Studies on Cellular Site of Calcium Action in Promoting Pollen Growth

Abstract

Studies were conducted to determine cellular site of Ca action in promoting pollen growth of Crinum asiaticum and a few other species. The following experimental results have strongly indicated that Ca binding takes place in pectins of the pollen tube walls. This appeared to increase the wall rigidity and to regulate permeability of the pollen cells thereby enhancing pollen growth. Radioactive Ca incorporation was observed exclusively in the pollen tube wall regions. The promoting action of Ca on pollen growth disappeared when pectinase was supplemented to the media. This was not the case with cellulase and other enzymes used. Methyl donors promoted pollen growth, and the promotion was more than doubled if Ca ions were present. Ethionine, on the other hand, inhibited tube elongation and exhibited no Ca effect. Growth of pollen tubes in oscillaled liquid media during elongation was poorer than growth in standing media. The Ca effect was also reduced when pollen was oscillated. The observations made of the reduced rate of 45Ca incorporation when pollen was washed in water, and the hydroxylamineferric chloride test, have indicated that a considerable portion of these pectins are cold‐water soluble.

Pollen tube tip growth

“…This picture is considerably more complex than that available to Kwack (1967) over 20 years ago when he concluded that the calcium effect was only perceived by the cell walls and could be negated by pectinase treatment. We now know that loss of the internal gradients of calcium are always accompanied by loss of an ability to sustain normal growth m vitro.…”

“…The outer wall layer is believed to be mostly pectin, since it is readily removed by pectinase (Kwack, 1967;Roggen & Stanley, 1971) which also I'emoves the fibrous wall at the tip (f3ashek . This tip zone is rich in uronic acids (Rae et al, 1985).…”

“…'Pubes transferred to, or grown in, high levels of calcium ions grow more slowly or cease growth altogether. According to the cell wall hypothesis caleium cross-links the pectins adding mechanical strength to the tip wall, which then more effectively resists the internal turgor pressure (Kwack, 1967;Heslop-Harrison, 1979;Shivanna & Johri, 1985). 'Phis idea is again taken from Work on sporophyte tissues, wbere calcium is believed to strengthen links between carbohydrate chains bearing carboxylic acid groups, especially the pectins of the middle lamella (Burstrom, 1968).…”

“…It seems likely Aat the arabinoxylan and galactoarabinoxylan groups found in extracts of whole pollen tubes (Vithanage et al, 1980) are also from this outer layer. Kwack (1967) incorporated ^''Ca into the media of pollen tubes growing under various "Conditions. LJnder normal growth conditions, radioautography was used to show that the radioactive ion was bound exclusively to the cell wall.…”

Calcium: A Central Regulator of Plant Growth and Development

“…It was shown in 1963 that Ca 21 must be present in the medium to support pollen tube growth in vitro (Brewbaker and Kwack, 1963). Using 45 Ca 21 , Kwack (1967) showed that incorporation occurred exclusively in the pollen tube wall; some of the autoradiographic images indicated an enhanced accumulation of Ca 21 in the apical region. Because the pollen tube cell wall, especially at the tip, is composed almost entirely of pectin, it is reasonable to assume that a Ca 21 /pectate interaction dominates the requirement for this ion.…”