Cytokinin biosynthesis regulation
Cytokinins are known to play very important roles in the regulation of plant development. They are involved in processes ranging from cell differentiation through to the control of senescence. Since their discovery in 1955 much work has been done looking into the form and function of cytokinins.
There are known to be two main classes of cytokinins; those with isoprenoid side chains and those with aromatic side chains. This section of the cytokinin section of plant biology advice takes a look at how the biosynthesis of cytokinins are regulated.
Nitrogen supply and cytokinin biosynthesis
It is well known that inorganic nitrogen has a huge impact upon the growth and development of plants. The cytokinin hormone is a key signalling factor used in analysing, and responding to, the nitrogen nutrient levels available to the plant. Cytokinins are able to travel via the xylem from the root to the shoots and acts as a long range signal of nitrogen availability. In Arabidopsis there is a molecular mechanism in place that controls nitrogen dependent biosynthesis; similar mechanism are expected across the plant kingdom. In Arabidopsis the regulation of the genes IPT3 and IPT5 are dependent on the availability of nitrogen. These genes code for adenosine phosphate-isopentenyltransferases and are located in the plastids. This strongly suggests that nitrogen levels have a major role to play in the biosynthesis of cytokinin in Arabidopsis.
Plant hormones and cytokinin biosynthesis regulation
Many of the genes that are known to be involved in the biosynthesis of cytokinins are themselves under the influence of plant hormones; these hormones include auxin, abscisic acid and even cytokinins themselves. Some of the main genes that are involved in cytokinin biosynthesis are IPT (encodes adenosine phosphate-isopentenyltransferase) and CKX (encodes cytokinin oxidase/dehydrogenase). it has been shown in Arabidopsis that the transcripts of some IPT genes accumulate in the presence of auxin in the roots, and some IPTs are down regulated by the existence of cytokinins. The CKX genes have been shown to be unregulated in maize roots in the presence of abscisic acids and cytokinins.
Miyawaki et al. (2004). Expression of cytokinin biosynthetic isopentenyltransferase genes in Arabidopsis: tissue specificity and regulation by auxin, cytokinin, and nitrate. Plant J. 37:128 to 138
Sakakibara (2006) Cytokinins: Activity, Biosynthesis, and translocation. Annu. Rev. Plant Bio. 57: 431 to 449
Takei et al (2002). Multiple routes communicating nitrogen availability from roots to shoots: a signal transduction pathway mediated by cytokinin. J. Exp. Bot. 53:971 to977
Takei et al. (2004). Arabidopsis CYP735A1 and CYP735A2 encode cytokinin hydroxylases that catalyze the biosynthesis of trans-zeatin. J. Biol. Chem. 279:41866 to 72