School of Natural & Environmental Sciences, Ridley Building, Newcastle University, Newcastle upon Tyne NE1 7RU, UK.
Department of Biochemistry and Molecular Biology, MS330, University of Nevada, Reno, NV 89557 USA.
J Plant Physiol. 2017 Nov;218:144-154. doi: 10.1016/j.jplph.2017.07.021. Epub 2017 Aug 5.
Nocturnal degradation of transitory starch is a limiting factor for the optimal function of crassulacean acid metabolism and must be coordinated with phosphoenolypyruvate carboxylase (PEPC)-mediated CO uptake to optimise carbon gain over the diel cycle. The aim of this study was to test the hypothesis that nocturnal carboxylation is coordinated with starch degradation in CAM via a mechanism whereby the products of these pathways regulate diel transcript abundance and enzyme activities for both processes. To test this hypothesis, a starch and CAM-deficient mutant of Mesembryanthemum crystallinum was compared with wild type plants under well-watered and saline (CAM-inducing) conditions. Exposure to salinity increased the transcript abundance of genes required for nocturnal carboxylation, starch and sucrose degradation in both wild type and mutant, but the transcript abundance of several of these genes was not sustained over the dark period in the low-carbohydrate, CAM-deficient mutant. The diel pattern of transcript abundance for PEPC mirrored that of PEPC protein, as did the transcripts, protein, and activity of chloroplastic starch phosphorylase in both wild type and mutant, suggesting robust diel coordination of these metabolic processes. Activities of several amylase isoforms were low or lacking in the mutant, whilst the activity of a cytosolic isoform of starch phosphorylase was significantly elevated, indicating contrasting modes of metabolic regulation for the hydrolytic and phosphorylytic routes of starch degradation. Externally supplied sucrose resulted in an increase in nocturnal transcript abundance of genes required for nocturnal carboxylation and starch degradation. These results demonstrate that carbohydrates impact on transcriptional and post-transcriptional regulation of nocturnal carboxylation and starch degradation in CAM.
夜间暂存淀粉的降解是肉质植物酸代谢最佳功能的限制因素，必须与磷酸烯醇丙酮酸羧化酶（PEPC）介导的 CO2 吸收相协调，以优化昼夜周期内的碳增益。本研究的目的是验证以下假设，即夜间羧化作用通过一种机制与 CAM 中的淀粉降解相协调，这种机制使这些途径的产物调节两种过程的昼夜转录丰度和酶活性。为了验证这一假设，将景天科植物的一个淀粉和 CAM 缺陷突变体与野生型植物在充分浇水和盐胁迫（CAM 诱导）条件下进行比较。盐胁迫增加了野生型和突变体中夜间羧化、淀粉和蔗糖降解所需基因的转录丰度，但在低碳水化合物、CAM 缺陷突变体中，这些基因的一些转录丰度在黑暗期内不能持续。PEPC 的昼夜转录丰度模式与 PEPC 蛋白的模式一致，叶绿体淀粉磷酸化酶的转录物、蛋白和活性也与野生型和突变体一致，这表明这些代谢过程具有稳健的昼夜协调作用。几种淀粉酶同工酶在突变体中的活性较低或缺失，而细胞质同工酶的淀粉磷酸化酶活性显著升高，表明淀粉降解的水解和磷酸化途径的代谢调节方式不同。外源蔗糖导致夜间羧化和淀粉降解所需基因的转录丰度增加。这些结果表明，碳水化合物影响 CAM 中夜间羧化和淀粉降解的转录和转录后调节。
