Discovery process of receptors that control pollen tube growth

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      (Contributed by Yan Zhuang)

      Hülskamp et al. observed that Arabidopsismutants defective in ovule development show lower pollen tube growthpreference, which concluded that some gene functions in the guidance of pollentube in female gametophyte1. In 2001, Higashiyama et al.applied an ultraviolet (UV) laser beam on different kinds of gametophytic cellsof T. fournieri, a species thatconfers a protruding female gametophyte2. They found that pollen tube guidance ceased completely when boththe two synergid cells were ablated, whereas abolishment of either of themcauses decreased attraction frequency2. These suggest that the pollen tube guidance signals are from both ofthe two synergid cells.

      Kasahara et al. first reported a gene encoding MYB98 specifically expressedin synergid cells, trichomes and endosperm, of which the mutation impairs pollentube guidance3.MYB98 is a transcription factor and the downstream functional gene was yetunknown at that time. Ma´rton et al. assumed that EA1 is essential forpollen tube guidance in Zea mays , nevertheless, no homolog genes werefound in dicots4.

      Okuda et al. extracted the RNA of separated synergid cells and found CRPswere highly expressed in these cells by analyzing expressed sequence tag5,thereafter they named the peptides LUREs. Intriguingly, the expression levelsof 56 CRPs decreased in myb98 mutantsas reported6. In vitro pollen tubeattraction assay was conducted to prove the function of LUREs in pollen tubeguidance5.There are homologs of LUREs in Arabidopsisand ectopicly expressed AtLURE1 in T.fournieri is able to guide Arabidopsispollen tubes to T. fournieri femalegametophyte7. Upto then, the receptor of LUREs in pollen tubes is still of mystery.

      In Arabidopsis, lost of LIP1 and LIP2, two receptor-like kinases,causes reduced pollen tube guidance8. LIP1and LIP2 locate on the membrane of pollen tube. However, the lack of extracellulardomains makes LIP1/2 more like co-receptors of a yet unknown receptor9.

      Wang et al. selected several RLKs highly expressed in pollen tubes and expressedtheir kinase-dead dominant negative forms10. Thepollen tube guidance was examined in these mutants. Finally they found thereceptors of LUREs, namely MDIS1/2 and MIK1/210. Onthe other hand, by pollen tube attraction assay, Takeuchi et al. defined PRK3/6 as the receptor of LUREs11. Theyadded AtLURE1 into the culture media and normal pollen tube showed wavy growthtracks while mutants unable to recognize LUREs did’t. They used this system toscreen 23 RLKs and found pollen tube growth of prk6 completely lost the wavy tracks. However, the seeminglyredundant functions of these receptors still confer questions about how LUREsdifferentially bind to distinct receptors and the next step cascade triggeredby LUREs-receptors interactions.

Reference:

1.  Hulskamp, M., Schneitz, K. & Pruitt, R. E.Genetic Evidence for a Long-Range Activity That Directs Pollen Tube Guidance inArabidopsis. Plant Cell 7, 57-64 (1995).

2.  Higashiyama, T. et al. Pollen tube attraction by thesynergid cell. Science 293, 1480-1483 (2001).

3.  Kasahara, R. D.,Portereiko, M. F., Sandaklie-Nikolova, L., Rabiger, D. S. & Drews, G. N.MYB98 is required for pollen tube guidance and synergid cell differentiation inArabidopsis. Plant Cell 17, 2981-2992 (2005).

4.  Marton, M. L., Cordts, S.,Broadhvest, J. & Dresselhaus, T. Micropylar pollen tube guidance by eggapparatus 1 of maize. Science 307, 573-576 (2005).

5.  Okuda, S. et al. Defensin-like polypeptide LUREsare pollen tube attractants secreted from synergid cells. Nature 458, 357-361(2009).

6.  Jones-Rhoades, M. W.,Borevitz, J. O. & Preuss, D. Genome-wide expression profiling of theArabidopsis female gametophyte identifies families of small, secreted proteins.PLoS Genet. 3, 1848-1861 (2007).

7.  Takeuchi, H. &Higashiyama, T. A species-specific cluster of defensin-like genes encodesdiffusible pollen tube attractants in Arabidopsis. PLoS Biol. 10, e1001449(2012).

8.  Liu, J. et al. Membrane-bound RLCKs LIP1 and LIP2 are essential malefactors controlling male-female attraction in Arabidopsis. Curr. Biol. 23, 993-998(2013).

9.  Qu, L. J., Li, L., Lan, Z.& Dresselhaus, T. Peptide signalling during the pollen tube journey anddouble fertilization. J. Exp. Bot. 66, 5139-5150 (2015).

10. Wang, T. et al. A receptor heteromer mediatesthe male perception of female attractants in plants. Nature 531, 241-244(2016).

11. Takeuchi, H. &Higashiyama, T. Tip-localized receptors control pollen tube growth and LUREsensing in Arabidopsis. Nature 531, 245-248 (2016).

dongfy88

This is a very nice summary of recent research on pollen tube growth!