Tolerance of anaerobic conditions caused by flooding during germination and early growth in rice (Oryza sativa L.)

Abstract

Rice is semi-aquatic, adapted to a wide range of hydrologies, from aerobic soils in uplands to anaerobic and flooded fields in waterlogged lowlands, to even deeply submerged soils in flood-prone areas. Considerable diversity is present in native rice landraces selected by farmers over centuries. Our understanding of the adaptive features of these landraces to native ecosystems has improved considerably over the recent past. In some cases, major genes associated with tolerance have been cloned, such as SUB1A that confers tolerance of complete submergence and SNORKEL genes that control plant elongation to escape deepwater. Modern rice varieties are sensitive to flooding during germination and early growth, a problem commonly encountered in rainfed areas, but few landraces capable of germination under these conditions have recently been identified, enabling research into tolerance mechanisms. Major QTLs were also identified, and are being targeted for molecular breeding and for cloning. Nevertheless, limited progress has been made in identifying regulatory processes for traits that are unique to tolerant genotypes, including faster germination and coleoptile elongation, formation of roots and leaves under hypoxia, ability to catabolize starch into simple sugars for subsequent use in glycolysis and fermentative pathways to generate energy. Here we discuss the state of knowledge on the role of the PDC-ALDH-ACS bypass and the ALDH enzyme as the likely candidates effective in tolerant rice genotypes. Potential involvement of factors such as cytoplasmic pH regulation, phytohormones, reactive oxygen species scavenging and other metabolites is also discussed. Further characterization of contrasting genotypes would help in elucidating the genetic and biochemical regulatory and signaling mechanisms associated with tolerance. This could facilitate breeding rice varieties suitable for direct seeding systems and guide efforts for improving waterlogging tolerance in other crops.

Designing New Screening Methods and Physiological Dissection of Anaerobic Stress Tolerance in Rice

“…The enzyme alcohol dehydrogenase (ADH) via anaerobic fermentation metabolism has been reported by several researchers to supply ATP for seedling germination and survival under anaerobic stress conditions (Gibbs and Greenway, 1996;Gibbs et al, 2000;Jackson et al, 1982 Fig.5) with the correlation coefficient of r 2 = 0.53 was reported in this study similar to the findings of Miro and Ismail, (2013). This explains that higher the ADH activity higher will be the germination per cent and higher will be the survival rate and seedling establishment.…”

“…Hence, morpho-physiological traits attributed for anaerobic stress tolerance are germination per cent, greater shoot length by means of coleoptile elongation and higher vigor index. Since the rice seedlings escape low oxygen stress by coleoptile elongation rather than root emergence, the tolerant genotypes also recorded higher shoot: root ratio (Miro and Ismail, 2013).…”

“…All the genotypes taken for the study recorded 100 per cent germination percentage under control condition created for both the methods (Portray and Beaker). Fig 2, showed that the genotypes that recorded a germination percentage of more than 90 % under anaerobic condition were regarded as highly tolerant to low oxygen stress, as seed germination is a critical point in seedling establishment and subsequent plant vigor (Manigbas et al, 2008;Miro and Ismail, 2013 (Fig 3). Thus, the highly tolerant genotypes can be exploited as donors in crop improvement programmes aiming for tolerance to anaerobic stress conditions.…”

Involvement of Phenolic Compounds in Anaerobic Flooding Germination of Rice (<i>Oryza sativa</i> L.)

“…Although most of rice varieties are able to germinate under anaerobic or flooding conditions, the elongation of coleoptile highly links to the tolerance capacity of individual varieties [5,12,13]. In 5 cm flooding treatment, only "Koshihubo" variety showed a shoot height higher than water level after 14 days flood.…”

Plant responses to flooding

“…Rice adaptive plasticity to different hydrological regimes has allowed the selection and characterization of some genotypes that have been subsequently used to breed high yielding and tolerant modern varieties. Miro and Ismail (2013) present an overview of the current understanding of the mechanisms associated with tolerant traits such as anaerobic germination and early vigor in rice that can help to develop tolerant varieties for direct-seeding systems. Narsai and Whelan (2013) employed a meta-analyses of microarray data to compare global low oxygen transcriptomic responses of tolerant rice with the relatively intolerant Arabidopsis.…”