BackgroundBeing sessile organisms, plants are often exposed to a wide array of abiotic and biotic stresses. Abiotic stress conditions include drought, heat, cold and salinity, whereas biotic stress arises mainly from bacteria, fungi, viruses, nematodes and insects. To adapt to such adverse situations, plants have evolved well-developed mechanisms that help to perceive the stress signal and enable optimal growth response. Phytohormones play critical roles in helping the plants to adapt to adverse environmental conditions. The elaborate hormone signaling networks and their ability to crosstalk make them ideal candidates for mediating defense responses.ResultsRecent research findings have helped to clarify the elaborate signaling networks and the sophisticated crosstalk occurring among the different hormone signaling pathways. In this review, we summarize the roles of the major plant hormones in regulating abiotic and biotic stress responses with special focus on the significance of crosstalk between different hormones in generating a sophisticated and efficient stress response. We divided the discussion into the roles of ABA, salicylic acid, jasmonates and ethylene separately at the start of the review. Subsequently, we have discussed the crosstalk among them, followed by crosstalk with growth promoting hormones (gibberellins, auxins and cytokinins). These have been illustrated with examples drawn from selected abiotic and biotic stress responses. The discussion on seed dormancy and germination serves to illustrate the fine balance that can be enforced by the two key hormones ABA and GA in regulating plant responses to environmental signals.ConclusionsThe intricate web of crosstalk among the often redundant multitudes of signaling intermediates is just beginning to be understood. Future research employing genome-scale systems biology approaches to solve problems of such magnitude will undoubtedly lead to a better understanding of plant development. Therefore, discovering additional crosstalk mechanisms among various hormones in coordinating growth under stress will be an important theme in the field of abiotic stress research. Such efforts will help to reveal important points of genetic control that can be useful to engineer stress tolerant crops.

During the transition from vegetative to reproductive growth, the shoot meristem of flowering plants acquires the inflorescence identity to generate flowers rather than vegetative tissues. An important regulator that promotes the inflorescence identity in Arabidopsis is AGAMOUS-LIKE 24 (AGL24), a MADS-box transcription factor. Using a functional estradiol-inducible system in combination with microarray analysis, we identified AGL24-induced genes, including SUPPRESSOR OF OVEREXPRESSION OF CO 1 (SOC1), a floral pathway integrator. Chromatin immunoprecipitation (ChIP) analysis of a functional AGL24-6HA-tagged line revealed in vivo binding of AGL24-6HA to the regulatory region of SOC1. Mutagenesis of the AGL24 binding site in the SOC1 promoter decreased Pro SOC1 :GUS expression and compromised SOC1 function in promoting flowering. Our results show that SOC1 is one of the direct targets of AGL24, and that SOC1 expression is upregulated by AGL24 at the shoot apex at the floral transitional stage. ChIP assay using a functional SOC1-9myc-tagged line and promoter mutagenesis analysis also revealed in vivo binding of SOC1-9myc to the regulatory regions of AGL24 and upregulation of AGL24 at the shoot apex by SOC1. Furthermore, we found that as in other flowering genetic pathways, the effect of gibberellins on flowering under short-day conditions was mediated by the interaction between AGL24 and SOC1. These observations suggest that during floral transition, a positive-feedback loop conferred by direct transcriptional regulation between AGL24 and SOC1 at the shoot apex integrates flowering signals.

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Gibberellins (GAs) are a class of plant hormones involved in the regulation of flower development in Arabidopsis. The GA-deficient ga1-3 mutant shows retarded growth of all floral organs, especially abortive stamen development that results in complete male sterility. Until now, it has not been clear how GA regulates the late-stage development of floral organs after the establishment of their identities within floral meristems. Various combinations of null mutations of DELLA proteins can gradually rescue floral defects in ga1-3. In particular, the synergistic effect of rga-t2 and rgl2-1 can substantially restore flower development in ga1-3. We find that the transcript levels of floral homeotic genes APETALA3 (AP3), PISTILLATA (PI), and AGAMOUS (AG) are immediately upregulated in young flowers of ga1-3 upon GA treatment. Using a steroid-inducible activation of RGA, we further demonstrated that these floral homeotic genes are transcriptionally repressed by RGA activity in young flowers whereas the expression of LEAFY (LFY) and APETALA1 (AP1) is not substantially affected. In addition, we observed the partial rescue of floral defects in ga1-3 by overexpression of AG. Our results indicate that GA promotes the expression of floral homeotic genes by antagonizing the effects of DELLA proteins, thereby allowing continued flower development.F lower development starts with the specification of floral meristem identity, and of floral organ identity within nascent floral meristems flanking an inflorescence meristem. During this process, LEAFY (LFY) stands out as a major regulator in both integrating upstream floral inductive signals and controlling three classes of downstream floral homeotic genes, called A, B, and C function genes, respectively. These classes of downstream genes control floral organ identity in discrete domains (1-3). Although it has been clear that combinatorial activation of A, B, and C function genes governs the identity of different floral organs (3-5), little is known about how a young floral bud with established floral organ identities continues to develop into a mature flower. It is possible that floral homeotic genes play consistent roles in initiation and further promotion of floral organ growth because the region-specific expression of these genes is present throughout the whole process of flower development (6-11). Indeed, the C function gene AGAMOUS (AG) has been suggested to function not only in the early specification of stamen and carpel identity, but also in the late patterning of carpel structures (12). It is noteworthy that, as a major promoter of floral homeotic genes, LFY is not expressed in flowers after stage 5 (13). If floral homeotic genes are the regulators required for both floral organ identity and their continued development, the promotion of floral homeotic genes at later stages of flower development should be regulated by mechanisms other than LFY-dependent ones. The gibberellin (GA)-deficient ga1-3 mutant develops flowers with retarded growth of all floral organs despite their...