Mesophyll conductance variability of rice aquaporin knockout lines at different growth stages and growing environments

The plasma membrane subfamily of aquaporins [plasma membrane intrinsic proteins (PIPs)], which facilitates the CO₂ diffusion across membranes, is proposed to play an important role in mesophyll conductance to CO₂ (g_m ), a major limiting factor of photosynthesis. However, recent studies implied no causal relationship between g_m and PIPs because they failed to repeat the previous observed differences in g_m between PIP knockout lines and their wild-type. The contrasting results on the role of PIPs in g_m were interpreted as the different growth conditions among studies, which has never been tested. Here, we assessed whether the differences in g_m between wild-type and PIP knockout lines, Ospip1;1, Ospip1;2 and Ospip2;1, varied with growth condition (field versus pot condition) and growth stages in rice. Under field conditions, no differences were observed in plant performance, photosynthetic rate (A) and g_m between PIP knockout lines and the wild-type. However, in agreement with previous studies, two out of three knockout lines showed significant declines in tiller number, plant height, A and g_m under pot conditions. Moreover, we found that the differences in A and g_m between PIP knockout lines and the wild-type varied with the growth stage of the plants. Our results showed that the differences in g_m between PIP knockout lines and wild-type were depending on the growth environments and stage of the plants, and further efforts are required to reveal the underlying mechanisms.