Seasonal changes in day length influence flowering time in many plant species. In Arabidopsis, flowering is accelerated by exposure to long day (LD). Those inductive photoperiods are perceived in leaves [1] and initiate a long-distance signaling mediated by CO and FT. CO is expressed in the phloem according to a circadian rhythm [2-4]. Only under LD does CO induce FT expression as high levels of CO in the evening coincide with the external light that stabilizes CO protein [4, 5]. Subsequently, FT protein travels through the phloem to the shoot apex where, together with FD, it initiates flowering [6-12]. Despite the photoperiodic induction, a mechanism of floral repression is needed to avoid precocious flowering. We show that TEMPRANILLO genes (TEM1 and TEM2) act as novel direct FT repressors. Molecular and genetic analyses suggest that a quantitative balance between the activator CO and the repressor TEM determines FT levels. Moreover, developmental TEM downregulation marks the timing of flowering, as it shifts the CO/TEM balance in favor of CO activity, allowing FT transcript to reach the threshold level required to trigger flowering. We envision that this might be a general mechanism between long-day plants to ensure a tight regulation of flowering time.

In Arabidopsis, FLOWERING LOCUS T (FT) promotes flowering in response to long days in the photoperiod pathway, while signalling downstream gibberellin (GA) perception is critical for flowering under short days. Previously we have established that the TEMPRANILLO (TEM) genes have a pivotal role in the direct repression of FT. Here we show that TEM genes directly regulate the expression of the GA 4 biosynthetic genes GA 3-oxidase1 and 2 (GA3OX1 and GA3OX2). Plants overexpressing TEM genes resemble GA-deficient mutants, and conversely, TEM downregulation give rise to elongated hypocotyls perhaps as a result of an increase in GA content. We consistently find that TEm1 represses GA3OX1 and GA3OX2 by directly binding a regulatory region positioned in the first exon. our results indicate that TEM genes seem to link the photoperiod and GA-dependent flowering pathways, controlling floral transition under inductive and non-inductive day lengths through the regulation of the floral integrators.

Plant hormones are small-molecule signaling compounds that are collectively involved in all aspects of plant growth and development. Unlike animals, plants actively regulate the spatial distribution of several of their hormones. For example, auxin transport results in the formation of auxin maxima that have a key role in developmental patterning. However, the spatial distribution of the other plant hormones, including gibberellic acid (GA), is largely unknown. To address this, we generated two bioactive fluorescent GA compounds and studied their distribution in Arabidopsis thaliana roots. The labeled GAs specifically accumulated in the endodermal cells of the root elongation zone. Pharmacological studies, along with examination of mutants affected in endodermal specification, indicate that GA accumulation is an active and highly regulated process. Our results strongly suggest the presence of an active GA transport mechanism that would represent an additional level of GA regulation. . Plants regulate hormone response pathways at multiple levels including hormone biosynthesis, metabolism, perception, and signaling. In the case of auxin, elegant studies have shown that the regulation of auxin distribution through the action of specific transporters also has a central role in plant development (2, 3). The recent isolation of transporters for other hormones (4-6) suggests that the spatial distribution of these compounds may also be regulated.Gibberellins (GAs) are a class of tetracyclic diterpenoid hormones that regulate many developmental processes such as seed germination, root and shoot elongation, flowering and fruit patterning (7-9). Over the years, more than 130 GAs have been identified, of which only a few, such as GA 1 , GA 3 , and GA 4 , are bioactive (8). Much progress has been made in understanding how plants control GA response through regulation of biosynthesis, metabolism, and signaling (10)(11)(12)(13)(14). Although experiments with radiolabeled GAs as well as grafting studies have established that GAs move through the plant (15-18), little is known about either the mechanisms of transport or the distribution of GA.To address these questions, we generated fluorescently tagged GA. The labeled GAs retained much of their bioactivity and thus were used as fluorescent GA surrogates to study the distribution of GA in the Arabidopsis root system. ResultsFluorescently Labeled GAs Are Bioactive. Four derivatives of fluorescein (Fl)-labeled GA 3 were synthesized (SI Appendix, Figs. S1-S4), varying primarily in the length of the linker between Fl and GA 3 (Fig. 1A and SI Appendix, Fig. S5A). Conjugation to GA 3 through amide formation on C6 was based on previous reports demonstrating its stability in vitro and in vivo (19). The four molecules were compared for their GA bioactivity. GAs are essential for seed germination in many plants including Arabidopsis (20,21). The GA biosynthesis mutant ga1 germinates poorly, whereas exogenous application of GA 3 fully restores germination levels (22, 23). Whereas molecules ...

Many processes critical to plant growth and development are regulated by the hormone auxin. Auxin responses are initiated through activation of a transcriptional response mediated by the TIR1/AFB family of F-box protein auxin receptors as well as the AUX/IAA and ARF families of transcriptional regulators. However, there is little information on how auxin regulates a specific cellular response. To begin to address this question, we have focused on auxin regulation of cell expansion in the Arabidopsis hypocotyl. We show that auxin-mediated hypocotyl elongation is dependent upon the TIR1/AFB family of auxin receptors and degradation of AUX/IAA repressors. We also use microarray studies of elongating hypocotyls to show that a number of growth-associated processes are activated by auxin including gibberellin biosynthesis, cell wall reorganization and biogenesis, and others. Our studies indicate that GA biosynthesis is required for normal response to auxin in the hypocotyl but that the overall transcriptional auxin output consists of PIF-dependent and -independent genes. We propose that auxin acts independently from and interdependently with PIF and GA pathways to regulate expression of growth-associated genes in cell expansion.

SummaryIn addition to the role of the cell wall as a physical barrier against pathogens, some of its constituents, such as pectin-derived oligogalacturonides (OGA), are essential components for elicitation of defence responses. To investigate how modifications of pectin alter defence responses, we expressed the fruit-specific Fragaria · ananassa pectin methyl esterase FaPE1 in the wild strawberry Fragaria vesca. Pectin from transgenic ripe fruits differed from the wild-type with regard to the degree and pattern of methyl esterification, as well as the average size of pectin polymers. Purified oligogalacturonides from the transgenic fruits showed a reduced degree of esterification compared to oligogalacturonides from wild-type fruits. This reduced esterification is necessary to elicit defence responses in strawberry. The transgenic F. vesca lines had constitutively activated pathogen defence responses, resulting in higher resistance to the necrotropic fungus Botrytis cinerea. Further studies in F. vesca and Nicotiana benthamiana leaves showed that the elicitation capacity of the oligogalacturonides is more specific than previously envisaged.