A Maize Thiamine Auxotroph Is Defective In Meristem Maintenance And Leaf Blade Development

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Plants undergo organogenesis throughout their life cycle via the perpetuation of stem cell pools called shoot meristems (SMs). SM maintenance requires the coordinated equilibrium between stem cell division and differentiation, and is feedback-regulated by integrated networks of gene expression, hormonal signaling, and metabolite sensing. Here I show that maize mutant bladekiller1 (blk1) is defective in meristem maintenance and leaf-blade development. The blk1 mutants exhibit a progressive reduction in SM size, resulting in premature shoot termination. Molecular markers for stem cell maintenance (including knotted1, Zmwuschel1-1, and abphyll1) and organ initiation (including ZmPIN1a~YFP and narrow sheath1) reveal that these meristematic functions are progressively compromised in mutant plants, especially in inflorescence and floral SMs. Positional cloning of the blk1 mutation identified a predicted missense mutation in a highly-conserved amino acid encoded by the maize thiamine biosynthesis2 (thi2) gene located on chromosome 3L. Consistent with chromosome dosage studies suggesting that blk1 is a null allele of thi2, biochemical analyses confirm that the non-mutant THI2 enzyme co-purifies with a thiazole substrate whereas the mutant enzyme does not. A nearly identical paralog of thi2, named thiamine biosynthesis1 (thi1) is located on chromosome 8. Although both paralogs are expressed ubiquitously, transcript accumulation of thi2 is significantly more abundant than thi1 in vegetative and inflorescence SMs. Heterologous expression studies reveal that THI2 is targeted to chloroplasts, providing additional evidence for the synthesis of thiamine in plastids. All blk1 mutant phenotypes are rescued by exogenous thiamine supplementation, suggesting that blk1 is thiamine auxotroph. These data reveal that the inhibition of thiamine accumulation blocks the proliferative growth of stem cell populations in the maize shoot, and provide additional evidence for the integral role of carbohydrate metabolism and signaling during meristem development.

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