A Botanist Discovers a Plant That Lacks the Ability to Form

A botanist discovers a plant that lacks the ability to form starch grains in root cells, yet the roots still grow downward. This evidence would refute the long-standing hypothesis that ________.

A) falling amyloplasts trigger gravitropism
B) starch accumulation triggers the negative phototropic response of roots
C) starch grains block the acid-growth response in roots
D) starch is converted to auxin, which causes the downward bending in roots
E) starch and downward movement are necessary for thigmotropism