Department of Plant Biotechnology, Institute of Biology, University of Stuttgart, 70569, Stuttgart, Germany.
Plant J. 2014 Jul;79(1):82-91. doi: 10.1111/tpj.12555. Epub 2014 Jun 20.
The knock-out mutation of plastidial phosphoglucomutase (pgm) causes a starchless phenotype in Arabidopsis thaliana, and results in a severe growth reduction of plants cultivated under diurnal conditions. It has been speculated that high soluble sugar levels accumulating during the light phase in leaf mesophyll might cause a reduction of photosynthetic activity or that shortage of reduced carbon during the night is the reason for the slow biomass gain of pgm. Separate simultaneous measurements of leaf net photosynthesis and root respiration demonstrate that photosynthetic activity per unit fresh weight is not reduced in pgm, whereas root respiration is strongly elevated. Comparison with a mutant defective in the dominating vacuolar invertase (AtβFruct4) revealed that high sucrose concentration in the cytosol, but not in the vacuole, of leaf cells is responsible for elevated assimilate transport to the root. Increased sugar supply to the root, as observed in pgm mutants, forces substantial respiratory losses. Because root respiration accounts for 80% of total plant respiration under long-day conditions, this gives rise to retarded biomass formation. In contrast, reduced vacuolar invertase activity leads to reduced net photosynthesis in the shoot and lowered root respiration, and affords an increased root/shoot ratio. The results demonstrate that roots have very limited capacity for carbon storage but exert rigid control of supply for their maintenance metabolism.
质体磷酸葡萄糖变位酶（pgm）的敲除突变导致拟南芥淀粉缺失表型，并导致在昼夜条件下培养的植物生长严重减少。有人推测，在光照阶段叶片叶肉中积累的高可溶性糖水平可能导致光合作用活性降低，或者夜间还原碳短缺是 pgm 生物量缓慢增加的原因。分别对叶片净光合作用和根呼吸的同步测量表明，pgm 中单位鲜重的光合作用活性没有降低，而根呼吸则强烈升高。与在主导液泡转化酶（AtβFruct4）中缺陷的突变体的比较表明，叶片细胞中胞质溶胶而不是液泡中的高蔗糖浓度是导致向根中运输同化产物增加的原因。向根中提供的增加的糖供应，如在 pgm 突变体中观察到的，导致大量呼吸损失。因为根呼吸在长日照条件下占植物总呼吸的 80%，这导致生物量形成的延迟。相比之下，降低液泡转化酶活性会导致地上部净光合作用降低和根呼吸降低，并提供增加的根/冠比。结果表明，根具有非常有限的碳储存能力，但对其维持代谢的供应施加严格的控制。
