ABA and the Floral Transition - Abscisic Acid: Metabolism, Transport and Signaling - page 355

18.1 Introduction
Plasticity in the switch to flowering shows the ability of plants to modify an inbuilt
developmental programme according to environmental cues. To maximise repro-
ductive success, the environmentally-sensitive processes of fertilization, embryo-
genesis and seed development must occur in coincidence with the most favourable
environmental conditions.
Variations in day length (photoperiod) or temperature play an important role in
the switch to flowering. In species like Arabidopsis (i.e. long day plants) flower-
ing is strongly accelerated under long day photoperiods (LDs). While photoperiod
represents a major environmental cue affecting the floral transitions, less is known
regarding the role of water-derived signals. This is interesting as water availabil-
ity plays a major role in plant adaptation and distribution. Water scarcity triggers
ABA accumulation in plants, which in turn causes stomatal closure and induces
expression of stress-related genes (Shinozaki and Yamaguchi-Shinozaki 2007 ).
ABA thus coordinates different networks of environmental and developmental
pathways leading to stress avoidance. Water deprivation is also known to trigger
flowering in different plant species, also referred to as drought escape-DE- (Xu
et al. 2005 ; Lafitte et al. 2006 ; Sherrard and Maherali 2006 ; Ivey and Carr 2012 ;
Franks et al. 2007 ; Franks 2011 ; Sharp et al. 2009 ; Srivastava et al. 2000 ). The
onset of DE forces plants to flower early and produce seeds before the environ-
ment becomes too dry to survive (Verslues and Juenger 2011 ). Since it is well
recognized that drought conditions trigger ABA accumulation in plants, ABA sig-
nalling might underpin the observed variations in DE responses.
The transition to flowering occurs at the shoot apical meristem (SAM), a
dome-like structure comprising a group of undifferentiated cells responsible for
the generation of all lateral primordial (leaves and buds) as well as the inter-
nodes (Sussex 1989 ; Huala and Sussex 1993 ). Plants are able to detect internal
and external cues and integrate these signals in the SAM. At the floral transi-
tion, the Arabidopsis vegetative SAM shifts from producing leaves/shoots to
flowers (Schultz and Haughn 1993 ). Four main pathways have been shown to
contribute to the floral transition including the photoperiodic, the vernalization,
the autonomous and the gibberellin pathways (Simpson et al. 1999 ; Bernier
1993 ; Martínez-Zapater et al. 1994 ). Furthermore, temperature and plant age
play an important role in fine-tuning flowering according to environmental
fluctuations or endogenous cues (Blázquez et al. 2003 ; Balasubramanian et al.
2006 ; Wang et al. 2009 ; Kumar and Wigge 2010 ; Seo et al. 2009 ). These signals
are integrated and share common components with the four major pathways
described above.
In this chapter, we will critically analyse the evidence for and against ABA
controlling the floral transition. We will highlight flowering genes and pathways
emphasising their relevance to ABA signalling. Most data on ABA action derive
from experiments in the model organism Arabidopsis thaliana (Thale cress)
although data from other plant species will also be included in our discussion.