Lipids are required for directional pollen-tube growth.

Successful pollination and fertilization are absolute requirements for sexual reproduction in higher plants. Pollen hydration, germination and penetration of the stigma by pollen tubes are influenced by the exudate on wet stigmas and by the pollen coat in species with dry stigmas. The exudate allows pollen tubes to grow directly into the stigma, whereas the pollen coat establishes the contact with the stigma. Pollen tubes then grow into the papillae, which are covered by a cuticle. The components of the exudate or pollen coat that are responsible for pollen tube penetration are not known. To discover the role of the exudate, we tested selected compounds for their ability to act as functional substitutes for exudate in the initial stages of pollen-tube growth on transgenic stigmaless tobacco plants that did not produce exudate. Here we show that lipids are the essential factor needed for pollen tubes to penetrate the stigma, and that, in the presence of these lipids, pollen tubes will also penetrate leaves. We propose that lipids direct pollen-tube growth by controlling the flow of water to pollen in species with dry and wet stigmas.

Recent developments in the molecular genetics and biology of self-incompatibility.

A summary of recent work on molecular aspects of self-incompatibility in Nicotiana alata is presented. The amino acid sequences of style proteins corresponding to different S-alleles of N. alata have a high level of homology in some regions and are variable in other regions. The regions of homology include N-terminal sequences as well as most of the glycosylation sites and cysteine residues. The glycosyl substituents may consist of a number of 'glycoforms'. The isolated style S-glycoproteins inhibit in vitro growth of pollen tubes. The S-glycoproteins tested inhibited the growth of pollen of several S-genotypes, and there was some specificity in the interaction. Heat treatment of the isolated S-glycoproteins dramatically increased their activity as inhibitors of pollen tube growth, although the specificity in the interaction was lost. The nature of the S-allele products in pollen is not yet established.

Inhibition of in Vitro Pollen Tube Growth by Isolated S-Glycoproteins of Nicotiana alata.

Pollen from three S-genotypes of Nicotiana alata was grown in vitro in the presence of S-glycoproteins isolated from styles of the same three genotypes. Pollen germination was not affected by the presence of the S-glycoproteins, but pollen tube growth of all genotypes was inhibited. S2 pollen was preferentially inhibited by the S2-glycoprotein and S3 pollen by the S3-glycoprotein. The S6-glycoprotein preferentially inhibited growth of both S2 and S6 pollen over S3 pollen. Heat treatment dramatically increased the inhibitory activity of the S-glycoproteins as inhibitors both of pollen germination and tube growth; after heat treatment, S-allele specificity of pollen tube inhibition was not detected.

A comparison of translocation of labelled assimilate by normal and lignified sieve elements in wheat leaves.

Leaves of Triticum aestivum L. were exposed to (14)CO2. The (14)C activity in lignified sieve elements was not above background levels, whereas it was significnatly higher in normal sieve elements and companion cells.

Leaf structure and translocation of dry matter in a C3 and a C 4 grass.

Dry weight analyses and (14)CO2 were used to study translocation in leaves of the C3 grass Lolium temulentum L. and the C4 grass Panicum maximum Jacq. and the results related to the distribution and amount of phloem in the lamina. The rate of specific mass transfer rose from the tips to the bases of leaf blades, in both species high rates were recorded. Major veins were responsible for the bulk of longitudinal translocation and minor veins were important in collecting and loading photosynthate. Transverse veins stored (14)C-assimilate and may have coordinated the functioning of the longitudinal veins. The bearing of the results on the mechanism of translocation is discussed.