Cellular and transcriptomic analyses reveal two-staged chloroplast biogenesis underpinning photosynthesis build-up in the wheat leaf

Abstract

Background The developmental gradient in monocot leaves has been exploited to uncover leaf developmental gene expression programs and chloroplast biogenesis processes. However, the relationship between the two is barely understood, which limits the value of transcriptome data to understand the process of chloroplast development. Results Taking advantage of the developmental gradient in the bread wheat leaf, we provide a simultaneous quantitative analysis for the development of mesophyll cells and of chloroplasts as a cellular compartment. This allows us to generate the first biologically-informed gene expression map of this leaf, with the entire developmental gradient from meristematic to fully differentiated cells captured. We show that the first phase of plastid development begins with organelle proliferation, which extends well beyond cell proliferation, and continues with the establishment and then the build-up of the plastid genetic machinery. The second phase is marked by the development of photosynthetic chloroplasts which occupy the available cellular space. Using a network reconstruction algorithm, we predict that known chloroplast gene expression regulators are differentially involved across those developmental stages. Conclusions Our analysis generates both the first wheat leaf transcriptional map and one of the most comprehensive descriptions to date of the developmental history of chloroplasts in higher plants. It reveals functionally distinct plastid and chloroplast development stages, identifies processes occurring in each of them, and highlights our very limited knowledge of the earliest drivers of plastid biogenesis, while providing a basis for their future identification.

Chloroplast development in green plant tissues: the interplay between light, hormone, and transcriptional regulation

“…The role of this gene has not been explored beyond fruits of tomato and pepper. Loudya et al (2021) recently demonstrated that chloroplast proliferation, build-up of chloroplast protein import and increases in chloroplast genetic machinery (genome replication, transcription and translation) can all be completed before greening of the chloroplast. Therefore, apart from the individual candidate genes mentioned earlier it should be noted that identifying regulators that act before greening (during the first 'plastid' or 'structural establishment' phase) is fundamental to complete our understanding of chloroplast development.…”

“…The fully formed thylakoid membrane therefore contains proteins and pigment–protein complexes including Photosystem II, the cytochrome b 6 f complex, Photosystem I and the adenosine triphosphate (ATP) synthase (Staehelin & DeWit, 1984). Whether from etioplasts in dark‐grown cotyledon cells (Pipitone et al ., 2021) or from proplastids in meristematic cells (Loudya et al ., 2021), two stages of chloroplast biogenesis can be identified: the first has been referred to as the ‘plastid phase’, or photosynthesis‐enabling ‘structural establishment phase’, while the second is the ‘chloroplast’ or ‘greening phase’, and during deetiolation this is also the ‘chloroplast proliferation phase’.…”

“…Although the process of chloroplast division is tightly correlated with cell size, it is considered largely independent of cell division as it continues even when cell division has stopped (Jarvis & López‐Juez, 2013; Loudya et al ., 2021). As several genes involved in chloroplast division are activated by light and repressed in the dark (Mohammed et al ., 2018) it appears that along with chloroplast biogenesis, chloroplast division in developing leaves is regulated by light.…”

“…chloroplast biogenesis) thylakoids are formed and the photosynthetic machinery arranged within the thylakoid (Jarvis & López‐Juez, 2013). Imaging, transcriptomic and proteomic analyses at increasing resolutions (Li et al ., 2010; Majeran et al ., 2010; Loudya et al ., 2021) have begun to define the processes and helped place genetically‐identified components in context. During the plastid build‐up stage, machineries required for import of nuclear encoded proteins and for the synthesis of proteins encoded in the chloroplast DNA become established.…”

“…When dark‐grown cotyledons containing abundant nongreen chloroplast precursors called etioplasts initiate greening in the light, the conversion of etioplasts into chloroplasts is followed by the division of those chloroplasts as the cells harbouring them expand (Pipitone et al ., 2021). By contrast, during development of linear monocotyledonous leaves, in which undifferentiated cells with a very small number of proplastids differentiate into photosynthetic cells with many chloroplasts, plastid division precedes the process of photosynthetic build‐up (Loudya et al ., 2021). The abundance of etioplasts in cells of dark‐grown cotyledons probably reflects the fact that etioplasts may have also developed following division of proplastids in embryonic cells.…”