Barneix Atilio J
IBYF-CONICET-FAUBA, Av. San Martín 4453, 1417 Buenos Aires, Argentina.
J Plant Physiol. 2007 May;164(5):581-90. doi: 10.1016/j.jplph.2006.03.009. Epub 2006 May 11.
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 50-70% of the final grain N is accumulated before flowering and later remobilized to the grain, N fertilization being 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 quickly, 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, NO(3)(-) uptake transporters are depressed when the plant concentration of some amino acids, such as glutamine, is high. It has also been 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, 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.
小麦在谷物中独具特色，因其面粉具有烘焙品质，而这取决于其蛋白质的类型和浓度。因此，谷物蛋白质浓度（GPC）是小麦国际市场价格的主要决定因素之一。超过50%-70%的最终籽粒氮在开花前积累，随后再转运至籽粒中，施用氮肥是用于提高GPC的常见做法。然而，在氮肥增量施用后，GPC达到最大值，然后保持不变，作物的氮吸收或再转运并未增加，从而降低了氮肥效率。尽管调节根系氮吸收的遗传和分子机制正在迅速得到阐明，但氮从叶片向籽粒转运的调节和生理机制仍不太清楚。在本综述中，讨论了氮向籽粒转运可能涉及的调控点以及提高GPC的困难。已经证明，籽粒中的蛋白质合成受源限制，并且籽粒只能积累由韧皮部提供的氨基酸所限制的蛋白质。还表明，可输出到韧皮部的氨基酸/糖比例没有限制。另一方面，当植物中某些氨基酸（如谷氨酰胺）的浓度较高时，NO(3)(-)吸收转运体受到抑制。还表明，高氮供应会增加细胞分裂素浓度，防止叶片衰老和蛋白质水解。基于这些信息，推测在籽粒充实期间，当植物氮素状况充足时，有两个主要调控点。一方面，由于组织中氨基酸浓度高，根系中的氮吸收转运体受到抑制，氮吸收较低。另一方面，高氨基酸浓度使细胞分裂素水平保持较高，抑制叶片蛋白质降解并减少氨基酸向韧皮部的输出。因此，尽管氮供应充足，GPC仍无法提高。
