Question

Describe and discuss the experiments that confirmed that FT is the mobile flowering compound known as ‘florigen’. Please explain as detail as possible. Thank you

Answer 1

Unambiguous evidence for signal transmission can derive from a range of different experimental approaches.

The main strategies aim to detect

(i) movement of the putative signal molecule,

(ii) a phenotypic change,

(iii) altered expression of a molecular target.

To distinguish local signalling in the SAM from signals arriving from distant sources, one of the most powerful tools is grafting, which combines donor and receiver tissues either differing genetically or in prior treatment such as inductive photoperiod.

Alternatively, spatially regulated gene expression can be tested, for example by employing the phloem-specific SUC2 or minor vein phloem-specific GAS1 promoters .

The evidence for floral signal transport in the phloem is based both on signal velocity and directionality . In this context, it is important to define what is meant by ‘delivery’. In the sense of molecules with signalling functions, it is essential first for the signal to be loaded into the phloem stream via companion cells and then move from this site towards the SAM. Arrival at its final destination within the SAM requires post-phloem transport, involving local cell-to-cell migration within the shoot meristem. The simple presence of a particular RNA, protein or small molecule hormone in sampled phloem sap is insufficient on its own to assess functional delivery.

A further caveat concerns interpretation of negative data from grafting experiments.

If vascular transport is moving a putative signal molecule from donor to receiver, it is essential to demonstrate that the flow direction is as intended. Where both scion and rootstock carry leaves, prediction of phloem flow is not straightforward and is best tested by labelled CO₂ or sugars to the donor side and confirming that label is detectable in the receiver tissues . In one recent case testing graft transmission of FT effects in poplar, the lack of response could be due to incongruence between source–sink flow and donor–receiver directionality .

Frequently, receiver shoot defoliation is included as part of the experimental manipulation: this will indeed enhance donor–receiver flow, but may also have local effects on the receiver itself, from causing stress-induced flowering to the removal of other signalling molecules.