Whole genome duplication-polyploidy and its impact on photosynthetic cells and traits in Arabidopsis arenosa

Abstract

Whole genome duplication (WGD), which leads to polyploidy, has significant implications for evolution and agriculture. Research consistently shows that polyploid plants tend to be more robust and stress-resistant compared to their diploid counterparts, with better photosynthetic rates, particularly under drought conditions. This is especially relevant and timely in the current context of rising global temperatures and resulting water stress. However, changes in cell morphology in plant leaves induced by WGD, including those of stomatal guard cells and mesophyll cells, may lead to variable changes in photosynthetic rates and other photosynthetic traits, such as gas exchange, under normal conditions. In addition, It is not known whether the observed variation in photosynthetic rates and gas exchange under normal conditions can also lead to variation in the content of principal end products of photosynthesis, such as transitory starch. Despite variable findings, there has been no comprehensive comparison of photosynthetic traits under normal and/or drought conditions in the whole plant across diploid, neo-polyploids, and established polyploids within a single species. This study, thus, takes a novel approach leveraging on naturally ploidy-variable species Arabidopsis arenosa to conduct a comparative analysis across diploids, neo-polyploids, and established polyploids with the goal of examining the effect of WGD and subsequent evolution on the morphology of photosynthesis-relevant cells, the photosynthetic performance of the whole plant under normal and drought conditions, and transitory starch content. This comprehensive comparison allows us not only to understand what changes occur as a result of WGD but also what challenges selection has worked to remedy. The Chapter 1 of this work reveals that WGD can indeed lead to changes in the morphological traits of stomatal guard cells, spongy and palisade mesophyll cells, and in most cases, cell size increases. However, over evolutionary time, this increase in size could change based on the functionality of these cells. Some traits, such as the sizes of stomatal guard cells and spongy mesophyll cells, may return to diploid-like levels, while others, such as palisade mesophyll cell size and stomata density, may not return to diploid-like levels after WGD. Nevertheless, in higher ploidies such as neo-octaploid and neo-hekkaidecaploids, the size and shape of stomatal guard cells tend to become variable. This variability suggests that as stomatal guard cells grow larger, proper cell division and differentiation may become more difficult, which can lead to such variations in size and shape. The Chapter 2 assessed the impact of WGD and subsequent evolution on stomata, particularly in relation to the challenges and solutions posed to stomatal function and photosynthesis under normal and drought conditions. We show that some stomatal traits change upon WGD, but we find that increased drought tolerance of photosynthesis is a later-evolved feature in the established tetraploids. We also show that a naturally-evolved allele of a calcium transporter, ACA8, which has known roles in regulating stomatal dynamics, and experienced a selective sweep in the tetraploid, leads to increased water use efficiency. Homozygotes for the tetraploid allele also have improved ability to maintain photosynthesis under drought relative to homozygotes for the diploid allele. We hypothesise that improved Ca2+ signalling in guard cells might have evolved to compensate for the observed WGD-triggered cell size increase, and now, together with reduced cell size, contributes to improved performance of established tetraploids under drought. Given that WGD and subsequent post-polyploidy modification can alter cell size and content, such as the number of chloroplasts, the site for transitory starch production and even photosynthesis itself, in Chapter 3, I carried out the first-ever evaluation to determine whether such changes can affect transitory starch content. I found no significant difference in mesophyll starch quantities when comparing across 2X, neo-tetraploid F1 and established tetraploid despite significant differences in mesophyll cell size. However, I did observe a significant increase in stomatal transitory starch in chimeric neo-tetraploid with diploid roots and tetraploid shoots, while it was significantly reduced in the non-chimeric neo-tetraploid F1 and neo-triploids, which lack potentially adaptive alleles found in the established tetraploid. These findings suggest that the increase in starch amount is adaptive, and evidence for selection having targeted genes (PTST3 and AMY3) involved in starch metabolism in A. arenosa may provide candidate loci responsible for the shift. Conclusively, the findings of this thesis underscore the connection between WGD and photosynthetic cells and trait changes in A. arenosa. As photosynthesis remains the Earth’s primary energy source and many polyploids show improved photosynthetic traits relative to diploids, particularly under environmental stress conditions like drought, a detailed understanding of how photosynthetic traits are modified in polyploids will likely fuel future plant breeding approaches aiming to improve crop yield through photosynthesis.