A chemical genetics approach reveals a role of brassinolide and cellulose synthase in hypocotyl elongation of etiolated Arabidopsis seedlings

The development of juvenile seedlings after germination is critical for the initial establishment of reproductive plants. Ethylene plays a pivotal role in the growth of seedlings under light or dark during early development. Previously, we identified small molecules sharing a quinazolinone backbone that suppressed the constitutive triple response phenotype in dark-grown eto1-4 seedlings. We designated these small molecules as ACS inhibitor quinazolinones (acsinones), which were uncompetitive inhibitors of 1-aminocyclopropane-1-carboxylic acid synthase. To explore the additional roles of acsinones in plants, we screened and identified 19 Arabidopsis mutants with reduced sensitivity to acsinone7303, which were collectively named revert to eto1 (ret) because of their recovery of the eto1 phenotype. A map-based cloning approach revealed that CELLULOSE SYNTHASE6 (CESA6) and DE-ETIOLATED2 (DET2) were mutated in ret8 (cesa6^ret8;eto1-4) and ret41 (det2^ret41;eto1-5), respectively. Etiolated seedlings of both ret8 and ret41 exhibit short hypocotyls and roots. Ethylene levels were similar in etiolated cesa6^ret8 and det2-1 and in eto1 mutants treated with acsinone7303. Chemical inhibitors of ethylene biosynthesis and perception did not significantly suppress the etiolated phenotype of cesa6^ret8 and det2^ret41. However, together with eto1, cesa6^ret8 and det2^ret41 exhibited an enhanced phenotype in the hypocotyls and apical hooks of etiolated seedlings. These results confirm that, in addition to ethylene, cellulose synthesis and brassinolides can independently contribute to modulate hypocotyl development in young seedlings.