41. The following diagram represents a longitudinal section through an Arabidopsis shoot apical meristem (SAM) and leaf primordium at its flank. The dorsal (D) and ventral (V) domains are marked. The D and V genes are expressed in the dorsal and ventral domains, respectively Consider the following statements describing the phenotypes of leaf polarity.
A. Loss of D function makes the leaf ventralized whereas its overexpression dorsalizes the leaf.
B. Loss of V function makes the leaf dorsalized whereas its overexpression ventralizes the leaf.
C. Loss of microRNA miR166 dorsalizes the leaf whereas its overexpression ventralizes the leaf.
D. miR166 functions by inhibiting its target mRNA.
Which one of the following functional models best describes the above results?

The functional model that best describes the relationship among leaf polarity genes D, V, and miR166 based on the statements given is:

• (A) Loss of D function makes the leaf ventralized whereas its overexpression dorsalizes the leaf.

• (B) Loss of V function makes the leaf dorsalized whereas its overexpression ventralizes the leaf.

• (C) Loss of microRNA miR166 dorsalizes the leaf whereas its overexpression ventralizes the leaf.

• (D) miR166 functions by inhibiting its target mRNA.

These statements align with the known genetic and molecular control of leaf dorsoventral polarity:

• The D genes (e.g., HD-ZIP III family genes like PHABULOSA, PHAVOLUTA) specify adaxial/dorsal identity.

• The V genes specify ventral/abaxial identity.

• miR166 is expressed in the abaxial domain and restricts the expression of adaxializing HD-ZIP III genes by targeting their mRNA for degradation.

• Loss of miR166 results in enhanced adaxial/dorsal identity (dorsalized leaf), while overexpression causes ventralization by suppressing D gene expression.​

Thus, the functional model is:

• D genes promote dorsal identity, V genes promote ventral identity.

• miR166 negatively regulates D gene expression to maintain ventral identity.

• Loss-of-function or gain-of-function mutations in these genes reciprocally affect leaf polarity.

Introduction

Leaf polarity in plants, defined by the distinct dorsal (adaxial) and ventral (abaxial) surfaces, is critical for proper leaf function and development. Genetic factors including class III HD-ZIP transcription factors (D genes), ventral identity genes (V genes), and the microRNA miR166 form an integrated regulatory network controlling this polarity.

Molecular Mechanisms Underlying Leaf Polarity

• D genes (Class III HD-ZIPs): Expressed on leaf adaxial side, promote dorsal identity, and lamina development.​

• V genes: Expressed on abaxial/ventral leaf side, promote ventral identity opposing D genes.​

• miR166: Localizes on the abaxial side and restricts HD-ZIP III mRNA, preventing ectopic dorsal identity on the ventral side.​

• miR166 functions through RNA-induced silencing complex (RISC) mediating degradation of target HD-ZIP III transcripts.​

Phenotypic Effects of Genetic Alterations

• Loss of D gene function leads to ventralized, abaxialized leaves lacking typical dorsal characteristics.​

• Overexpression of D genes results in enhanced dorsal traits.​

• Loss of V gene function dorsalizes the leaf, with increased adaxial features.​

• Loss of miR166 leads to dorsalization due to unchecked D gene expression, while its overexpression ventralizes leaves by suppressing D genes.​

Summary Table: Gene and miRNA Functions in Leaf Polarity

Conclusion

The interaction between D genes, V genes, and miR166 microRNA establishes leaf dorsoventral polarity in Arabidopsis with miR166 restricting dorsal gene expression on the ventral side. The model reflects phenotype changes observed in gene loss and gain of function scenarios.