Development of An In Vitro Method to Evaluate the Potential of Micronutrient Import to Rice Grains.

Micronutrient accumulation in cereal grains is a function of membrane and long distance transport processes which are spatially and temporally coordinated throughout the plant.  As with any nutrient, import of these minerals into grain requires passage through tissues defining the maternal-filial barrier.  Unlike many nutrients, however, the mechanisms controlling micronutrient uptake into seeds is not well understood.  To better define these regulatory controls and estimate the potential for seed tissues to import nutrients of interest from the maternal plant, we have optimized a panicle feeding method in rice which isolates the physiological processes associated with nutrient accumulation in reproductive tissues from those occurring elsewhere.  Based on the concept of Singh and Jenner’s (1981) wheat panicle culture method, this design relies on detachment of the rice panicle and in vitro stem feeding through xylem vasculature using an optimized nutrient solution that more closely matches the macro and micronutrient requirements of intact panicles.  To simulate in vivo grain development, we provide recommendations on the timing of panicle detachment, assay duration and other physiological/ biochemical conditions which maximize nutrient uptake to provide consistent phenotypic measurements.   This assay provides flexibility in manipulating input micronutrients thereby enabling direct correlation of seed micronutrient content to vascular source nutrient concentrations.  We discuss application of this assay in the phenotypic evaluation of grain nutrition across rice cultivars.