Post anthesis heat stress in wheat: is the reduction in grain size a consequence of premature maturation of the outer layers of the grain?


Wheat is one of the most widely consumed foods in the world, both due to its high adaptability and to the unique processing properties of its grain. The mature wheat grain (caryopsis) consists of the products of fertilization, which are the embryo and the endosperm and represent the next generation and the main storage tissue, respectively, and by the surrounding seed coat and pericarp tissues that formerly made up the carpel wall. The value of a wheat crop is ultimately the result of its endosperm yield and quality, with the first being a function mainly of grain number and grain size and the second depending mainly on endosperm protein content and composition. Post-anthesis heat stress, even when applied in the form of short period of high temperatures can considerably reduce the potential grain size of wheat and this project aims to establish if this effect could result, at least in part, from modifications occurring in the cell walls of the testa and/or pericarp tissue and caused by their premature maturation.


Wheat Grain Development sml

As caryopsis development proceeds, following fertilization, the endosperm undergoes a phase of very rapid expansion with continues, although at a slower rate, until it reaches its maximum size, at about 18-20 daa. Endosperm development is accompanied by histological changes of the surrounding maternal  tissues, specific for each of the different layers and consisting of a combination of degeneration, reabsorption, wall thickening, and compression, which ultimately allow for this tissues to  “accommodate” endosperm expansion and allow the grain to achieve its size potential.

Grains which have been exposed to heath stress during their development are smaller than non-stressed grain and they also mature earlier. Our hypothesis is that these effects are the consequence of perturbations induced by heath stress on the normal kinetic of maturation of the maternal tissues relatively to the filial tissues and resulting in the premature maturation of the pericarp respect to the endosperm.  Given that the ability of a tissue to expand is largely a function of the plasticity of its composing cell walls, we will be exploring and comparing the cell wall composition of  testa, pericarp and aleurone layer in stressed and non-stressed wheat grain, using a combination of immunofluorescence microscopy and biochemical analyses, while using transcriptome to determine the rate of development of their endosperm and pericarp and test directly the hypothesis that the pericarp is more accelerated by heat stress in development than endosperm.

The project aims at determining at which stage during grain development heat stress is likely to cause the most significant reduction in grain size and the biological basis for this reduction by furthering our understanding of how the morphological changes occurring in the endosperm and in the surrounding maternal tissues impact on each other during grain development. The outcomes of this research may be of use to wheat crop physiologist to develop better pre-harvest yield prediction equations for heat- stressed crops, and to breeders to help them develop crops that are more resilient to post-anthesis stress.


Dr Paola Tosi, Dr Rowan Mitchell


The University of Reading & Rothamsted Research