The greatest storage protein of endosperm tissue in maize and its wild relatives is zein. This protein accumulates in spheroidal organelles termed protein or zein bodies. With the intention of finding out if there are differences between species as to the size of the protein bodies, measurements were carried out in three maize varieties (one dent and two flint), in perennial teosinte (Z. perennis) and in Tripsacum dactyloides (2n=72).
Microscopic observations were effected upon hard endosperm cuts of mature kernels. The kernels were thin sectioned and destarched with alpha-amylase, and the endosperm tissue was stained with Coomassie Brilliant Blue R 250. Excepting the staining method, the procedure employed for the preparation and observation of the samples is similar to that described by Wolf and Koo (Stain Tech. 45:277, 1970). The size of the zein bodies was evaluated by measuring the largest diameter, and was expressed in micrometers (um). Protein body sizes were measured in three zones: (1) the first to the fourth layer of cells beneath the aleurone layer; (2) from the fifth to the tenth layer of cells; and (3) from the eleventh layer of cells towards the inside of the kernel. In each zone and for each sample approximately 200 zein bodies were measured. The results obtained may be seen in Tables 1 and 2.
The high variability which exists with respect to size in the zein bodies within each analyzed zone should be emphasized. The ample variation range is approximately similar in all samples studied, and there are not only size variations within zones but in the same zone. The high number of measurements effected (200 per each zone and 600 for each sample) show that the zein bodies have a very variable size, a character which has not been reported previously.
This is probably due to the fact that the work reported by other authors was based on results carried out with dyes which were not specific (i.e., iodine). Coomassie's high detectability and specificity allows the identification with great precision of the zein bodies embedded in the protein matrix and allows even the smallest (not visible by means of other dyes) to be seen. The dye employed is therefore very appropriate for the staining of the zein bodies.
The first zone of cells, which appears to be the most representative, shows that the dent line possesses larger zein bodies (see Table 2). Flint maize has zein bodies which do not differ significantly from the perennial teosinte, and are of smaller size than the dent maize. In all species there is a general tendency indicating that the size of the zein bodies decreases from zone 1 to zone three, that is to say, from the most superficial layers of cells inward. Although in some cases there exist significant differences (see Table 2), the sizes of the zein bodies are not an appropriate index to distinguish the Maydeae. In sum, one may say that the endosperm structure of the different species is very similar and that the greatest differences are a consequence (as far as proteins) of the different proportion found in the different species, as previously expressed (MNL 56:106, 1982).
Erratum: In the last Newsletter (MNL 57:70, 1983) we committed an error in the magnification factor of the microscopic camera. Consequently, the true values for Tripsacum dactyloides (2n=72) are: the size of the zein bodies vary from 1.2 to 2.4 um in diameter (average 1.8 um); the starch grains vary from 10.2 to 13.5 um in diameter (average 12.0 um).
Luis Maximo Bertoia and Jorge Luis Magoia
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