Nitric oxide: a multitasked signaling gas in plants.

Nitric oxide (NO) is a gaseous reactive oxygen species (ROS) that has evolved as a signaling hormone in many physiological processes in animals. In plants it has been demonstrated to be a crucial regulator of development, acting as a signaling molecule present at each step of the plant life cycle. NO has also been implicated as a signal in biotic and abiotic responses of plants to the environment. Remarkably, despite this plethora of effects and functional relationships, the fundamental knowledge of NO production, sensing, and transduction in plants remains largely unknown or inadequately characterized. In this review we cover the current understanding of NO production, perception, and action in different physiological scenarios. We especially address the issues of enzymatic and chemical generation of NO in plants, NO sensing and downstream signaling, namely the putative cGMP and Ca(2+) pathways, ion-channel activity modulation, gene expression regulation, and the interface with other ROS, which can have a profound effect on both NO accumulation and function. We also focus on the importance of NO in cell-cell communication during developmental processes and sexual reproduction, namely in pollen tube guidance and embryo sac fertilization, pathogen defense, and responses to abiotic stress.

Targeting of pollen tubes to ovules is dependent on nitric oxide (NO) signaling.

The guidance signals that drive pollen tube navigation inside the pistil and micropyle targeting are still, to a great extent, unknown. Previous studies in vitro showed that nitric oxide (NO) works as a negative chemotropic cue for pollen tube growth in lily (Lilium longiflorum). Furthermore, Arabidopsis thaliana Atnos1 mutant plants, which show defective NO production, have reduced fertility. Here, we focus in the role of NO in the process of pollen-pistil communication, using Arabidopsis in-vivo and lily semi-vivo assays. Cross-pollination between wild-type and Atnos1 plants shows that the mutation affects the pistil tissues in a way that is compatible with abnormal pollen tube guidance. Moreover, DAF-2DA staining for NO in kanadi floral mutants showed the presence of NO in an asymmetric restricted area around the micropyle. The pollen-pistil interaction transcriptome indicates a time-course-specific modulation of transcripts of AtNOS1 and two Nitrate Reductases (nr1 and nr2), which collectively are thought to trigger a putative NO signaling pathway. Semi-vivo assays with isolated ovules and lily pollen further showed that NO is necessary for micropyle targeting to occur. This evidence is supported by CPTIO treatment with subsequent formation of balloon tips in pollen tubes facing ovules. Activation of calcium influx in pollen tubes partially rescued normal pollen tube morphology, suggesting that this pathway is also dependent on Ca(2+) signaling. A role of NO in modulating Ca(2+) signaling was further substantiated by direct imaging the cytosolic free Ca(2+) concentration during NO-induced re-orientation, where two peaks of Ca(2+) occur-one during the slowdown/stop response, the second during re-orientation and growth resumption. Taken together, these results provide evidence for the participation of NO signaling events during pollen-pistil interaction. Of special relevance, NO seems to directly affect the targeting of pollen tubes to the ovule's micropyle by modulating the action of its diffusible factors.

Signalling by tips.

New molecules, including protein kinases, lipids and molecules that have neurotransmitter activities in animals have emerged as important players in tip-growing cells. Transcriptomics analysis reveals that the largest single class of genes expressed in pollen tubes encode signal transducers, reflecting the necessity to decode complex and diverse pathways that are associated with tip growth. Many of these pathways may use common intracellular second messengers, with ions and reactive oxygen species emerging as two major common denominators in many of the processes involved in tip growth. These second messengers might influence the actin cytoskeleton through known interactions with actin-binding proteins. In turn, changes in the dynamic properties of the cytoskeleton would define the basic polarity events needed to shape and modify tip-growing cells.

Nitric oxide is involved in growth regulation and re-orientation of pollen tubes.

Nitric oxide (NO) controls diverse functions in many cells and organs of animals. It is also produced in plants and has a variety of effects, but little is known about their underlying mechanisms. In the present study, we have discovered a role for NO in the regulation of pollen tube growth, a fast tip-growing cellular system. Pollen tubes must be precisely oriented inside the anatomically complex female ovary in order to deliver sperm. We hypothesized that NO could play a role in this guidance and tested this hypothesis by challenging the growth of pollen tubes with an external NO point source. When a critical concentration was sensed, the growth rate was reduced and the growth axis underwent a subsequent sharp reorientation, after which normal growth was attained. This response was abrogated in the presence of the NO scavenger CPTIO and affected by drugs interfering in the cGMP signaling pathway. The sensitivity threshold of the response was significantly augmented by sildenafil citrate (SC), an inhibitor of cGMP-specific phosphodiesterases in animals. NO distribution inside pollen tubes was investigated using DAF2-DA and was shown to occur mostly in peroxisomes. Peroxisomes are normally excluded from the tip of pollen tubes and little if any NO is found in the cytosol of that region. Our data indicate that the rate and orientation of pollen tube growth is regulated by NO levels at the pollen tube tip and suggest that this NO function is mediated by cGMP.