Shoot branching is a major determinant of plant architecture and is highly regulated by endogenous and environmental cues. Two classes of hormones, auxin and cytokinin, have long been known to have an important involvement in controlling shoot branching. Previous studies using a series of mutants with enhanced shoot branching suggested the existence of a third class of hormone(s) that is derived from carotenoids, but its chemical identity has been unknown. Here we show that levels of strigolactones, a group of terpenoid lactones, are significantly reduced in some of the branching mutants. Furthermore, application of strigolactones inhibits shoot branching in these mutants. Strigolactones were previously found in root exudates acting as communication chemicals with parasitic weeds and symbiotic arbuscular mycorrhizal fungi. Thus, we propose that strigolactones act as a new hormone class-or their biosynthetic precursors-in regulating above-ground plant architecture, and also have a function in underground communication with other neighbouring organisms.

Bioactive gibberellins (GAs) are diterpene plant hormones that are biosynthesized through complex pathways and control diverse aspects of growth and development. Biochemical, genetic, and genomic approaches have led to the identification of the majority of the genes that encode GA biosynthesis and deactivation enzymes. Recent studies have highlighted the occurrence of previously unrecognized deactivation mechanisms. It is now clear that both GA biosynthesis and deactivation pathways are tightly regulated by developmental, hormonal, and environmental signals, consistent with the role of GAs as key growth regulators. In some cases, the molecular mechanisms for fine-tuning the hormone levels are beginning to be uncovered. In this review, I summarize our current understanding of the GA biosynthesis and deactivation pathways in plants and fungi, and discuss how GA concentrations in plant tissues are regulated during development and in response to environmental stimuli.

The hormone-mediated control of plant growth and development involves both synthesis and response. Previous studies have shown that gibberellin (GA) plays an essential role in Arabidopsis seed germination. To learn how GA stimulates seed germination, we performed comprehensive analyses of GA biosynthesis and response using gas chromatography-mass spectrometry and oligonucleotide-based DNA microarray analysis. In addition, spatial correlations between GA biosynthesis and response were assessed by in situ hybridization. We identified a number of transcripts, the abundance of which is modulated upon exposure to exogenous GA. A subset of these GA-regulated genes was expressed in accordance with an increase in endogenous active GA levels, which occurs just before radicle emergence. The GA-responsive genes identified include those responsible for synthesis, transport, and signaling of other hormones, suggesting the presence of uncharacterized crosstalk between GA and other hormones. In situ hybridization analysis demonstrated that the expression of GAresponsive genes is not restricted to the predicted site of GA biosynthesis, suggesting that GA itself, or GA signals, is transmitted across different cell types during Arabidopsis seed germination.

Previous work showed that PHYTOCHROME-INTERACTING FACTOR3-LIKE5 (PIL5), a light-labile basic helix-loop-helix protein, inhibits seed germination by repressing GIBBERELLIN 3beta-HYDROXYLASE1 (GA3ox1) and GA3ox2 and activating a gibberellic acid (GA) catabolic gene (GA2ox2). However, we show persistent light-dependent and PIL5-inhibited germination behavior in the absence of both de novo GA biosynthesis and deactivation by GA2ox2, suggesting that PIL5 regulates not only GA metabolism but also GA responsiveness. PIL5 increases the expression of two GA repressor (DELLA) genes, GA-INSENSITIVE (GAI) and REPRESSOR OF GA1-3 (RGA/RGA1), in darkness. The hypersensitivity of gai-t6 rga-28 to red light and the suppression of germination defects of a rga-28 PIL5 overexpression line show the significant role of this transcriptional regulation in seed germination. PIL5 also increases abscisic acid (ABA) levels by activating ABA biosynthetic genes and repressing an ABA catabolic gene. PIL5 binds directly to GAI and RGA promoters but not to GA and ABA metabolic gene promoters. Together, our results show that light signals perceived by phytochromes cause a reduction in the PIL5 protein level, which in turn regulates the transcription of two DELLA genes directly and that of GA and ABA metabolic genes indirectly.

This report provides direct evidence that strigolactone (SL) positively regulates drought and high salinity responses in Arabidopsis. Both SL-deficient and SL-response [more axillary growth (max)] mutants exhibited hypersensitivity to drought and salt stress, which was associated with shoot-rather than root-related traits. Exogenous SL treatment rescued the drought-sensitive phenotype of the SL-deficient mutants but not of the SL-response mutant, and enhanced drought tolerance of WT plants, confirming the role of SL as a positive regulator in stress response. In agreement with the drought-sensitive phenotype, max mutants exhibited increased leaf stomatal density relative to WT and slower abscisic acid (ABA)-induced stomatal closure. Compared with WT, the max mutants exhibited increased leaf water loss rate during dehydration and decreased ABA responsiveness during germination and postgermination. Collectively, these results indicate that cross-talk between SL and ABA plays an important role in integrating stress signals to regulate stomatal development and function. Additionally, a comparative microarray analysis of the leaves of the SL-response max2 mutant and WT plants under normal and dehydrative conditions revealed an SL-mediated network controlling plant responses to stress via many stress-and/or ABA-responsive and cytokinin metabolism-related genes. Our results demonstrate that plants integrate multiple hormone-response pathways for adaptation to environmental stress. Based on our results, genetic modulation of SL content/response could be applied as a potential approach to reduce the negative impact of abiotic stress on crop productivity.hormonal regulation | plant adaptation | transcriptome analysis

Exposure of imbibed seeds to low temperature (typically 48 8C) is widely used to break seed dormancy and to improve the frequency of germination. However, the mechanism by which temperature accelerates germination is largely unknown. Using DNA microarray and gas chromatography-mass spectrometry analyses, we found that a subset of gibberellin (GA) biosynthesis genes were upregulated in response to low temperature, resulting in an increase in the level of bioactive GAs and transcript abundance of GA-inducible genes in imbibed Arabidopsis thaliana seeds. Using a loss-of-function mutant, the cold-inducible GA biosynthesis gene, AtGA3ox1, was shown to play an essential role in mediating the effect of low temperature. Besides temperature, AtGA3ox1 also is positively regulated by active phytochrome and negatively regulated by GA activity. We show that both red light and GA deficiency act in addition to low temperature to elevate the level of AtGA3ox1 transcript, indicating that multiple signals are integrated by the AtGA3ox1 gene to control seed germination. When induced by low temperature, AtGA3ox1 mRNA was detectable by in situ RNA hybridization in an additional set of cell types relative to that in red light-induced seeds. Our results illustrate that the GA biosynthesis and response pathways are activated during seed imbibition at low temperature and suggest that the cellular distribution of bioactive GAs may be altered under different light and temperature conditions.

Recent studies using highly branched mutants of pea, Arabidopsis and rice have demonstrated that strigolactones, a group of terpenoid lactones, act as a new hormone class, or its biosynthetic precursors, in inhibiting shoot branching. Here, we provide evidence that DWARF14 (D14) inhibits rice tillering and may act as a new compo-nent of the strigolactone-dependent branching inhibition pathway. The d14 mutant exhibits increased shoot branch-ing with reduced plant height like the previously characterized strigolactone-deficient and -insensitive mutants d10 and d3, respectively. The d10-1 d14-1 double mutant is phenotypically indistinguishable from the d10-1 and d14-1 single mutants, consistent with the idea that D10 and D14 function in the same pathway. However, unlike with d10, the d14 branching phenotype could not be rescued by exogenous strigolactones. In addition, the d14 mutant contained a higher level of 2'-epi-5-deoxystrigol than the wild type. Positional cloning revealed that D14 encodes a protein of the alpha/beta-fold hydrolase superfamily, some members of which play a role in metabolism or signaling of plant hormones. We propose that D14 functions downstream of strigolactone synthesis, as a component of hormone signaling or as an enzyme that participates in the conversion of strigolactones to the bioactive form.

PHYTOCHROME INTERACTING FACTOR 3-LIKE5 (PIL5) is a basic helix-loop-helix transcription factor that inhibits seed germination by regulating the expression of gibberellin (GA)- and abscisic acid (ABA)-related genes either directly or indirectly. It is not yet known, however, whether PIL5 regulates seed germination solely through GA and ABA. Here, we used Chromatin immunoprecipitation-chip (ChIP-chip) analysis to identify 748 novel PIL5 binding sites in the Arabidopsis thaliana genome. Consistent with the molecular function of PIL5 as a transcription regulator, most of the identified binding sites are located in gene promoter regions. Binding site analysis shows that PIL5 binds to its target sites mainly through the G-box motif in vivo. Microarray analysis reveals that phytochromes regulate a large number of genes mainly through PIL5 during seed germination. Comparison between the ChIP-chip and microarray data indicates that PIL5 regulates 166 genes by directly binding to their promoters. Many of the identified genes encode transcription regulators involved in hormone signaling, while some encode enzymes involved in cell wall modification. Interestingly, PIL5 directly regulates many transcription regulators of hormone signaling and indirectly regulates many genes involved in hormone metabolism. Taken together, our data indicate that PIL5 inhibits seed germination not just through GA and ABA, but also by coordinating hormone signals and modulating cell wall properties in imbibed seeds.