Pores for Thought: Can Genetic Manipulation of Stomatal Density Protect Future Rice Yields?

Abstract

Rice (Oryza sativa L.) contributes to the diets of around 3.5 billion people every day and is consumed more than any other plant. Alarmingly, climate predictions suggest that the frequency of severe drought and high-temperature events will increase, and this is set to threaten the global rice supply. In this review, we consider whether water or heat stresses in cropsespecially ricecould be mitigated through alterations to stomata; minute pores on the plant epidermis that permit carbon acquisition and regulate water loss. In the short-term, water loss is controlled via alterations to the degree of stomatal "openness", or, in the longer-term, by altering the number (or density) of stomata that form. A range of molecular components contribute to the regulation of stomatal density, including transcription factors, plasma membrane-associated proteins and intercellular and extracellular signaling molecules. Much of our existing knowledge relating to stomatal development comes from research conducted on the model plant, Arabidopsis thaliana. However, due to the importance of cereal crops to global food supply, studies on grass stomata have expanded in recent years, with molecular-level discoveries underscoring several divergent developmental and morphological features. Cultivation of rice is particularly water-intensive, and there is interest in developing varieties that require less water yet still maintain grain yields. This could be achieved by manipulating stomatal development; a crop with fewer stomata might be more conservative in its water use and therefore more capable of surviving periods of water stress. However, decreasing stomatal density might restrict the rate of CO 2 uptake and reduce the extent of evaporative cooling, potentially leading to detrimental effects on yields. Thus, the extent to which crop yields in the future climate will be affected by increasing or decreasing stomatal density should be determined. Here, our current understanding of the regulation of stomatal development is summarised, focusing particularly on the genetic mechanisms that have recently been described for rice and other grasses. Application of this knowledge toward the creation of "climate-ready" rice is discussed, with attention drawn to the lesser-studied molecular elements whose contributions to the complexity of grass stomatal development must be understood to advance efforts.

Functional Analysis of the Rice Type-B Response Regulator RR22

“…Ectopic overexpression of RR22 also resulted in a decrease in the number of adjacent stomatal cell files. This effect on stomata development may involve the genes SHR and SCR, because these partner transcription factors positively regulate the formation of stomatal cell files in rice (Schuler et al, 2018;Buckley et al, 2019;Wu et al, 2019) and, based on studies in Arabidopsis, cytokinin can inhibit SCR expression (Zhang et al, 2013). These results suggest that, in addition to affecting cell size and proliferation, that alterations in the cytokinin transcriptional response can also affect the developmental architecture of the rice leaf.…”

RSD1 Is Essential for Stomatal Patterning and Files in Rice

“…Although global climatic variability is a serious threat to food security, genetic engineering of stomatal development will enable us to create stress-tolerant crops (Serna and Fenoll, 2002;Korres et al, 2017). By controlling stomatal development and reducing stomatal density, rice can control water loss and make it easier to survive under drought conditions (Buckley et al, 2019). The lack of RSD1 led to a reduction of stomatal density and the leaf water loss rate in rice.…”

“…The lack of RSD1 led to a reduction of stomatal density and the leaf water loss rate in rice. The effect of stomatal density on plants has been applied to create drought-resistant crops (Buckley et al, 2019). Recent research has shown that excessive expression of EPF genes in wheat and rice can significantly improve water use efficiency without affecting plant yield when stomatal density is reduced (Caine et al, 2019;Dunn et al, 2019).…”

Emerging Molecular Strategies for Improving Rice Drought Tolerance

“…Reducing stomata density can help restricting water loss; however, it also reduces leaf CO 2 levels by hindering CO 2 exchange between the plant and the environment [ 18 ]. A recent study implied developing future-ready water-use efficient rice by targeting stomatal density using epidermal patterning factor OsEPF1.…”