Starch phosphorylase 2 is essential for cellular carbohydrate partitioning in maize

Abstract

Carbohydrate partitioning is essential for plant growth and development, and its hindrance will result in excess accumulation of carbohydrates in source tissues. Most of the related mutants in maize (Zea mays L.) display impaired whole-plant sucrose transport, but other mechanisms affecting carbohydrate partitioning have seldom been reported. Here, we characterized chlorotic leaf3 (chl3), a recessive mutation causing leaf chlorosis with starch accumulation excessively in bundle sheath chloroplasts, suggesting that chl3 is defective in carbohydrate partitioning. Positional cloning revealed that the chl3 phenotype results from a frameshift mutation in ZmPHOH, which encodes starch phosphorylase 2. Two mutants in ZmPHOH exhibited the same phenotype as chl3, and both alleles failed to complement the chl3 mutant phenotype in an allelism test. Inactivation of ZmPHOH in chl3 leaves reduced the efficiency of transitory starch conversion, resulting in increased leaf starch contents and altered carbohydrate metabolism patterns. RNA-seq revealed the transcriptional downregulation of genes related to photosynthesis and carbohydrate metabolism in chl3 leaves compared to the wild type. Our results demonstrate that transitory starch remobilization is very important for cellular carbohydrate partitioning in maize, in which ZmPHOH plays an indispensable role.

Starch degradation in the bean fruit pericarp is characterized by an increase in maltose metabolism

“…Meanwhile, the action of PHO2 on cytosolic glucan produces glucose 1‐P (Fettke et al, 2005 ). The fact that silencing of the genes encoding chloroplast‐targeted β‐amylases resulted in starch accumulation (Weise et al, 2006 ) and that DPE2 (Chia et al, 2004 ) and PHO2 (Qin et al, 2022 ) mutants had elevated starch, maltose, and hexose levels but low sucrose levels support this hypothesis.…”