A) Chromoplast
(B) Amyloplast
(C) Chloroplast
(D) Etioplast

Amyloplasts control gravitropism in roots of higher plants, serving as statoliths that sediment to detect gravity direction.​

Correct Answer

The correct option is (B) Amyloplast. These starch-filled plastids in root columella cells act as gravity sensors by sedimenting downward, triggering auxin redistribution for downward root growth.​

Option Analysis

(A) Chromoplast

Chromoplasts store pigments like carotenoids and xanthophylls, providing color to flowers, fruits, and roots such as carrots. They aid pollination and seed dispersal but play no role in gravity sensing.​

(B) Amyloplast

Amyloplasts, dense due to starch granules, function as statoliths in root cap columella cells of higher plants. Upon reorientation, they sediment along the gravity vector, initiating signal transduction that relocalizes auxin transporters like PIN3 and PIN7, creating an auxin gradient for positive gravitropism.​

(C) Chloroplast

Chloroplasts contain chlorophyll for photosynthesis in green tissues, mainly leaves and stems. They contribute to shoot negative gravitropism in some contexts but lack the density for statolith function in roots.​

(D) Etioplast

Etioplasts form in dark-grown seedlings as precursors to chloroplasts, featuring prolamellar bodies. They transition to chloroplasts upon light exposure and do not sediment as statoliths for gravitropism.​

Gravitropism in roots of higher plants ensures downward growth for anchorage and nutrient uptake, primarily controlled by amyloplasts as sedimenting statoliths in columella cells. This process, key for CSIR NET Life Sciences, involves starch-filled amyloplasts detecting gravity and triggering auxin asymmetry.​

Gravitropism Mechanism

Amyloplasts sediment in root statocytes upon tilt, interacting with actin networks to activate mechanosensitive channels and relocalize PIN proteins. This creates lateral auxin flow, inhibiting elongation on the lower side for curvature. Starchless mutants confirm amyloplast density is essential.​

Role of Other Organelles

• Chromoplast: Pigment storage for coloration, irrelevant to gravitropism.​

• Chloroplast: Photosynthetic in shoots, minor shoot gravitropism role via starch but not root statoliths.​

• Etioplast: Dark-grown precursors to chloroplasts, no gravity-sensing function.​

Exam Relevance

For competitive exams like CSIR NET, recognize amyloplasts distinguish root positive gravitropism from shoot negative via endodermal amyloplasts.​