Growth and Development of higher plants

Balancing growth and development in roots and shoots is of vital importance for plants to adequately adapt to environmental conditions. Although root-shoot relationships are of prime importance all over plant’s life, roots remained the ‘hidden half’ for long, because shoot development was more easily studied. This particularly holds true in the field of flowering.
The time at which flowering occurs is governed by environmental cues such as day length and temperature, and is influenced by endogenous signals related to the age of the plant. Classical physiological experiments demonstrated that environmental signals that influence flowering are perceived in different tissues. For example, day length (photoperiod) is detected in expanded leaves, causing systemic signals (‘the floral stimulus’) to travel to the shoot apical meristem (SAM) to trigger flowering. In contrast, vernalisation, the acquisition of competence to flower that results from exposure to extended periods of cold (winter conditions), is perceived by meristematic tissues, hence might not involve long-distance signals and be SAM autonomous.
 
Arabidopsis thaliana is a facultative long-day (LD), vernalisation-requiring plant. Mutant analyses identified major genes involved in controlling its flowering time (reviewed in Bernier & Périlleux, 2005):  
-    In the ‘photoperiod pathway’, CONSTANS (CO), which encodes a zinc finger transcription factor, is controlled at the transcriptional level by the circadian clock and exposure to light, and at the post-transcriptional level by stabilization of the protein in response to light. The combination of these regulatory mechanisms results in accumulation of CO protein specifically under LD. CO then activates the transcription of FLOWERING LOCUS T (FT), which encodes a protein with similarities to Raf kinase inhibitor proteins. The activation of FT by CO appears to occur specifically in the phloem, where CO and FT are both expressed. It was recently demonstrated that the product of FT induces a graft transmissible signal that is transported through the phloem and FT itself is now regarded as a major component of the floral stimulus.
- Both the vernalisation- and the autonomous- pathways act by relieving the inhibition of flowering by FLOWERING LOCUS C (FLC), which is expressed in the SAM. The reduction in FLC expression by cold involves chromatin remodelling of FLC and requires a battery of specific proteins (VIN3, VRN2, VRN1). FLC is also repressed by the products of genes of the ‘autonomous pathway’, such as FCA, an RNA-binding protein whose abundance is controlled by alternative splicing of its transcripts.
All pathways ultimately converge to regulate in the SAM transcription of a set of integrator genes, particularly SUPPRESSOR OF OVEREXPRESSION OF CONSTANS 1 (SOC1), which is one of the earliest MADS-box gene expressed in the SAM at floral transition. Both SOC1 and FT are repressed by FLC.

From this short survey, a simplifying view may be that all genes act at flowering where they are expected to: 1) in the photoperiodic pathway, CO and FT are expressed in the vasculature of the leaves; FT then triggers a long-distance signal (maybe its own product) to activate SOC1 in the SAM; 2) in the vernalisation pathway, repression of FLC in the SAM allows activation of SOC1 and the shift to floral morphogenesis. However, a careful examination of published pictures led to the conclusion that several genes controlling flowering time, such as FLC, FCA, VIN3 are also expressed in the root apical meristem (RAM), although their function there has been poorly investigated. Very recently, it was reported that mis-expression of CO, FLC or SOC1 in the roots does no rescue the flowering phenotype of knock-out co, flc, or soc1 mutants, respectively but this approach does not address the question of which role the roots may have during flowering of wild type plants. Furthermore, there is no evidence that the long-distance signalling that includes FT going from leaves to SAM does not pass through the roots. It is known from other studies that roots are part of a signalling network controlling floral transition. In Sinapis alba, a close relative of Arabidopsis, P2 reported that nutrients and hormones are flowering signals that follow a complex ‘shoot-to-root-to-shoot’ physiological loop to integrate C- and N-assimilate availability. Flowering is consistently stimulated by an increase in the C/N ratio of the phloem sap and several critical events of SAM transition are triggered by the application of sucrose or cytokinins. The aim of WP4 will be to analyse how the root system contributes to the molecular events leading to floral transition in Arabidopsis. This is feasible thanks to the availability of an hydroponic system that has been designed in the lab of P2 for synchronous growth and flowering of Arabidopsis, up to mature stage of development.