Programmed cell death or desiccation tolerance: two possible routes for wheat endosperm cells

The fate of cells in the endosperm of developing wheat kernels was investig ated under normal conditions and upon premature slow drying on the cut ear. To follow the changes in membrane integrity and cellular ultrastructure, a n electron spin resonance (ESR) spin probe technique and low temperature sc anning electron microscopy (LTSEM) were used. ESR data indicated that durin g development, the relative amount of cells with intact membranes decreased and became almost zero at the stage of mass maturity, i.e. several days be fore the onset of maturation drying. This suggests that the death of starch y endosperm cells is a developmental phenomenon rather than one induced by water loss. Even at 8 days after anthesis (daa), early in the differentiati on phase, a considerable proportion of endosperm cells had already lost pla sma membrane integrity. Comparison of ESR data with LTSEM micrographs revea led that the loss of plasma membrane integrity occurred early in the starch accumulation process, from which it was concluded that the programmes of s tarch accumulation and developmental death are simultaneously switched on. When the differentiation into starchy endosperm was arrested by premature s low drying of kernels on the cut ear, meristematic cells (aleurone initials ) acquired desiccation tolerance as assessed by the combination of the spin probe technique and LTSEM. In contrast, fast drying caused immediate death of these cells. Thus, meristematic endosperm cells have the competence to acquire desiccation tolerance. This occurs upon premature slow drying or du ring normal differentiation into aleurone cells, but is lost when the cells differentiate into starchy endosperm.