BioPL 3420Spring 2016Lecture Topic 24Answer to Assigned Study QuestionGametophytes, Pollination, Seeds and FruitsPlant Physiology LecturesBioPL 3420Spring 2016Lecture Topic 24 Answer to Assigned Study QuestionGametophytes, Pollination, Seeds and FruitsStudy Question. Pollen tubes are an excellent example of tip growth, localized growthat a single point on the surface of the cell. However, it is clear that the direction of tipgrowth is critical to the success of pollen in fertilization. The text on pages 635-637describes how the rate of pollen tube growth might be regulated, and the direction ofpollen tube growth is influenced by both physical and chemical factors as it growsthrough the stigma. It is pretty clear that Ca2+ ions are involved. Let’s assume thatlocalization of high intracellular Ca2+ is what determines where and how quickly the tipgrows. What I would like you to do is to hypothesize how extracellular signals (from thematernal tissues or the ovule might alter the location of this high intracellular Ca2+ andthus the direction of tip growth. Then suggest one type of mutation that might alter thisdirectional response.Answer: It is pretty clear that locally high intracellular Ca2+ is a key factor in determining thelocation of tip growth. Localized high intracellular Ca2+ requires two components: openCa2+ channels at the point of tip growth, and Ca2+ pumps (located a short distancefrom the open channels to keep the high Ca2+ from extending beyond the tip. Let’s firstassume that the Ca2+ channels and pumps are located throughout the tip region butare active only at the center of the growing tip In order to change the direction of growthof the tip (to turn toward the micropyle of the ovule from the transmitting tract, forexample) would thus require the coordinated change in which channels are open andwhich are closed. Similarly there must be a change in which pumps are on or off. Thiswould move the location of the high Ca2+ concentration and the direction of tip growthwould be changed. This must be controlled by directional information from outside thetip via signal transduction pathways. Alternately, the location of the high intracellular Ca2+ might be controlled by the locationof the Ca2+ channels and pumps, which means that the response to the extracellularsignals must change the location of these transport proteins in the plasma membrane ofthe growing tip.One might expect that a phenotype of a mutant that does not respond to extracellularsignaling would be either a very straight or a random direction of tube growth, and thatsuch a mutation could occur in any step in the signaling pathway between the receptorfor the chemical signal that “steers” tube growth, to the factors that control channel andpump activity (first scenario) or that control the location of the channels and pumps inthe tip (second