Investigations on maize kernel development and the relevance of endogenous hypoxia

Maize kernels are the largest cereal grains and their endosperm is severely oxygen deficient during grain filling. The causes, dynamics, and mechanisms of acclimation to hypoxia are minimally understood. We here demonstrate that hypoxia develops in the small, growing endosperm, but not the nucellus, and becomes the standard state in modern maize (B73, popcorn, sweet corn), mutants (sweet4c, glossy6, waxy), and non-domesticated wild relatives (Teosinte and Tripsacum species). By applying magnetic resonance imaging, X-ray micro-computed tomography, and infrared microspectroscopy, we uncovered an interconnected void space at the chalazal pericarp, diffusion barriers, and steep gradients in tissue hydration as the major determinants of oxygen supply to the endosperm. Upon perturbation of oxygen supply, reciprocal shifts in gene expression occur with key elements controlling mitochondrial functions (23.6 kDa HSP, VDAC2) and multiple signalling pathways (core hypoxia genes, cyclic nucleotide metabolism, ethylene synthesis). Metabolite profiling revealed oxygen-dependent shifts in mitochondrial pathways ascorbate metabolism, starch synthesis, and auxin degradation. The basal endosperm and its transfer function is not constrained by hypoxia. Long-term elevation of exogenous oxygen supply during main filling stage caused a more rapid rate of kernel development. Overexpression of phytoglobins did not cause consistent and marked changes in major traits of mature kernels and demonstrated that this kind of modulation of phytoglobins is not a suitable strategy to increase yield. Altogether, evidence here supports a mechanistic framework for the establishment of, and acclimation to hypoxia in the maize endosperm. Moreover, the successful establishment of a method to map sucrose in maize in a high spatial resolution provides us with further opportunities to improve our understanding of processes underlying assimilate allocation inside the kernel.