Does a thick leaf gas film protect against submergence on rice?

The saying “Rice is Life” refers to its importance and the fact the rice feeds half the world. The third most constraining environmental stress for rice is submergence (e.g. the entire plant underneath water). Some rice genotypes strategically limit stem elongation and enter low-oxygen quiescence syndrome to maintain their energy supplies during submergence and post-submergence recovery. 

A group of seven scientists led by Drs Chakraborty and Akankhya Guru ICAR- National Rice Research Institute (India) and Indira Gandhi Agricultural University (India), studied the importance of genetic background, leaf gas film and hydrophobicity of 12 rice genotypes whilst submerging them. The researchers found that rice genotypes with SUB1 genes have thicker leaf gas film and more epicuticular wax in comparison to non-SUB1 genotypes,  which allows the differentiation of submergence tolerant varieties. 

Rice leaves are coated by a form of hydrophobic wax which creates a leaf gas film. This film helps with respiration and photosynthesis under water. Rice plants have the thickest leaf gas film on both sides of the leaves amongst all cereal crops. The key gene to submerge tolerance is SUB1A-1 within the SUB1 QTL region. The gene suppresses the action of the plant hormone ethylene which restricts stem elongation.

The researchers grew 12 rice genotypes in 2017 and 2018 at ICAR. In total, 240 pots containing three, 25-day-old plants were submerged under 100 cm water. The leaf gas film was removed from half of the pots by brushing the leaves with cotton balls soaked in TritonX-100 surfactant. After 14 days of submergence, leaf gas film thickness, tissue porosity and leaf density was measured. The rate of leaf gas film depletion was measured in seven consecutive days of each genotype separately along with leaf hydrophobicity and epicuticular wax content. Researchers also recorded plant survival, elongation ability, ethylene and total chlorophyll content. Genotyping and gene expression of the Leaf Gas Film1 (LGF1) gene associated with epicuticular biosynthesis were used to also differentiate submergence tolerant and susceptible genotypes.

All of the 12 genotypes had the SUB1 QTL region but six of them had the SUB1 gene. The leaf gas film thickness was significantly higher in SUB1 genotypes but leaf porosity and leaf density did not show any clear patterns between the genotypes. By the fifth day of submergence, the leaf gas film disappeared in non-SUB1 genotypes whilst SUB1 genotypes retained the film longer. The epicuticular wax content and the expression of its responsible gene were also key to differentiate submergence tolerant genotypes. The removal of the leaf gas film lowered plant survival and significantly altered the levels of ethylene production in these genotypes. 

“Faster induction of submergence induced genes and increased accumulation of ethylene upon LGF removal suggests that presence of LGF not only acts as a physical barrier for stress perception but also serves as a medium for ethylene ventilation during the submergence period”, Chakraborty, Guru and colleagues wrote. “Thus, removal of it results in partial loss of well-known quiescence function of SUB1 and overall submergence tolerance ability in rice.”