Steroids are required for epidermal cell fate establishment in Arabidopsis roots

Abstract

The simple structure of Arabidopsis roots provides an excellent model system to study epidermal cell fate specification. Epidermal cells in contact with 2 underlying cortical cells differentiate into hair cells (H cells; trichoblasts), whereas cells that contact only a single cortical cell differentiate into mature hairless cells (N cells; atrichoblasts). This position-dependent patterning, in combination with the constrained orientation of cell divisions, results in hair and nonhair cell files running longitudinally along the root epidermis. Here, we present strong evidence that steroid hormones called brassinosteroids (BRs) are required to maintain position-dependent fate specification in roots. We show that BRs are required for normal expression levels and patterns of WEREWOLF (WER) and GLABRA2 (GL2), master regulators of epidermal patterning. Loss of BR signaling results in loss of hair cells in H positions, likely as a consequence of reduced expression of CAPRICE (CPC), a direct downstream target of WER. Our observations demonstrate that in addition to their well-known role in cell expansion, BRs play an essential role in directing cell fate.

Fig. 1.

BRs regulate expression levels of root epidermal cell markers. (A) Quantitative RT-PCR using wild-type roots treated with 10 nM BL shows increased expression of WER and GL2. (B) Treatment with cycloheximide (CHX) does not inhibit the induction of WER by BL. (C) The bri1 roots show decreased expression of WER, GL2, and CPC. Results are represented as mean ± standard error (n = 3 replicates).

Fig. 2.

Disruptions in the BR pathway lead to aberrant levels and patterning of GL2. (A–C) GL2∷GUS expression in roots of wild type (A) and bri1 mutants (B and C). (B) The bri1 roots were stained for 20 min, the same duration as shown for wild type. (C) The bri1 roots stained for 40 min. (D and E) GL2 shows an increased randomness of expression pattern on roots treated with BRZ (E) compared with the control roots (D). (F) BL treatment does not affect the pattern of GL2 expression. (G and H) Cross-section of GL2∷GUS plants in either wild-type (G) or bri1 background (H). (Scale bars: A–F, 50 μm; G and H, 25 μm.)

Fig. 3.

BRI1 regulates expression of CPC but not EGL3. (A and B) CPC∷GUS expression is reduced significantly in bri1-116 (B) in comparison with wild type (A). (C and D) The patterning expression of EGL3, a marker of H cell fate, is similar in wild type (C) and in bri1 (D). (Scale bars: 50 μm.)