Nutritional restriction triggers callose accumulation on the sieve plates of the funiculus of developing bean seeds

Seed development represents a very high investment of metabolic resources that plants are not always able to fulfill, and nutrient remobilization is widely used to support seed development under stressful conditions. Phaseolus vulgaris fruits removed from the plant at 20 days after anthesis (DAA) respond to nutrient restriction by recycling pod material. At the same time, the available resources are channeled to a few seeds in order to produce some viable seeds. By analyzing carbohydrate levels and the distribution of sucrose [U-14C] between the pod tissue and seed sections (funiculus, seed coat and embryo), in combination with the structural analysis of phloem sieve plates; we were able to identify that nutrient flow is reduced at the funiculus level. The accumulation of callose resulted in a large reduction in the diameter of the sieve pores that eventually will stop nutrient flow. It produces the death of most seeds, but also increases the probability of some seeds to successfully completing their development during severe nutrient restriction.