Variation in maize chlorophyll biosynthesis alters plant architecture.

Chlorophyll is a tetrapyrrole metabolite essential for photosynthesis in plants. The first committed step of chlorophyll biosynthesis is catalyzed by a multimeric enzyme Magnesium chelatase, the subunit I of which is encoded by the oil yellow1 (oy1) gene in maize (Zea mays). A range of chlorophyll contents and net CO2 assimilation rates can be achieved in maize by combining a semi-dominant mutant allele of oy1 (Oy1-N1989) and a cis-regulatory modifier named very oil yellow1 (vey1) that varies between different inbred lines. We previously demonstrated that these allelic interactions can delay reproductive maturity. In this study, we demonstrate that multiple gross morphological traits respond to a reduction in chlorophyll. We found that stalk width, number of lateral branches (tillers), and branching of the inflorescence decline with a decrease in chlorophyll level. Chlorophyll deficit suppressed tillering in multiple maize mutants including teosinte branched1, Tillering1, and grassy tillers1. In contrast to these traits, plant height showed a non-linear response to chlorophyll levels. Weak suppression of Oy1-N1989 by vey1B73 resulted in a significant increase in mutant plant height. By contrast, enhancement of the severity of the Oy1-N1989 phenotype by the vey1Mo17 allele resulted in reduced plant height. We demonstrate that the effects of reduced chlorophyll contents on plant growth and development are complex and depend on the trait being measured. We propose that the lack of chlorophyll exerts growth control via energy balance sensing, which is upstream of the known genetic networks for branching and architecture.