38. Extensive molecular genetic studies on miR156, miR172, SPL genes and AP2- like genes have yielded the following functional model on the juvenile → adult → reproductive transition in Arabidopsis

Based on these results, the following schematic diagram has been proposed to predict the expression kinetics of these genetic factors:

Which of the following combinations is most likely to be correct?
(1) a-miR156; b-SPL genes; c -miR172; d – AP2 like genes
(2) a-miR156; b-miR172; c-:-SPL genes; d – AP2 like genes
(3) a-miR 172; b-SPL genes; c-AP2 like genes; d-miR156
(4) a-miR156; b-AP2 like genes; c. miR172; d-SPL genes

The most likely correct combination is:

(1) a – miR156; b – SPL genes; c – miR172; d – AP2 like genes

Explanation:

• miR156 (a) is highly expressed during the juvenile phase and represses SPL (SQUAMOSA PROMOTER BINDING PROTEIN-LIKE) genes (b) to maintain juvenility.​

• As miR156 expression decreases, SPL gene expression increases and promotes the expression of miR172 (c), which promotes the adult phase.​

• miR172 negatively regulates AP2-like transcription factors (d), which repress adult phase traits and flowering, allowing progression to reproductive development.​

Introduction

In Arabidopsis thaliana, vegetative phase change and the transition to reproductive development is tightly regulated by a genetic regulatory network involving microRNAs and transcription factors. The sequential interplay of miR156, SPL genes, miR172, and AP2-like genes orchestrates the timing and progression of developmental phases from juvenile to adult and finally to reproductive stages.

Functional Dynamics of the Genetic Factors

• miR156 (a): Highly abundant in juvenile plants, miR156 suppresses SPL transcription factors to maintain the juvenile state.​

• SPL genes (b): As plants age, the miR156 levels decline, leading to increased SPL gene expression which promotes adult traits and triggers the expression of miR172.​

• miR172 (c): This miRNA accumulates during the adult phase and targets AP2-like transcription factors for repression, thereby facilitating floral transition and adult phase traits.​

• AP2-like genes (d): These act as repressors of adult traits and flowering; downregulation by miR172 permits phase progression.​

Regulatory Feedback and Phase Transition

• The system forms feedback loops where SPL genes promote miR172, and miR172 suppresses AP2-like genes, while miR156 maintains suppression of SPL genes during juvenility.​

• These interactions ensure coordinated temporal control, underpinning complex developmental transitions.

Summary Table: Genetic Factors in Arabidopsis Phase Transition

Conclusion

The developmental transition in Arabidopsis from juvenile to adult to reproductive stages is regulated sequentially by miR156, SPL genes, miR172, and AP2-like genes, consistent with option (1).