The question

The biology of chloroplasts underpins the biology of whole plants, and the two most-important impacts of plants for humanity: as food source and carbon sink. Chloroplasts are biology’s “solar cells” and, on land, leaves are nothing but biology’s “solar panels”. Yet a surprising number of aspects of chloroplast biology remain poorly understood.

Chloroplast development is under the control of the plant cell’s nucleus. All cells in a plant carry the same nuclear genetic information, yet different cell types in different organs carry vastly different chloroplast complements. How does this occur?

In fact all cells in a plant begin life as naïve “stem cells” carrying a few “simple” organelle precursors called proplastids. Some, called mesophyll cells, making up the bulk of leaves, go on to be filled with many complex chloroplasts. How do such cells turn chloroplast development on? And when do they stop, at different points for different cell types?

We are a small, experienced laboratory with vibrant, young energy. These are the very fundamental questions the laboratory is trying to help find answers for.

How do we do this? Two main approaches:

Genetics and molecular cell biology (initially in Arabidopsis thaliana)

Plants require light as their energy source for growth, and therefore need to optimise light capture. They do so by sensing their light environment and modifying their development accordingly. Arabidopsis thaliana, a very useful laboratory model plant, has lent itself to the search for mutations in light sensory mechanisms. In mutant plants greening is defective. Other model species have allowed the identification of master switch genes, transcriptional  regulators, which play a role in the process of greening. We carry out searches for mutants which disrupt chloroplast development, or which “correct” the impact of previous mutations, reversing the outcome, the phenotype.

As an example, we had identified a series of CAB (LHCB)-underexpressed, cue mutants, impaired specifically in the greening light response (Vinti et al. 2005). The gene carrying the lesion in one of them, cue8, has turned out to encode a protein, TIC100, with a previously-disputed role (Loudya et al. 2022). Thanks to the cue8, the protein is now confirmed to play a role in the import of other proteins into chloroplasts, through the inner plastid envelope. One suppressor of cue8, suppressor of tic100 1 (soh1) turns out to carry a second lesion in the TIC100 gene itself which corrects the impact of the first lesion. The analysis of cue8 has also been hugely insightful in our understanding of a process of plastid-to-nucleus communication, which we now see as one of chloroplast developmental progress-reporting to the nucleus (Loudya et al. 2020).

Some of the mutants we identified in Arabidopsis, boosting chloroplast development, are now being replicated in rice plants.

We have also devoted much of our past attention to understanding the developing of the “solar panels”, leaves themselves. Specifically we explored the fact that the stem cell pool which develops them, the meristem, is arrested in the dark, leaves developing only in the light.

Our current conclusion is that the light switch is a complex one, which operates through hormone transport impacting hormonal responses, and through energy (sugar) signalling operating on the meristem, that in turn possibly through the control of sugar transport itself (Mohammed et al. 2018, Doczi et al. 2019).

Analysis of the developing cereal leaf (wheat and ancient wheat)

In cereals, including wheat, rice, maize, stem cells producing leaves sit as a “meristem” at the base of the plant, just below ground. Leaf “primordia” form at the flanks of the meristem, cells undertake programme of differentiation decisions, proliferate rapidly and begin to differentiate. This results in a gradient of developmental stages, including of chloroplast greening, from base to tip of young leaves. We have exploited this gradient, using a combination of  microscopy, molecular cell biology, extensive, global gene expression analysis complemented with selected protein biochemistry, to obtain probably the most detailed picture to date of the “biography of a chloroplast” (Loudya, Mishra et al. 2021).

This biography has helped us understand the roles of previously-known transcriptional regulators in chloroplast development, but also their limits, and it has revealed that we are probably still missing the key “master switches”.

The search for those “master switches” continues, currently using single cell-based techniques.

What for?

As biological solar cells, our ability to control whether, or how far, cells accumulate developed chloroplasts would open a huge range of possibilities in relation to crop productivity. For example attempts are underway to introduce into rice a modified form of turbo-charged photosynthesis called C4. One of the challenges it faces is the need for a specialised type of cell, the “bundle sheath” to develop a greater chloroplast content. And, on the contrary, chloroplasts are themselves expensive to build, and tunning their cellular content to particular conditions or light levels within crop canopies my result in improved efficiencies of, for example, fertiliser use.

Cited

• Dóczi R et al. (2019). Frontiers in Plant Science 10:202. Pubmed 30891050
• Loudya N et al. E (2020) Philosophical Transaction of the Royal Society B Biology 375: 20190400. Pubmed 32362263
• Loudya N, Mishra P et al. (2021) Genome Biology 22:1-30. Pubmed 33975629
• Loudya N, et al. (2022) Plant Cell 34: 3028–3046. Pubmed: 35640571. (Featured as front journal cover).
• Mohammed B et al. (2018) Plant Physiology 176: 1365-1381. Pubmed 29197077
• Vinti G et al. (2005) Plant Mol. Biol. 57: 343-357. Pubmed 15830126

Our collaborators

• Dr. Naresh Loudya and Dr. Priyanka Mishra (Indian Institute of Science, Bangalore, formerly at Royal Holloway)
• Prof. Laszlo Bogre and Prof. Alessandra Devoto (Royal Holloway)
• Prof. Paul Jarvis (University of Oxford)
• Dr. Keiichi Mochida (Riken Institute, Yokohama, Japan)
• Prof. Julian Hibberd (University of Cambridge)
• Prof. Steve Kelly and Dr. Ross Hendron (Oxford Uni. and Wild Bioscience Ltd.)
• Prof. Jane Langdale (Oxford Uni.)
• Prof. José Luis Micol (Universidad Miguel Hernández, Elche, Spain)
• Prof. Matthew Terry (University of Southampton)