A specific protective mechanism against chloroplast photo-reactive oxygen species in phosphate-starved rice plants [RNA-seq]

Phosphorus (Pi) starvation prevents a good match between light energy absorption and photosynthetic carbon metabolism. Photosynthetic electron-transport chain switches to use molecular oxygen as an electron carrier, generating photo-reactive oxygen species (photo-ROS) in chloroplast. In rice (Oryza sativa), DEEP GREEN PANICLE1 (DGP1) is robustly up-regulated in response to Pi-deficiency stress. DGP1 decreases the DNA-binding activities of the photosynthetic activators GLK1/2 on the genes involved in chlorophyll biosynthesis, light harvesting and electron transport. This Pi-starvation-induced mechanism dampens electron transport rates (ETRI and ETRII) and alleviates the electron-excessive stress in mesophyll cells. Meanwhile, DGP1 hijacks glycolytic enzymes GAPC1/2/3, redirecting glucose metabolism toward pentose phosphate pathway with superfluous NADPH production. Phenotypically, light irradiation induces O2– accumulation in Pi-starved WT leaves, but was observably accelerated in dgp1 mutant and impaired in GAPCsRNAi line and glk1glk2 double mutant. Interestingly, overexpression of DGP1 in rice caused hyposensitivity to the ROS-inducers (catechin and methyl viologen) and dgp1 mutant shows a similar inhibitory growth with the WT plants. We conclude that DGP1 gene serves as a specific antagonizer against Pi-starvation-induced photo-ROS, which integrates light-absorbing and anti-oxidative systems by orchestrating transcriptional and metabolic regulations, respectively. Overall design: We examined the transcriptional data in leaves of glk1glk2 and dgp1 mutants with their WT (DJ) plants under normal growth or low-Pi stress (10 µM KH2PO4).

磷（Pi）胁迫会破坏光能吸收与光合碳代谢之间的稳态匹配。光合电子传递链会转而以分子氧作为电子载体，在叶绿体中生成光活性活性氧（photo-ROS）。在水稻（Oryza sativa）中，深绿穗1（DEEP GREEN PANICLE1, DGP1）会在缺磷胁迫下被显著上调表达。DGP1会降低光合激活因子GLK1/2对叶绿素生物合成、捕光及电子传递相关基因的DNA结合活性。这一由缺磷诱导的调控机制会抑制电子传递速率（ETRI和ETRII），并缓解叶肉细胞内的电子过量胁迫。同时，DGP1会结合糖酵解酶GAPC1/2/3，将葡萄糖代谢重定向至磷酸戊糖途径，从而产生过量的烟酰胺腺嘌呤二核苷酸磷酸（NADPH）。

表型层面，光照会诱导缺磷野生型（WT）叶片中超氧阴离子（O2·–）的积累，而这一过程在dgp1突变体中显著加速，在GAPCsRNAi干扰株系与glk1glk2双突变体中则受到明显抑制。值得注意的是，水稻中DGP1过表达会使其对活性氧（ROS）诱导剂（儿茶素与甲基紫精）产生低敏感性，而dgp1突变体的生长抑制情况与野生型对照（DJ）相似。综上，我们认为DGP1基因可作为拮抗缺磷诱导光活性活性氧的特异性因子，其分别通过转录调控与代谢调控，整合光能吸收与抗氧化系统。

实验设计概述：我们在正常生长条件或低磷胁迫（10 µM 磷酸二氢钾，KH2PO4）下，检测了glk1glk2突变体、dgp1突变体及其野生型对照（DJ）叶片的转录组数据。