Targeting of Pollen Tubes to Ovules Is Dependent on Nitric Oxide (NO) Signaling

Abstract

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.

Defects in Peroxisomal 6-Phosphogluconate Dehydrogenase Isoform PGD2 Prevent Gametophytic Interaction in Arabidopsis thaliana

“…However, limiting NO synthesis in plastids seems to mainly harm the female gametophyte, usually showing as degenerated embryos in developing siliques. This resulted in about 40% less seed production by noa1 mutants as compared with wild-type plants (Prado et al, 2008), which we did not observe for the pgd2 mutants. Moreover, in a triple mutant with additional lack of nitrate reductase (NIA) activity in the cytosol (nia1 nia2 noa1-2), the accumulated NO deficiency resulted in a higher portion of aborted embryos but not in male sterility León, 2010a, 2010b).…”

“…Recently, NADPH production in peroxisomes was shown to be important for the biosynthesis of nitric oxide (NO) in roots facing abiotic stress, either induced by salt (NaCl; Corpas et al, 2009) or heavy metals (cadmium; Corpas and Barroso, 2014a). Whether peroxisomal NADPH may be needed for NO synthesis, which plays an important role during fertilization (Prado et al, 2004(Prado et al, , 2008, remained unclear, mainly because, aside from the OPPP, NADPH in peroxisomes also can be produced by other enzymes, such as NADP-dependent isocitrate dehydrogenase (NADP-ICDH; At1g54340) with peroxisomal targeting signal type 1 (PTS1) motif SRL Leterrier et al, 2012) or stress-related phosphorylation of NAD(H) via the NAD kinase isoform NADK3 (Chai et al, 2006) with PTS1 motif SRY (Waller et al, 2010). Hence, under which conditions OPPP reactions in peroxisomes may be necessary or even essential remained to be determined.…”

Cyclic nucleotide-gated channel 18 is an essential Ca 2+ channel in pollen tube tips for pollen tube guidance to ovules in Arabidopsis

“…Synergid cells, central cells, egg cells, and egg apparatus are all involved in pollen tube guidance, probably by secreting different attractants (9)(10)(11)(12)(13)(14)(15). Additionally, nitric oxide (NO) and phytosulfokine peptides have also been implicated in both preovular and ovular pollen tube guidance (16)(17)(18). Thus, pollen tubes could be guided by diverse attractants in a single plant species.…”

Assessing Nitric Oxide (NO) in Higher Plants: An Outline

“…Some of those processes include seed and pollen germination [42][43][44][45][46], root development and plant growth [47][48][49], stomata movement [50,51], senescence [52,53], flower development [54], and fruit ripening [55][56][57][58].…”