Whole-Plant Dynamic System of Nitrogen Use for Vegetative Growth and Grain Filling in Rice Plants (Oryza sativa L.) as Revealed through the Production of 350 Grains from a Germinated Seed Over 150 Days: A Review and Synthesis

Abstract

A single germinated rice (Oryza sativa L) seed can produce 350 grains with the sequential development of 15 leaves on the main stem and 7–10 leaves on four productive tillers (forming five panicles in total), using nitrogen (N) taken up from the environment over a 150-day growing season. Nitrogen travels from uptake sites to the grain through growing organ-directed cycling among sequentially developed organs. Over the past 40 years, the dynamic system for N allocation during vegetative growth and grain filling has been elucidated through studies on N and 15N transport as well as enzymes and transporters involved. In this review, we synthesize the information obtained in these studies along the following main points: (1) During vegetative growth before grain-filling, about half of the total N in the growing organs, including young leaves, tillers, root tips and differentiating panicles is supplied via phloem from mature source organs such as leaves and roots, after turnover and remobilization of proteins, whereas the other half is newly taken up and supplied via xylem, with an efficient xylem-to-phloem transfer at stem nodes. Thus, the growth of new organs depends equally on both N sources. (2) A large fraction (as much as 80%) of the grain N is derived largely from mature organs such as leaves and stems by degradation, including the autophagy pathway of chloroplast proteins (e.g., Rubisco). (3) Mobilized proteinogenic amino acids (AA), including arginine, lysine, proline and valine, are derived mainly from protein degradation, with AA transporters playing a role in transferring these AAs across cell membranes of source and sink organs, and enabling their efficient reutilization in the latter. On the other hand, AAs such as glutamine, glutamic acid, γ-amino butyric acid, aspartic acid, and alanine are produced by assimilation of newly taken up N by roots and and transported via xylem and phloem. The formation of 350 filled grains over 50 days during the reproductive stage is ascribed mainly to degradation and remobilization of the reserves, previously accumulated over 100 days in the sequentially developed vegetative organs.

Ornithine δ‐aminotransferase is critical for floret development and seed setting through mediating nitrogen reutilization in rice

“…Rice is a major food crop worldwide, its yield is greatly influenced by nitrogen using efficiency, especially during the reproductive growth stage (Yoneyama et al, 2016). Due to the importance of nitrogen for plant growth, nitrogen utilization has always been a topic of study (Tabuchi et al, 2005;Sun et al, 2014a;Chen et al, 2016).…”

“…In the vegetative growth stage, about half of the total nitrogen in the growing organs is supplied via phloem from mature source organs (e.g. leaves and roots), while the other half is acquired from the environment (Yoneyama et al, 2016). During the grain filling stage, most grain nitrogen (as much as 80%) is derived from mature leaves and shoots by reutilization (Yoneyama et al, 2016).…”

Nitrogen Use Efficiency in Rice

“…Thus, Rubisco is an excellent N storage molecule, and its autophagic degradation in rice leaves may contribute to an efficient and rapid N remobilization by facilitating protein degradation for N mobilization in senescent leaves [70]. Rubisco is also involved in photorespiratory losses which can be as high as 20% of the total carbon fixation in C 3 plants and also liberates ammonia, which is required for reassimilation [105].…”

“…Up to 95% of seed protein is derived from amino acids that are exported to the seed after the degradation of existing proteins in leaves [14], and the rest is supplemented from the soil and late top-dressed fertilizers [70]. Gln and asparagine (Asn) are major forms of total amino acids in phloem and xylem sap of rice plants [14,71].…”

Modelling of ammonia volatilisation in fertilised and flooded rice systems

“…NH 3 cycling within crop canopies appears to depend simultaneously on structure and density of the crop canopy (Denmead et al, 2008), opening of stomata affected by crop water status (Chaves et al, 2016), rate and timing of N fertiliser application (Table 2, Chapter 1), floodwater or soil properties which may affect availability of NH 3 susceptible to volatilisation , and the whole-plant N dynamics, in particular rate of N uptake, level of glutamine synthetase activity that affects assimilation of NH 4 + by the crop that was shown to vary among cultivars, and the N remobilisation from senescing leaves to growing leaves or grain which may differ among plants (Kumagai et al, 2011, Yoneyama et al, 2016. All these factors may contribute to the contradicting observations in fertilised and flooded rice systems compared to the other crop systems, and may also affect the ratio between NH 3 volatilisation from the floodwater and NH 3 gas exchange through the stomatal pathway reported by .…”