The mechanism of rhythmic ethylene production in sorghum. The role of phytochrome B and simulated shading

Abstract

Mutant sorghum (Sorghum bicolor [L.] Moench) deficient in functional phytochrome B exhibits reduced photoperiodic sensitivity and constitutively expresses a shade-avoidance phenotype. Under relatively bright, high red:far-red light, ethylene production by seedlings of wild-type and phytochrome B-mutant cultivars progresses through cycles in a circadian rhythm; however, the phytochrome B mutant produces ethylene peaks with approximately 10 times the amplitude of the wild type. Time-course northern blots show that the mutant's abundance of the 1-aminocyclopropane-1-carboxylic acid (ACC) oxidase mRNA SbACO2 is cyclic and is commensurate with ethylene production, and that ACC oxidase activity follows the same pattern. Both SbACO2 abundance and ACC oxidase activity in the wild-type plant are very low under this regimen. ACC levels in the two cultivars did not demonstrate fluctuations coincident with the ethylene produced. Simulated shading caused the wild-type plant to mimic the phenotype of the mutant and to produce high amplitude rhythms of ethylene evolution. The circadian feature of the ethylene cycle is conditionally present in the mutant and absent in the wild-type plant under simulated shading. SbACO2 abundance in both cultivars demonstrates a high-amplitude diurnal cycle under these conditions; however, ACC oxidase activity, although elevated, does not exhibit a clear rhythm correlated with ethylene production. ACC levels in both cultivars show fluctuations corresponding to the ethylene rhythm previously observed. It appears that at least two separate mechanisms may be involved in generating high-amplitude ethylene rhythms in sorghum, one in response to the loss of phytochrome B function and another in response to shading.

Figure 1

Diurnal SbACO2 mRNA abundance from sorghum grown under a 12-h/12-h photoperiod, 31°C/22°C thermoperiod. The first sample was from 5-d-old plants (58M is phyB-1, 100M is PHYB). White and black bars indicate light/warm and dark/cool periods, respectively.

Figure 2

Diurnal/circadian SbACO2 mRNA abundance from sorghum grown under a 12-h/12-h photoperiod, 31°C/22°C thermoperiod until 8 am of d 6, then in constant light at 27°C. The first sample was from 5-d-old plants (58M is phyB-1, 100M is PHYB). White and black bars indicate light/warm and dark/cool periods, respectively.

Figure 3

Diurnal/circadian ACC oxidase activity from sorghum grown under a 12-h/12-h photoperiod, 31°C/22°C thermoperiod until 8 am of d 6, then in constant light at 27°C. The first sample was from 5-d-old plants (58M is phyB-1, 100M is PHYB); n = 4, means ± se. White and black bars indicate light/warm and dark/cool periods, respectively. FW, Fresh weight.

Figure 4

Diurnal ACC levels from sorghum grown under a 12-h/12-h photoperiod, 31°C/22°C thermoperiod until 8 am of d 6, then in constant light at 27°C. The first sample was from 5-d-old plants (58M is phyB-1, 100M is PHYB); n = 4, means ± se. White and black bars indicate light/warm and dark/cool periods, respectively. FW, Fresh weight.

Figure 5

Diurnal SbACO2 mRNA abundance from sorghum grown under a simulated high-shade 12-h/12-h photoperiod, 31°C/22°C thermoperiod. The first sample was from 5-d-old plants (58M is phyB-1, 100M is PHYB). Gray and black bars indicate shaded light/warm and dark/cool periods, respectively.

Figure 6

Diurnal/circadian SbACO2 mRNA abundance from sorghum grown under a simulated high-shade 12-h/12-h photoperiod, 31°C/22°C thermoperiod until 8 am of d 6, then in constant light at 27°C. The first sample was from 5-d-old plants (58M is phyB-1, 100M is PHYB). Gray and black bars indicate shaded light/warm and dark/cool periods, respectively.

Figure 7

Ethylene production from sorghum grown under a simulated high-shade 12-h/12-h photoperiod, 31°C/22°C thermoperiod until 8 am of d 6, then given constant simulated high shade at 27°C. The first sample was from 5-d-old plants (58M is phyB-1, 100M is PHYB). n = 5, means ± se. Gray and black bars indicate shaded light/warm and dark/cool periods, respectively.

Figure 8

Diurnal/circadian SbCABII mRNA abundance from sorghum grown under either “normal light” (A) or simulated high shade (B) with a 12-h/12-h photoperiod, 31°C/22°C thermoperiod until 8 am of d 6, then in constant light at 27°C. The first sample was from 5-d-old plants (58M is phyB-1, 100M is PHYB). White (gray) and black bars indicate light (shaded light)/warm and dark/cool periods, respectively.

Figure 9

Diurnal/circadian ACC oxidase activity from sorghum grown under a simulated high-shade 12-h/12-h photoperiod, 31°C/22°C thermoperiod until 8 am of d 6, then in constant light at 27°C. The first sample is from 5-d-old plants (58M is phyB-1, 100M is PHYB). n = 4, means ± se. Gray and black bars indicate shaded light/warm and dark/cool periods, respectively. FW, Fresh weight.

Figure 10

Diurnal ACC levels from sorghum grown under a simulated high-shade 12-h/12-h photoperiod, 31°C/22°C thermoperiod until 8 am of d 6, then in constant light at 27°C. The first sample was from 5-d-old plants (58M is phyB-1, 100M is PHYB). n = 4, means ± se. Gray and black bars indicate shaded light/warm and dark/cool periods, respectively. FW, Fresh weight.

Figure 11

Ethylene production from sorghum grown under a simulated high-shade 12-h/12-h photoperiod, 31°C/22°C thermoperiod until 8 am of d 6, then given constant simulated high shade at 31°C. The first sample was from 5-d-old plants (58M is phyB-1, 100M is PHYB). n = 5, means ± se. Gray and black bars indicate shaded light/warm and dark/cool periods, respectively.