Frost, Abscisic Acid, and Desiccation Hasten Embryo Development in Brassica napus

Abstract

Seed development in canola (Brassica napus) following a mild nonlethal freeze was examined with respect to abscisic acid (ABA) levels, desiccation, and expression of LEA.76 and isocitrate lyase (ICL) transcripts. Plants with seed of 70 and 55% moisture contents were frozen to -50C for 3 hours, and seed development followed after thawing. In addition, similar processes were compared during induction of extreme desiccation tolerance by application of ABA in Brassica microspore-derived haploid embryos in culture. A mild freeze/thaw caused a premature switch in seed developmental direction from predesiccation to desiccation as indicated by an immediate and accelerated loss of seed moisture to levels similar to the mature seed in 7 instead of 35 days, and by elevated ABA levels and induction of low levels of LEA.76 and ICL transcripts. Similarly, addition of ABA to haploid embryos in culture resulted in the induction of desiccation tolerance and low levels of late embryogenesis-abundant (LEA) but not ICL transcripts. In contrast, normal seed development and desiccation of ABA-treated (desiccation-tolerant) embryos resulted in the induction of ICL and very high levels of LEA.76 transcripts. Similarly, desiccation of control (desiccation-sensitive) embryos resulted in very high levels of LEA.76 transcripts. These results indicate that although LEA-type proteins have been implicated in the development of desiccation tolerance, high transcript levels of LEA.76 were not observed in the induction of desiccation tolerance either by a hastening of the maturation process in the developing Brassica seed, or by the exogenous application of ABA to Brassica haploid embryos in culture.

Non-lethal freezing effects on seed degreening in Brassica napus

“…While moisture content of stressed seed, 24 h after the stress, was generally found to be $10% less than non-stressed seed at each DAF (other than 24 and 48 DAF), this loss was not found to be significant. These data confirm the earlier report of Johnson-Flanagan et al (1992), who also reported no difference in seed moisture content between stressed and non-stressed seed 24 h after a nonlethal freezing stress.…”

“…Correlations between changes in moisture, chlorophyll and ABA content of developing canola silique/seeds, as a result of a non-lethal freezing stress, have been previously reported (Johnson-Flanagan et al 1992;Green et al 1998). In part, we have confirmed these studies and taken the investigation a step further by determining chlorophyll a/b composition, visualizing moisture distribution, and measuring ABA and ABA-metabolite composition, during seed Fig.…”

“…n=2 for 1 and 3 day measurements and n=1 for 7 day measurement. Open bar, control; filled bar, treatment Flanagan et al 1992), ABA levels were lower 1, 3 and 7 days after exposure to a non-lethal freezing stress. The greatest decrease in ABA level was seen in stressed 32 DAF seeds, that is seed most susceptible to chlorophyll retention, where ABA concentrations were reduced eightfold over control levels, immediately after the stress and remained low over the time period of the experiment.…”

Enhancing the carotenoid content of Brassica napus seeds by downregulating lycopene epsilon cyclase

“…This earlier maturation may be the reason why the transgenic seeds had slightly lower fatty acid contents. Fatty acid accumulation is one of the last metabolic processes to be deactivated by seed desiccation and shorter maturation times lead to lower oil concentrations (Johnson-Flanagan et al 1992;Si and Walton 2004). Increasing ABA levels would induce earlier maturation and result in the earlier termination of storage molecule accumulation.…”

Microspore Culture of Winter Oil Rapeseed (Brassica Napus): II. Embryo Conversion, Plant Regeneration and Double Haploid Production ofBrassica Napus L.

“…Huang et al ( 1991) showed that culturing embryos at low temperature from 5 to 10°C for 28 days stimulate direct embryo germination and plant development. The application of ABA treatment alone without desiccation improved embryo germination in rapeseed (8). In order to improve plant regeneration Iqbal 31 ( 1993) stimulated embryo resting or maturation phase before germination by ABA and cold treatment.…”