Iron insufficiency in floral buds impairs pollen development by disrupting tapetum function

Reduction of crop yield due to iron (Fe) deficiency has always been a concern in agriculture. How Fe insufficiency in floral buds affects pollen development remains unexplored. Here, plants transferred to Fe-deficient medium at the reproductive stage had reduced floral Fe content and viable pollen and showed a defective pollen outer wall, all restored by supplying floral buds with Fe. A comparison of differentially expressed genes (DEGs) in Fe-deficient leaves, roots, and anthers suggested that changes in several cellular processes were unique to anthers, including increased lipid degradation. Co-expression analysis revealed that ABORTED MICROSPORES (AMS), DEFECTIVE IN TAPETAL DEVELOPMENT AND FUNCTION1, and BASIC HELIX-LOOP-HELIX 089/091/010 encode key upstream transcription factors of Fe deficiency-responsive DEGs involved in tapetum function and development, including tapetal ROS homeostasis, programmed cell death, and pollen outer wall formation-related lipid metabolism. Analysis of RESPIRATORY-BURST OXIDASE HOMOLOG E (RBOHE) gain- and loss-of-function under Fe deficiency indicated that RBOHE- and Fe-dependent regulation cooperatively control anther reactive oxygen species levels and pollen development. Since DEGs in Fe-deficient anthers were not significantly enriched in genes related to mitochondrial function, the changes in mitochondrial status under Fe deficiency, including respiration activity, density, and morphology, were probably because the Fe amount was insufficient to maintain proper mitochondrial protein function in anthers. To sum up, Fe deficiency in anthers may affect Fe-dependent protein function and impact upstream transcription factors and their downstream genes, resulting in extensively impaired tapetum function and pollen development.