Salicylic Acid Regulates Pollen Tip Growth through an NPR3/NPR4-Independent Pathway
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Cell polarity
Tip growth
Pollen tubes expand by tip growth and extend directionally toward the ovule to deliver sperms during pollination. They provide an excellent model system for the study of cell polarity control and tip growth, because they grow into uniformly shaped cylindrical cells in culture. Mechanisms underlying tip growth are poorly understood in pollen tubes. It has been demonstrated that ROP1, a pollen‐specific member of the plant‐specific Rop subfamily of Rho GTPases, is a central regulator of pollen tube tip growth. Recent studies in pollen from Arabidopsis and other species have revealed a ROP‐mediated signalling network that is localized to the apical PM region of pollen tubes. The results provide evidence that the localization of this signalling network establishes the site for tip growth and the localized activation of this signalling network regulates the dynamics of tip F‐actin. These results have shown that the ROP1‐mediated dynamics of tip F‐actin is a key cellular mechanism behind tip growth in pollen tubes. Current understanding of the molecular basis for the regulation of the tip actin dynamics will be discussed.
Tip growth
Cell polarity
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Tip growth
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Pollen tubes expand by tip growth and extend directionally toward the ovule to deliver sperms during pollination. They provide an excellent model system for the study of cell polarity control and tip growth, because they grow into uniformly shaped cylindrical cells in culture. Mechanisms underlying tip growth are poorly understood in pollen tubes. It has been demonstrated that ROP1, a pollen‐specific member of the plant‐specific Rop subfamily of Rho GTPases, is a central regulator of pollen tube tip growth. Recent studies in pollen from Arabidopsis and other species have revealed a ROP‐mediated signalling network that is localized to the apical PM region of pollen tubes. The results provide evidence that the localization of this signalling network establishes the site for tip growth and the localized activation of this signalling network regulates the dynamics of tip F‐actin. These results have shown that the ROP1‐mediated dynamics of tip F‐actin is a key cellular mechanism behind tip growth in pollen tubes. Current understanding of the molecular basis for the regulation of the tip actin dynamics will be discussed.
Tip growth
Cell polarity
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Tip growth
Live cell imaging
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During sexual reproduction, pollen grains land on the stigma, rehydrate and produce pollen tubes that grow through the female transmitting-tract tissue allowing the delivery of the two sperm cells to the ovule and the production of healthy seeds. Because pollen tubes are single cells that expand by tip-polarized growth, they represent a good model to study the growth dynamics, cell wall deposition and intracellular machineries. Aiming to understand this complex machinery, we used a low throughput chemical screen approach in order to isolate new tip-growth disruptors. The effect of a chemical inhibitor of monogalactosyldiacylglycerol synthases, galvestine-1, was also investigated. The present work further characterizes their effects on the tip-growth and intracellular dynamics of pollen tubes.Two small compounds among 258 were isolated based on their abilities to perturb pollen tube growth. They were found to disrupt in vitro pollen tube growth of tobacco, tomato and Arabidopsis thaliana. We show that these 3 compounds induced abnormal phenotypes (bulging and/or enlarged pollen tubes) and reduced pollen tube length in a dose dependent manner. Pollen germination was significantly reduced after treatment with the two compounds isolated from the screen. They also affected cell wall material deposition in pollen tubes. The compounds decreased anion superoxide accumulation, disorganized actin filaments and RIC4 dynamics suggesting that they may affect vesicular trafficking at the pollen tube tip.These molecules may alter directly or indirectly ROP1 activity, a key regulator of pollen tube growth and vesicular trafficking and therefore represent good tools to further study cellular dynamics during polarized-cell growth.
Tip growth
Sexual reproduction
Plant reproduction
Double fertilization
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Pollen tubes elongate within the pistil to transport sperms to the female gametophytes for fertilization. Pollen tubes grow at their tips through a rapid and polarized cell growth process. This tip growth process is supported by an elaborate and dynamic actin cytoskeleton and a highly active membrane trafficking system that together provide the driving force and secretory activities needed for growth. A polarized cytoplasm with an abundance of vesicles and tip-focused Ca(2+) and H(+) concentration gradients are important for the polar cell growth process. Apical membrane-located Rho GTPases regulate Ca(2+) concentration and actin dynamics in the cytoplasm and are crucial for maintaining pollen tube polarity. Pollen tube growth is marked by periods of rapid and slow growth phases. Activities that regulate and support this tip growth process also show oscillatory fluctuations. How these activities correlate with the rapid, polar, and oscillatory pollen tube growth process is discussed.
Tip growth
Cell polarity
Double fertilization
Cytoplasmic streaming
Polarity (international relations)
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We have recently characterised NET2A as a pollen-specific actin-binding protein that binds F-actin at the plasma membrane of growing pollen tubes. However, the role of NET2 proteins in pollen development and fertilisation have yet to be elucidated. To further characterise the role of Arabidopsis NET2 proteins in pollen development and fertilisation, we analysed the subcellular localisation of NET2A over the course of pollen grain development and investigated the role of the NET2 family using net2 loss-of-function mutants. We observed NET2A to localise to the F-actin cytoskeleton in developing pollen grains as it underwent striking structural reorganisations at specific stages of development and during germination and pollen tube growth. Furthermore, net2 loss-of-function mutants exhibited striking morphological defects in the early stages of pollen tube growth, arising from frequent changes to pollen tube growth trajectory. We observed defects in the cortical actin cytoskeleton and actin-driven subcellular processes in net2 mutant pollen tubes. We demonstrate that NET2 proteins are essential for normal actin-driven pollen development highlighting an important role for the NET2 family members in regulating pollen tube growth during fertilisation.
Tip growth
Profilin
Actin-binding protein
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Abstract During reproduction in flowering plants, pollen grains form a tube that grows in a polarized fashion through the female tissues to eventually fertilize the egg cell. These highly polarized pollen tubes have a rapid rate of growth that is supported by a tip-focused delivery of membrane and cell wall components. To gain a better understanding of how this growth is regulated, we investigated the function RABA4D, a member of the Arabidopsis thaliana RabA4 subfamily of Rab GTPase proteins. Here, we show that RABA4D was expressed in a pollen-specific manner and that enhanced yellow fluorescent protein (EYFP)-RabA4d-labeled membrane compartments localized to the tips of growing pollen tubes. Mutant pollen in which the RABA4D gene was disrupted displayed bulged pollen tubes with a reduced rate of growth in vitro and displayed altered deposition of some cell wall components. Expression of EYFP-RabA4d restored wild-type phenotypes to the raba4d mutant pollen tubes, while expression of EYFP-RabA4b did not rescue the raba4d phenotype. In vivo, disruption of RABA4D resulted in a male-specific transmission defect with mutant raba4d pollen tubes displaying aberrant growth in the ovary and reduced guidance at the micropyle. We propose that RabA4d plays an important role in the regulation of pollen tube tip growth.
Rab
Tip growth
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Plant Rac‐like GTPases have been classified phylogenetically into two major groups—class I and class II. Several pollen‐expressed class I Rac‐like GTPases have been shown to be important regulators of polar pollen tube growth. The functional participation by some of the class I and all of the class II Arabidopsis Rac‐like GTPases in pollen tube growth remains to be explored. It is shown that at least four members of the Arabidopsis Rac GTPase family are expressed in pollen, including a class II Rac, AtRac7. However, when over‐expressed as fusion proteins with GFP, both pollen‐ and non‐pollen‐expressed AtRacs interfered with the normal pollen tube tip growth process. These observations suggest that these AtRacs share similar biochemical activities and may integrate into the pollen cellular machinery that regulates the polar tube growth process. Therefore, the functional contribution by individual Rac GTPase to the pollen tube growth process probably depends to a considerable extent on their expression characteristics in pollen. Among the Arabidopsis Racs, GFP‐AtRac7 showed association with the cell membrane and Golgi bodies, a pattern distinct from all previously reported localization for other plant Racs. Over‐expressing GFP‐AtRac7 also induced the broadest spectrum of pollen tube growth defects, including pollen tubes that are bifurcated, with diverted growth trajectory or a ballooned tip. Transgenic plants with multiple copies of the chimeric Lat52‐GFP‐AtRac7 showed severely reduced seed set, probably many of these defective pollen tubes were arrested, or reduced in their growth rates that they did not arrive at the ovules while they were still receptive for fertilization. These observations substantiate the importance of Rac‐like GTPases to sexual reproduction.
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