42. Genetic screens for mutants affecting development of leaf trichomes have led to the discovery of genes regulating trichome patterning – especially trichome density and spacing, as depicted in the figure below.42. Genetic screens for mutants affecting development of leaf trichomes have led to the discovery of genes regulating trichome patterning – especially trichome density and spacing, as depicted in the figure below.

The following statements are made in this regard:
A. GLABRAI (GLI) mutant plant will show fewer or no trichomes.
B. Cells that form trichomes strongly express the GLABRA2 (GL2) and TRYPTICON (TRY) genes.
C. TRY protein acts as a positive regulator of trichome cell differentiation in the surrounding cells.
D. Addition of exogenous JA will reduce the number of leaf trichomes.
Which one of the following options represents the combination of all correct statements?
(1) A and B                                                    (2) B and C
(3) C and D                                                    (4) A and D

The correct combination of all correct statements regarding leaf trichome patterning genes is:

(1) A and B

Explanation:

• (A) GLABRA1 (GL1) mutant plants show fewer or no trichomes. This gene encodes an R2R3 MYB transcription factor essential for trichome initiation; loss-of-function mutations cause glabrous (trichome-less) phenotypes.​

• (B) Cells forming trichomes strongly express GLABRA2 (GL2) and TRYPTICON (TRY) genes. GL2 regulates trichome development and spacing, while TRY encodes a single-repeat R3 MYB and is expressed in trichome and non-trichome cells; it participates in lateral inhibition for spacing.​

• (C) TRY protein acts as a positive regulator of trichome cell differentiation in surrounding cells — Incorrect. TRY is a negative regulator that suppresses trichome formation in neighboring cells, promoting proper spacing.​

• (D) Addition of exogenous jasmonic acid (JA) reduces leaf trichome numbers — Incorrect. JA actually promotes trichome initiation and increases trichome density in Arabidopsis and other plants.​

Introduction

Trichomes are hair-like epidermal structures on plant leaves that play roles in defense and adaptation. In Arabidopsis thaliana, genetic studies have uncovered a complex regulatory network involving transcription factors and hormones that control trichome initiation, density, and spatial distribution.

Key Genetic Factors in Trichome Development

• GLABRA1 (GL1): An R2R3 MYB transcription factor essential for initiating trichome development; mutants lack or have reduced trichomes.​

• GLABRA2 (GL2): A homeodomain protein expressed in trichome-forming cells, regulating their expansion and proper spacing.​

• TRYPTICON (TRY): Functions as an inhibitor to ensure proper spacing by repressing trichome fate in neighboring cells, thereby acting as a negative regulator.​

Hormonal Influence: Jasmonic Acid

• Jasmonic acid (JA) is a phytohormone that positively regulates trichome initiation and density.​

• Exogenous JA application increases rather than decreases trichome numbers in Arabidopsis and other species.

Mechanistic Overview

• Trichome development involves an activator complex (GL1, GL3/EGL3, and TTG1) that promotes GL2 expression driving trichome cell fate.​

• TRY acts in lateral inhibition by moving between cells to repress activator complex activity in adjacent cells to maintain spacing.​

Summary Table: Trichome Regulatory Components

Conclusion

The regulatory network for leaf trichome patterning in Arabidopsis centers on positive factors like GL1 and GL2 for trichome formation and negative regulators like TRY for spacing control. Jasmonic acid positively regulates this process, and the correct statements regarding these components are (A) and (B).