Polyamines are small ubiquitous molecules that have been involved in nearly all developmental processes, including the stress response. Nevertheless, no direct evidence of a role of polyamines in the wound response has been described. We have studied the expression of genes involved in polyamine biosynthesis in response to mechanical injury. An increase in the expression of the arginine decarboxylase 2 (ADC2) gene in response to mechanical wounding and methyl jasmonate (JA) treatment in Arabidopsis was detected by using DNA microarray and RNA gel-blot analysis. No induction was observed for the ADC1 gene or other genes coding for spermidine and spermine synthases, suggesting thatADC2 is the only gene of polyamine biosynthesis involved in the wounding response mediated by JA. A transient increase in the level of free putrescine followed the increase in the mRNA level forADC2. A decrease in the level of free spermine, coincident with the increase in putrescine after wounding, was also observed. Abscisic acid effected a strong induction onADC2 expression and had no effect on ADC1expression. Wound-induction of ADC2 mRNA was not prevented in the JA-insensitive coi1 mutant. The different pattern of expression of ADC2 gene in wild-type and coi1 mutant might be due to the dual regulation ofADC2 by abscisic acid and JA signaling pathways. This is the first direct evidence of a function of polyamines in the wound-response, and it opens a new aspect of polyamines in plant biology.

Summary: Gibberellins (GAs) play a critical role in fruit‐set and fruit growth. Gibberellin is perceived by its nuclear receptors GA INSENSITIVE DWARF1s (GID1s), which then trigger degradation of downstream repressors DELLAs. To understand the role of the three GA receptor genes (GID1A, GID1B and GID1C) in Arabidopsis during fruit initiation, we have examined their temporal and spatial localization, in combination with analysis of mutant phenotypes. Distinct expression patterns are revealed for each GID1: GID1A is expressed throughout the whole pistil, while GID1B is expressed in ovules, and GID1C is expressed in valves. Functional study of gid1 mutant combinations confirms that GID1A plays a major role during fruit‐set and growth, whereas GID1B and GID1C have specific roles in seed development and pod elongation, respectively. Therefore, in ovules, GA perception is mediated by GID1A and GID1B, while GID1A and GID1C are involved in GA perception in valves. To identify tissue‐specific interactions between GID1s and DELLAs, we analyzed spatial expression patterns of four DELLA genes that have a role in fruit initiation (GAI, RGA, RGL1 and RGL2). Our data suggest that GID1A can interact with RGA and GAI in all tissues, whereas GID1C–RGL1 and GID1B–RGL2 interactions only occur in valves and ovules, respectively. These results uncover specific functions of each GID1–DELLA in the different GA‐dependent processes that occur upon fruit‐set. In addition, the distribution of GA receptors in valves along with lack of expression of GA biosynthesis genes in this tissue, strongly suggests transport of GAs from the developing seeds to promote fruit growth.

Background: Ovule lifespan is an important factor in determining the ability to set fruits and produce seeds. Once ovule senescence is established, fruit set capacity in response to gibberellins (GAs) is lost. We aimed to elucidate whether ethylene plays a role in controlling ovule senescence and the fruit set response inArabidopsis.Results: Ethylene response inhibitors, silver thiosulphate (STS) and 1-methylcyclopropene (1-MCP), were able to delay the loss of pistil response to GA₃. In addition, ethylene insensitive mutantsein2-5andein3-1showed delayed loss of pistil response, as in plants treated with STS and 1-MCP, while constitutive mutantctr1-1displayed premature loss of response. The analysis of the expression of ethylene biosynthesis genes suggests that ethylene is synthesised in ovules at the onset of ovule senescence, while a transcriptional meta-analysis also supports an activated ethylene-dependent senescence upon the establishment of ovule senescence. Finally, aSAG12:GUSreporter line proved useful to monitor ovule senescence and to directly demonstrate that ethylene specifically modulates ovule senescence.Conclusions: We have shown that ethylene is involved in both the control of the ovule lifespan and the determination of the pistil/fruit fate. Our data support a role of the ovule in modulating the GA response during fruit set inArabidopsis. A possible mechanism that links the ethylene modulation of the ovule senescence and the GA₃-induced fruit set response is discussed.

Fruits of angiosperms can be divided into dry and fleshy fruits, depending on their dispersal strategies. Despite their apparently different developmental programmes, researchers have attempted to compare dry and fleshy fruits to establish analogies of the distinct biochemical and physiological processes that occur. But what are the common and specific phenomena in both biological strategies? Is valve dehiscence and senescence of dry fruits comparable to final ripening of fleshy fruits, when seeds become mature and fruits are competent for seed dispersal, or to over-ripening when advanced senescence occurs? We briefly review current knowledge on dry and fleshy fruit development, which has been extensively reported recently, and is the topic of this special issue. We compare the processes taking place in Arabidopsis (dry) and tomato (fleshy) fruit during final development steps using transcriptome data to establish possible analogies. Interestingly, the transcriptomic programme of Arabidopsis silique shares little similarity in gene number to tomato fruit ripening or over-ripening. In contrast, the biological processes carried out by these common genes from ripening and over-ripening programmes are similar, as most biological processes are shared during both programmes. On the other hand, several biological terms are specific of Arabidopsis and tomato ripening, including senescence, but little or no specific processes occur during Arabidopsis and tomato over-ripening. These suggest a closer analogy between silique senescence and ripening than over-ripening, but a major common biological programme between Arabidopsis silique senescence and the last steps of tomato development, irrespective of its distinction between ripening and over-ripening.

Ovule formation is a complex developmental process in plants with a strong impact on the production of seeds. Ovule primordia initiation is controlled by a gene network, including components of the signaling pathways of auxin, brassinosteroids and cytokinins. In contrast, gibberellins (GAs) and their signaling negative regulators DELLA proteins have never been shown to be involved in ovule initiation. Here we provide molecular and genetic evidences that point to DELLA proteins as novel players in the determination of ovule number in Arabidopsis and in species of agronomic interest like tomato and rapeseed, adding a new layer of complexity to this important developmental process. DELLA activity correlates positively with ovule number, acting as a positive factor for ovule initiation. In addition, ectopic expression of a dominant DELLA in the placenta is sufficient to increase ovule number. The role of DELLA proteins in ovule number seems not to be related to auxins transport or signaling in the ovule primordia. A possible crosstalk of DELLA proteins with the molecular and hormonal network controlling ovule initiation is discussed.