Defining the role of PIF3-like bHLH transcription factors in the integration of light and cold signalling in Arabidopsis

The period following seedling emergence is a particularly vulnerable stage in the plant life cycle. In Arabidopsis thaliana, the phytochrome-interacting factor (PIF) subgroup of basic-helix-loop-helix transcription factors has a pivotal role in regulating growth during this early phase, integrating environmental and hormonal signals. We previously showed that SPATULA (SPT), a PIF homolog, regulates seed dormancy. This project established that SPT-like gene PIF6 is important in the establishment of primary seed dormancy in Arabidopsis, and provided evidence that only one particular splice form of PIF6, PIF6-b is active (Penfield et al, Plant Mol Biol. 2010 73:89–95). SPT was shown to have wide-ranging roles, regulating gynoecium development (Foreman et al., Plant Signal Behav. 2011 6(4):471-6) and leaf cell expansion, in addition to germination (Josse et al., Plant Cell. 2011 23(4):1337-51).

In the control of cell expansion we demonstrated that SPT acts in an analogous manner to the gibberellin-dependent DELLAs which restrain seedling leaf expansion. However, although DELLAs are not required for SPT action, we showed that SPT is subject to negative control by DELLAs. Cross-regulation of SPT by DELLAs ensures that SPT protein levels are limited when DELLAs are abundant but rise following DELLA depletion. We showed that this regulation provides a means to prevent excessive growth suppression that would result from the dual activity of SPT and DELLAs, yet maintain growth restraint under DELLA-depleted conditions. We also provided evidence that SPT and DELLAs regulate common gene targets and illustrated that the balance of SPT and DELLA action depends on light quality signals in the natural environment.

Our previous work indicated that SPT was important for cold-activated seed germination. In this project we established that SPT-suppression of growth in the adult plant was temperature-dependent (Sidaway-Lee et al., 2010 20(16):1493-7). We demonstrated that when temperature falls SPT operates during the daytime to repress growth in adult plants. We were able to show that only daytime temperatures affect vegetative growth and that SPT couples morning temperature to growth rate. In seedlings, warm temperatures inhibit the accumulation of the SPT protein, and SPT autoregulates its own transcript abundance in conjunction with diurnal effects. Our data therefore inferred that SPT integrates time of day and temperature signalling to control vegetative growth rate.

This body of work established that transcription factor, SPT is a key regulator of gynoecium development and leaf cell expansion. As we had previously shown that SPT controlled seed germination, this new work suggests that SPT has pivotal role in wide-ranging responses through the plant lifecycle.



We defined the molecular mechanism through which SPT suppresses cell expansion. Furthermore, our research highlighted the molecular interplay between SPT and the DELLA proteins, that also repress growth. We showed that cross-regulation of SPT by DELLAs is an important feedback required to prevent excessive growth repression that would otherwise occur when both SPT and DELLAs were present.



Our research illustrated that other SPT-like genes had similar functions to SPT during seed germination. We also showed that SPT has a larger role at cool ambient temperatures and that SPT is important for regulating adult plant growth during the daily (daytime) growth surge. As plants are subject to a wide range of temperatures in the natural environment it is important to determine the temperature range over which pathway regulators operate in nature.

1. Identification of a key regulator (SPT) of plant leaf expansion.

2. Elucidation of the molecular mechanism through which SPT operates.

3. Demonstration the SPT and the unrelated DELLA proteins target common gene sets.

4. Demonstration that SPT operates in a counterbalance mechanism with the DELLA growth regulators to maintain control of leaf expansion. This cross-regulation is important to prevent excessive growth supression which would occur if SPT and DELLAs were present simultaneously.

5. Validated PIF6 (a SPT homologue) as a regulator of germination.

6. Identification of the active PIF6 splice variant.

7. Established that SPT operates at cool temperature environments.

8. Illustrated that SPT is photoperiod-regulated and that it is a major daytime suppressor of growth.