Strigolactones negatively regulate mesocotyl elongation in rice during germination and growth in darkness

Abstract

Strigolactones (SLs) are newly discovered plant hormones that regulate plant growth and development including shoot branching. They also stimulate symbiosis with arbuscular mycorrhizal fungi. Rice has at least three genes that are involved in SL synthesis (D10, D17/HTD1 and D27) and at least two genes that are involved in SL signaling (D3) and SL signaling or downstream metabolism (D14/D88/HTD2). We observed that mesocotyl elongation in darkness was greater in rice mutants defective in these genes than in the wild type. Exogenous application of a synthetic SL analog, GR24, rescued the phenotype of mesocotyl elongation in the SL-deficient mutants, d10-1, d17-1 and d27-1, in a dose-dependent manner, but did not affect mesocotyl lengths of the SL-insensitive mutants, d3-1 and d14-1. No significant differences in cell length were found between the d mutants and the wild type, except for some cells on the lower half of the d3-1 mesocotyl that were shortened. On the other hand, the number of cells in the mesocotyls was 3- to 6-fold greater in the d mutants than in the wild type. Treatment with GR24 reduced the number of cells in the d10-1 mesocotyl to the wild-type level, but did not affect the number of cells in the d3-1 and d14-1 mesocotyls. These findings indicate that SLs negatively regulate cell division, but not cell elongation, in the mesocotyl during germination and growth of rice in darkness.

Fig. 1

Comparison of the morphology of d mutant and wild-type seedlings grown in the dark. (A) Phenotypes of seedlings of d mutants and the wild type (WT) when germinated and grown for 8 d under darkness. Upper arrows and lower arrows indicate the positions of the coleoptilar node and a basal part of the seminal root, respectively. The mesocotyl is the tissues between the two arrows. Bars = 1 cm. (B) Higher magnification of the mesocotyl regions in (A). Some roots were removed to clearly see the mesocotyls. Bars = 1 mm. (C) Length of mesocotyls of 8-day-old seedlings. Values are mean ± SD, n = 15. (D) Length of coleoptiles of 8-day-old seedlings. Values are mean ± SD, n = 15.

Fig. 2

Effect of GR24 on mesocotyl elongation of 8-day-old dark-grown d mutant and wild-type (WT) seedlings. Each value represents mean ± SD, n = 10–15.

Fig. 3

Time-course of mesocotyl elongation of dark-grown d mutant and wild-type (WT) seedlings. Blue and red curves indicate untreated and GR24-treated seedlings, respectively. Each value represents mean ± SD, n = 10.

Fig. 4

Cell lengths in mesocotyls of dark-grown seedlings. (A) Definition of the upper part and the lower part of the mesocotyl. (B) Vertical length of cells in the upper and lower parts of mesocotyls in 8-day-old seedlings treated with (+GR24) or without (−GR24) 1 μM GR24. Each value represents mean ± SD, n = 400–450. (C) Representative tissue sections of mesocotyls from the wild type (WT), d3-1, d10-1 and d14-1. Bars = 50 μm.

Fig. 5

Cell numbers and effect of GR24 on cell numbers in a vertical line in mesocotyls of dark-grown seedlings. Values indicate the numbers of cells between the coleoptilar node and the basal part of the seminal root in dark-grown 8-day-old seedlings of d mutants and the wild type (WT). GR24 concentration was 1 μM. Each value represents mean ± SD, n = 30.