Rice (Oryza sativa) is one of the few plant species that can tolerate prolonged soil flooding or complete submergence thanks to an array of adaptive mechanisms. These include an ability to elongate submerged shoot organs at faster than normal rates and to develop aerenchyma, allowing the efficient internal transport of oxygen from the re-emerged elongated shoot to submerged parts. However, rice seeds are able to germinate anaerobically by means of coleoptile elongation. This cannot be explained in terms of oxygen transport through an emerged shoot. Differently from other cereal seeds that fail to induce the α-amylase enzymes required for starch degradation under anoxia, rice caryopses produce this enzyme, which allows starch degradation coupled to the fermentative metabolism and subsequent germination. The rapid depletion of soluble carbohydrates occurring during the first hours of germination under anoxia, together with a possible low-oxygen dependent change in calcium levels, leads to a signalling cascade that finally leads to α-amylase production. This process begins with the activation of a Calcineurin B-like (CBL), which targets the protein kinase CIPK15, which in turn triggers the SnRK1A pathway that induces the MYBS1 transcription factor which activates the starvation-inducible α-amylase gene RAmy3D. OsTPP7 encodes a trehalose-6-P-phosphate (T6P) phosphatase, which is non-functional in rice varieties that are unable to establish under submerged conditions. The presence of the OsTPP7 in rice accessions was correlated with increased sink strength of elongating coleoptiles, resulting in prolonged tolerance to complete submergence. High sucrose results in high T6P levels and consequently in repression of SnrK1 and downregulation of α-amylases. During anaerobic germination OsTPP7 misleads the seedling about its sugar status by converting T6P into trehalose. Subsequently the rice seedling can maintain a relative high sugar availability but low T6P levels, which, if high, would repress α-amylases. The subsequent intense flux of glucose from starch degradation is essential for fuelling glycolysis and lengthening of the coleoptile. Rice germination under anoxia is therefore the consequence of clever sugar management, that allows adept access to starch reserves.
Our activities focus on principally on rice, but also other cereals. We are interested on the characterization of molecular traits allowing successful germination under oxygen deprivation, that may represent a major step towards the successful selection of tolerant varieties.
- Nghi K.N., Tagliani A., Mariotti L., Weits D.A., Perata P., Pucciariello C. (2020) Auxin is required for the long coleoptile trait in japonica rice under submergence. New Phytologist, doi.org/10.1111/nph.16781;
- Pucciariello C. (2020) Molecular mechanisms supporting rice germination and coleoptile elongation under low oxygen. Plants 9: 1037;
- Nghi K.N., Tondelli A., Valè G. Tagliani A., Marè C., Perata P., Pucciariello C. (2019) Dissection of coleoptile elongation in japonica rice under submergence through integrated genome wide association mapping and transcriptional analyses. Plant, Cell & Environment 42: 1832.