Question

Growth of both stem and root is mediated by auxin, stem grows into air while root grows into soil. The reason is
A) Differential reaction of root and stem to auxin
B) Only the stem tip produces auxin
C) Only the root tip produces cytokinins
D) Only the tour cap can perceive gravity

Answer 1

Auxins stimulate cell elongation or expansion in Stems and coleoptiles. According to the acid growth hypothesis, auxin promotes elongation growth by increasing cell wall extensibility. Auxin stimulates proton pumping by activating the proton pump.

Complete answer:
Auxin inhibits elongation of the primary root, but stimulates root initiation on stem cutting and lateral root development. Lateral roots are commonly found above the root hair zone and originate from cells present in the pericycle.

Cell division is involved in formation, maintenance and growth of the shoot apical meristem. Cytokinins are positive regulators of cell division in the root apical meristem. They are involved in the regulation of both G1-S and G2-M transitions.

Cytokinesis and auxin stimulate morphogenesis in plants in both shoot and root initiation in the value. When auxin is present alone, or if the ratio of auxin to cytokinins is high, callus will initiate root formation. Conversely, a high cytokinins to auxin promotes shoot formation. Roughly equal amounts of auxin and cytokinins promotes proliferation of undifferentiated callus.

Hence the correct answer is option A.

Note: Both cytokinins and auxin also induce crown gall formation in dicot plants. Cytokinins also promote delay in leaf senescence. Cytokinins promote the conversion of etioplasts into chloroplast by stimulating chlorophyll synthesis.