High temperature stress and flowering in brassica napus L.

Abstract

High temperature stress (HTS) adversely affects reproduction in most plant species studied to date. HTS during flowering may result in an almost total inhibition of seed production in crop plants. Increasing our knowledge of the effects of HTS on seed production will aid the breeding of more thermotolerant crop plants and improve our understanding of the effects of stress on plants. An investigation of the effects of both drought and high temperature stress on the yields of barley, canola, flax, durum and spring wheat in five locations in Saskatchewan over a 25-year period was performed using multivariate analysis. Higher temperatures during June and July, when the plants were flowering, were correlated with reductions in yields of all the crops studied (except barley in June). A positive correlation between yields and precipitation during May and the winter preceding the growing season was observed.In growth chambers, Brassica napus silique and seed production were inhibited during a ramping HTS treatment. This was due to a decrease in pollen germinability rather than a reduction in the number of flowers produced. HTS also caused reductions in megagametophyte fertility and disrupted embryo and/or seed development.Transgenic plants were developed to overcome the effects of HTS on seed production. Two DNA constructs, one with the Arabidopsis thaliana LEAFY (AtLFY) promoter controlling A. thaliana HEAT SHOCK PROTEIN 101 (AtHSP101) ORF expression and another with the AtHSP101 promoter controlling AtLFY ORF expression, were inserted into B. napus. Other DNA constructs were made, using the constitutively expressed Cauliflower Mosaic Virus 35S or the synthetic EntCup4 promoters to control expression of the AtHSP101 or A. thaliana HEAT SHOCK TRANSCRIPTION FACTOR 3 (AtHSF3) ORFs. These constructs were inserted into both B. napus and A. thaliana. Transgenic plants were tested using a ramping temperature regime but were found not to have increased flower thermotolerance. During the manufacture of the DNA constructs it was determined that, in A. thaliana, 573 bp of AtHSP101 had been copied between Terminal Inverted Repeats of a Mu-Like Element (MULE). This fragment was named HSP101B. In some transgenic B. napus and A. thaliana lines, containing 2046 bp of the HSP101B upstream regulatory region controlling B-glucuronidase (GUS) expression, cold-inducible GUS expression was observed. Methylation may have a role in control of endogenous HSP101B transcription.