Fruit dehiscence in Brassicas
Dehiscence is a means by which some wild plants release their seeds. In Brassicas the mature fruit or 'pod', strictly a silique, releases seed by a sometimes explosive mechanism triggered by mechanical pressure and referred to as 'shatter'. This mechanism is a problem in Brassica crop plants and results in loss of seed, and hence loss of revenue, during harvesting. This problem is further compounded by the distribution of volunteers which contaminate future crops and the environment. The post-fertilisation development of the carpel wall of a number of Brassica species has been examined including, a range of Arabidopsis ecotypes and mutants, and fruits from two other Brassicas, Brassica napus and Brassica juncea, which exhibit differences in the dehiscence characteristic. These have been studied by a combination of cytological, cytochemical and molecular techniques. Following fertilisation, dehiscence zones form at the carpel margins, separating the carpel walls from the replum and forming two valves. Cells within the dehiscence zone exhibit reduced cellular cohesion due to breakdown of the middle lamella. Differentiation of the carpel wall layers results in a thickened exocarp, a senescing mesocarp, and modification of the endocarp layers in which the inner layer Enϸ lignifies whilst Enα collapses. It is proposed that the patterns of differentiation result in the development of the dehiscence mechanism. The dehiscence mechanism and pod 'shatter' is a result of; 1) weakening of valve attachment due to reduced cell cohesion in the dehiscence zone, and, 2) tensions which develop within the carpel walls due to desiccation and shrinkage of the mesocarp which is attached to a thickened, non-shrinking endocarp.The fruits from all of the Arabidopsis ecotypes examined exhibited a similar pattern of carpel wall development and similar dehiscence characteristics. Light microscopical examination of the fruits of Brassica napus and Brassica juncea which do not shatter as easily as those of Arabidopsis showed a different pattern of endocarp development in the post fertilised fruit. Enα tangential walls thickened considerably in the post-fertilised Brassica napus and Brassica juncea fruit, prior to the collapse of this cell layer. In Indian mustard, the Brassica juncea variety which had a non-shattering phenotype, the lignified walls of En6 were surrounded by a highly pectinised layer. This deposition of pectins confers more elasticity to the carpel walls, hence reducing the tensions which normally result in dehiscence and cause pod shatter. The model of the shattering and non-shattering phenotypes described in this study suggest a number of strategies which may be used to reduce the problems of pod shatter. These include modification of the separation layer to increase cellular cohesion, and modifications to the patterns of differentiation in the carpel wall to reduce the tensions which normally develop during fruit ripening.