Biomechanical, biochemical, and morphological mechanisms of heat shock-mediated germination in Carica papaya seed.
Carica papaya (papaya) seed germinate readily fresh from the fruit, but desiccation induces a dormant state. Dormancy can be released by exposure of the hydrated seed to a pulse of elevated temperature, typical of that encountered in its tropical habitat. Carica papaya is one of only a few species known to germinate in response to heat shock (HS) and we know little of the mechanisms that control germination in tropical ecosystems. Here we investigate the mechanisms that mediate HS-induced stimulation of germination in pre-dried and re-imbibed papaya seed. Exogenous gibberellic acid (GA3≥250 µM) overcame the requirement for HS to initiate germination. However, HS did not sensitise seeds to GA3, indicative that it may act independently of GA biosynthesis. Seed coat removal also overcame desiccation-imposed dormancy, indicative that resistance to radicle emergence is coat-imposed. Morphological and biomechanical studies identified that neither desiccation nor HS alter the physical structure or the mechanical strength of the seed coat. However, cycloheximide prevented both seed coat weakening and germination, implicating a requirement for de novo protein synthesis in both processes. The germination antagonist abscisic acid prevented radicle emergence but had no effect on papaya seed coat weakening. Desiccation therefore appears to reduce embryo growth potential, which is reversed by HS, without physically altering the mechanical properties of the seed coat. The ability to germinate in response to a HS may confer a competitive advantage to C. papaya, an opportunistic pioneer species, through detection of canopy removal in tropical forests.
Faculty of Life Sciences, University of Manchester, Oxford Road, Manchester M13 9PT, UK.
Rachel E
RE
Webster
Centre for Plant Sciences, University of Leeds, Woodhouse Lane, Leeds LS2 9JT, UK fbswmw@leeds.ac.uk.
Wanda M
WM
Waterworth
Royal Botanic Gardens, Kew, Wellcome Trust Millennium Building, Wakehurst Place, Ardingly, West Sussex RH17 6TN, UK.
Wolfgang
W
Stuppy
Centre for Plant Sciences, University of Leeds, Woodhouse Lane, Leeds LS2 9JT, UK.
Christopher E
CE
West
School of Biological, Biomedical and Environmental Sciences, The University of Hull HU6 7RX.
Roland
R
Ennos
Faculty of Life Sciences, University of Manchester, Oxford Road, Manchester M13 9PT, UK.
Clifford M
CM
Bray
Royal Botanic Gardens, Kew, Wellcome Trust Millennium Building, Wakehurst Place, Ardingly, West Sussex RH17 6TN, UK.
Hugh W
HW
Pritchard
D029441:Carica / Q000378:metabolism* / Q000502:physiology; D003513:Cycloheximide / Q000494:pharmacology; D003681:Dehydration; D018525:Germination / Q000187:drug effects / Q000502:physiology*; D005875:Gibberellins / Q000494:pharmacology; D018869:Heat-Shock Response / Q000502:physiology*; D006358:Hot Temperature; D057445:Plant Dormancy / Q000187:drug effects / Q000502:physiology; D010937:Plant Growth Regulators / Q000494:pharmacology; D011500:Protein Synthesis Inhibitors / Q000494:pharmacology; D012639:Seeds / Q000378:metabolism* / Q000502:physiology
D005875:Gibberellins; D010937:Plant Growth Regulators; D011500:Protein Synthesis Inhibitors; D003513:Cycloheximide; C007842:gibberellic acid

Carica papaya
; dormancy; germination; heat shock protein; seed coat; tropical crop.