Physiology and biochemistry of source-regulated protein accumulation in the wheat grain

BARNEIX, ATILIO.J.

Journal of Plant Physiology

ELSEVIER

Año: 2007 vol. 164 p. 581 - 590

0176-1617

Wheat is unique among cereals for the baking qualities of its flour, which are dependent upon the type and concentration of its proteins. As a consequence, the grain protein concentration (GPC) is one of the main determinants of wheat international market price. More than 50 to 70 % of the final grain N is accumulated before flowering and later remobilized to the grain, being N fertilization the common practice used to produce high GPC. However, after incremental additions of N fertilizer GPC reaches a maximum and then remains constant, without any increase in N uptake or remobilization by the crop, thus decreasing the efficiency of N fertilizer. Although the genetic and molecular mechanisms that regulate N uptake by the roots are being clarified fast, the regulation and physiology of N transport from the leaves to the grain remains less clear. In this review the possible regulatory points involved in N transport to the grain and the difficulties for increasing GPC are discussed. It has been demonstrated that protein synthesis in the grain is source-limited, and that the grain can accumulate protein limited only by the amino acids provided by the phloem. It has also been shown that there is no limitation in the amino acid/sugar ratios that can be exported to the phloem. On the other hand, NO3- uptake transporters are depressed when the plant concentration of some amino acids, like glutamine, is high. It was also shown that a high N supply increases cytokinins concentration, preventing leaf senescence and proteolysis.  Based on this information, it is postulated that there are two main regulatory points during grain filling when plant N status is ample. On the one hand the N uptake transporters in the roots are depressed due to the high amino acids concentration in the tissues, and N uptake is low. On the other hand, a high amino acids concentration keeps the cytokinins level high repressing leaf protein degradation and decreasing amino acid export to the phloem. As a consequence, GPC cannot be increased despite the ample N supply.