Maize pollen can be cultured in vitro with great difficulty: it germinates, but with a very poor tube growth, on simple media. This report describes several attempts made to improve in vitro pollen tube growth.
Complex media, such as Murashige and Skoog (1962, Physiol. Plant, 15:473-497) or Gengenbach and Green media (1975, Crop Sci. 15:645-649) even modified in a number of ways, block its development.
Different types of stylar extracts were added to the standard medium. Extracts of not-pollinated silks proved to have an inhibitory effect on pollen growth, which become normal again (equal or larger than the standard medium control) when extracts of pollinated styles or not-pollinated silks plus pollen grains were employed. In any case a substantial growth increase was not obtained.
Another attempt was made by supplementing the growth medium with some amino acids, whose role may be particularly important in pollen metabolism. These are asparagine and glutamine, considered important for nitrogen uptake, and threonine, tyrosine, proline, which in maize are present in clearly different amounts in the pollen and silks (Linskens and Pfahler, 1977, Theor. Appl. Genetics, 50:173-177): tyrosine almost exclusively in the silks, proline only in the pollen grain, threonine in different but negatively correlated amounts in the pollen and silks of different genotypes. Six inbred lines and two mutants of one of them were used in the trial. The presence of asparagine and glutamine generally showed a negative effect on the growth. The findings relating to the other three amino acids are reported in Table 1. Threonine effects were in general favorable, but not for all the genotypes; this is what could be expected in view of the different proportions of the content of this amino acid in the pollen and style of different genotypes. On the other hand, with regard to tyrosine and proline we obtained quite unexpected results. Tyrosine, practically absent in maize pollen, was ineffective or even inhibiting for pollen growth, while proline, which represents the predominant component in the amino acid pool of pollen grain, was revealed to be effective in improving germination and tube growth.
On the basis of these results, we explored the effect of a wide range of doses of proline (from 0 to 0.64%). The results (which are not here reported) indicate a linear effect, up to a maximum of 0.16%. Thus the germination and growth improvement which can be obtained is in fact greater than those reported in Table 1. These findings might be explained by the central role of proline in pollen metabolism for energy delivery, protein synthesis and particularly--as hydroxyproline--for tube wall building.
M. Sari Gorla and E. Croci
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