The roles of glk transcription factors in c3 and c4 plants

Chloroplast biogenesis and development is of crucial importance to photosynthesis efficiency and plant yield, and GOLDEN2-LIKE (GLK) transcription factors (TFs) are important regulators of chloroplast biogenesis and development in plants. A pair of GLK genes is located in many plant genomes. In C3 plants such as Arabidopsis, GLK1 and GLK2 act redundantly to regulate chloroplast development in a cellautonomous manner, while in some C4 grasses like maize, GLK1 and GLK2 are preferentially expressed in mesophyll and bundle sheath cells respectively, which is correlated with dimorphic chloroplast development in C4 plants. However, whether and how GLK-regulated chloroplast function differs across different plant species, particularly the genome-wide occupancy of GLK proteins in diverse plant species remains largely unexplored.In this study, GLK deficient mutant in tomato was generated using CRISPR technology, and it was observed that GLK deficiency resulted in pale green leaf in tomato. Transcriptome analysis indicated that most photosynthetic genes, including those in chlorophyll biosynthesis and light harvesting, are upregulated by GLK proteins in Arabidopsis and tomato. Comparative analysis of genome-wide GLK occupancy in diverse plant species revealed that binding targets of paralogous GLK1 and GLK2 substantially overlap in each species and photosynthetic genes are overrepresented in these overlapped target genes. In addition, GLK binding on photosynthetic genes is highly conserved across flowering plants. A subset of TF genes are conservatively bound by GLK proteins and they participate in multiple developmental processes, including flowering time control, leaf senescence, plant longevity, and stress responses, suggesting that GLK proteins might be involved in plant developmental processes apart from photosynthesis. Some eudicot-specific and monocot-specific GLK targets were identified, however, monocot GLK genes can still rescue Arabidopsis glk1 glk2 pale green leaf phenotype and ~40% of eudicot-specific GLK targets can be bound by ZmGLK1 in Arabidopsis, suggesting that GLK proteins retained the similar binding potential to their orthologs in other species, especially for the core targets involved in photosynthesis, and sequence variation in regulatory regions was responsible, at least in part, for the differential binding targets. Through the comparison of GLK binding targets in eudicot and monocot species that diverged more than 160 million years ago (Mya), this study revealed that the core set of orthologous genes is conservatively bound by orthologous TFs during evolution, while the TF binding sites may have evolved independently.葉綠體的生物發生和發育對於光合作用效率和植物產量至關重要，而GOLDEN2-LIKE（GLK）轉錄因子是植物中葉綠體的生物發生和發育的重要調節因子。 一對GLK 基因位於許多植物基因組中。 在擬南芥等C3 植物中，GLK1 和GLK2 以細胞自主方式冗餘地調節葉綠體的發育，而在玉米等C4草本植物中，GLK1 和GLK2 分別在葉肉細胞和維管束鞘細胞中偏好性表達，這與C4 植物中雙態葉綠體的發育相關。 然而，GLK 調節的葉綠體功能在植物物種之間是否存在差異以及差異如何，尤其是在各種植物物種中GLK 蛋白在基因組範圍內的結合位點仍未得到充分探討。本研究使用CRISPR 技術在番茄中產生了GLK 缺陷型突變體，並觀察到GLK 缺乏導致番茄的葉子呈淡綠色。轉錄組分析表明，大多數光合作用基因，包括參與葉綠素生物合成和捕光複合物的基因，都被擬南芥和番茄中的GLK 蛋白上調表達。 对不同植物物種中基因組範圍GLK 的結合情況比較分析表明，在每個物種中，同源的GLK1 和GLK2 結合的靶基因很大程度上重疊，並且在這些重疊的靶基因中光合基因被富集。此外，GLK 與光合作用基因的結合在被子植物中高度保守。 部分轉錄因子基因被GLK 蛋白保守地結合，它們參與多種發育過程，包括開花時間控制，葉片衰老，植物壽命和脅迫反應，表明除光合作用外，GLK 蛋白可能還參與一些其它的植物發育過程。本研究鑑定了一些特定於雙子葉植物和單子葉植物的GLK 靶標，但是，來源於單子葉植物的GLK基因仍可以互補擬南芥glk1 glk2淡綠色的葉表型，而且約40％的雙子葉植物特異性GLK 靶標可以被擬南芥中表達的ZmGLK1 結合。這些說明GLK 蛋白IV保留了對其他物種中直系同源的靶基因的相似的結合潛能，尤其是對於參與光合作用的核心靶標，而且調節區域中的序列變異至少部分程度上是造成差異性結合靶標的原因。通過比較在超過1.6 億年前分化的雙子葉植物和單子葉植物物種中GLK 的結合靶點，這項研究表明轉錄因子核心靶標的直系同源基因在進化過程中被直系同源的轉錄因子保守地結合，而轉錄因子的結合位點可能獨立地進化。Li, Jianjian.Ph.D. Chinese University of Hong Kong 2021.Includes bibliographical references (leaves ).Abstracts also in Chinese.Title from PDF title page (viewed on …)