The role of leaf structure in rice photosynthetic performance

Improving the efficiency of photosynthesis has been identified as a major aim in plant biology. As a result of previous work, a model of photosynthesis has been developed which incorporates the cellular anatomy of the leaf. Analysis of the model suggests that leaf cell size plays an important role in photosynthesis, particularly in how photosynthesis responds to an increase in CO2 level.

In Chapter 2, I aim to test the hypothesis that the change of cell size and shape alters photosynthesis by analysing a series of transgenic plants with altered leaf cell size. I have identified a series of transgenic rice plants (JL lines) and characterised their leaf anatomy, revealing variation in mesophyll and bundle sheath cell size. Interestingly, two lines; JL4 and JL27 showed a difference of lobing formation in mesophyll cells. Chlorophyll fluorescence-gas exchange analysis indicated that these line were impaired in photosynthesis.

In addition, the results from JL lines presented an unexpected relationship between mesophyll cell shape (cell lobing) and stomatal characteristics. This observation was followed up using transgenic rice plants with altered stomatal density. osEPF1_oe lines (low stomata number) and osEPFL9_oe lines (high stomata numbers) were used to analyse a potential link between stomatal function and the internal cell structure of the leaf. This work suggested that the degree of mesophyll lobing in transgenic rice is altered in response to the environment, which supports a link between the degree of cell lobing and gas diffusion through stomata.

In Chapter 3, I investigated the process of mesophyll cell lobing in rice, with the aim of identifying the earliest stage of mesophyll cell lobing, using rice leaf 6th of IR64 as a model for this study. The results indicated that the initiation of cell lobing occurred at an early stage of leaf development (P3/P4 stage) of leaf length less than 20 mm. This is similar to the stage when stomata differentiation occurs. This work supports the idea that MC lobing is regulated by stomatal function during leaf development.

In Chapter 4, I used another approach to test the idea from Chapter 2 that mesophyll cell lobing in rice is modulated by stomatal function. Using a series of rice plants gene-edited in the HT1 gene (which are expected to show abnormal stomatal opening to CO2 level, I investigated whether abnormal stomatal function in this mutant also led to altered mesophyll cell structure. The results indicated that mesophyll cell shape (lobiness) is responsive to stomata conductance and that increased gas flux could lead to increased cell lobing. Unexpectedly, the ht1 mutations also influenced leaf structure in many aspects complicating the interpretation of these data.

In conclusion, my research indicates that altered gas flux via stomata can affect mesophyll cell shape in rice. This adds to previous work in the area indicating that stomatal conductance can influence cell size and separation in the leaf mesophyll. However, there is still a lack of evidence to explain the mechanism by which stomatal function could lead to these changes, which needs to be explored in future research. For future work, it might also be interesting to further study mesophyll cell size and shape using 3D imaging techniques and compare it with the 2D imaging data reported in my work . In addition it would be interesting to explore the metabolic changes occurring as the mesophyll cells change size and shape, exploring the link to photosynthetic function and gas exchange. Overall, my results support a co-ordinated link between mesophyll cell size and shape, stomatal function and photosynthesis.