Cytokinin: a hormone with many roles
School of Biological Sciences, University of Canterbury, Private Bag 4800, Christchurch 8140, New Zealand.
In the early 1980’s, the cytokinins were declared to be "the hormone without a role". Now the cytokinins are known to be involved in most aspects of the plant life cycle. Early work showed that radio-labelled cytokinins travelled from the roots to the shoots. Our use of a root-specific Cu-inducible promoter driving cytokinin biosynthesis (via IPT) confirmed this mobility, as well as a role for cytokinin in releasing axillary buds. Further, we recently showed that root-produced cytokinins play a role in signalling the nitrogen status of the plant and suggest that the cytokinins are involved not only in the uptake of nitrogen but also in the homeostatic mechanism balancing uptake and assimilation of nitrogen with availability of carbohydrate in perennial ryegrass. During reproduction, cytokinin is limiting to pod/seed number. In pea as in arabidopsis, tipping the balance in favour of accumulation of cytokinin (by reducing cytokinin destruction) in the shoot apical meristem may hold the key to pod and seed number. Moreover, root-supplied cytokinin remains restricted to the maternal tissue and does not cross to filial tissue, as seeds form. Our gene expression studies confirmed that cytokinin is biosynthesised within the developing seed, and that there is a complex interplay between biosynthesis, metabolism and destruction during seed development, along with differential activity of various homoeologous gene family members in both tetraploid forage brassica and hexaploid wheat. Such detailed knowledge is needed to guide future gene editing for yield improvement. While cytokinins are intimately linked to cell division in cereals, in legumes they are implicated in enhancing sink activity. In double transgenic peas ectopically expressing both an amino acid transporter (AAP) and a sucrose transporter (SUT), we showed both elevated IPT expression and elevated endogenous cytokinins in the seed coat, alongside increased yield, clearly indicating, again, the intimate relationship between cytokinin, nitrogen and carbohydrate. The ability of cytokinins to delay senescence has been known for many years, as has the metabolism and/or the inactivation of cytokinin in senescing leaves. Unexpectedly, we have shown cytokinin biosynthesis increasing in both mature kiwifruit and in senescing leaves, and suggest that the free radical scavenging abilities of cytokinin may be associated with maintenance of mitochondrial membranes late into the senescence process. Several gall-forming bacteria produce cytokinins. However, for many years the multiple shoots and leafy galls produced by Rhodococcus fascians have been an enigma, because of the lack of elevated cytokinin in infected tissue. Both epiphytic and endophytic R. fascians enhance transporter gene expression but only the endophytic strains markedly affect morphology. We recently confirmed that endophytic R. fascians strains can produce a novel methylated cytokinin, and suggest that the interaction between this cytokinin, the new family of sugar transporters (SWEETS) and cell wall invertases enhances the multiple shoots formed following seed inoculation of pea by endophytic strains of R. fascians. The cytokinins are clearly a hormone with many roles.