Endosperm auxins

Results with plants show the azalog of indoleacetic acid to be a powerful auxin in Pisum (pea) and Avena (oat) growth.This and the propionic acid isomer also proved to be effective with Partheno-cissus tricuspidata (Boston ivy) tissue. 7-Azatryptophan was found to inhibit a-amylase synthesis in barley endosperm tissue. 

An auxin in apple endosperm (565) not identical with indoleacetic acid was identified as ethyl indoleacetate (863, cf. 697), but this may be an artifact due to esterification during isolation (377). 

Radiolabeled lAA applied to the endosperm of germinating maize seedlings is transported to the shoot [113]. About 1% of the radioactivity in the shoot remained as free lAA, 2% was esterified, and 97% was metabolized to compounds not hydrolyzable to IA A. [ C]-labeled lAInos [131] was used to demonstrate that labeled lAInos applied to the endosperm could also be transported to the shoot and then hydrolyzed to yield free lAA [98,114]. lAInos-galactose, however, did not move out of the kernel and into the shoot until the galactose moiety was removed by hydrolysis. The resultant lAInos was then transported into the shoot [132]. From these results it was concluded that lAInos is a specific seed auxin precursor and that lAInos is the primary form in which LAA is transported from kernel to shoot. 

It is assumed that lAA plays an important role in the very early stages of embryo development because in the complete absence of lAA there is abnormal embryo development (Haccius 1965). In the young seed the main source would be the developing endosperm and the suspensor. As the embryo matures auxin declines to very low levels (Eeuwens and Schwabe 1975). However, the amount of bound auxin greatly increases (Cohen and Bandurski 1982) in the endosperm. 

Symbols in b total gibberellins, o--o liquid endosperm, ---- embryo total auxins ... 

Seeds are a rich source not only of gibberellins and cytokinins but also of auxins. Relatively few generalizations can be made about auxin content, stage of endosperm development, and growth rate of embryos. During growth of apple seeds one finds two peaks of auxin accumulation (Luckwill et al. 

During seed development auxins accumulate in the nutritive tissues, i.e. in the endosperm and nucellus. This was shown firstly for the cereals (e.g. rye [78]) and later for apple [107] and garden pea [51] among others. In the garden pea the auxin produced in the endosperm was found to be 4-chloroindole acetic acid, whereas its methyl ester occurred mainly in the embryo [65]. The auxin extractable from the seed tissues is almost certainly produced there, presumably by the biosynthetic pathway which has been worked out from studies on other parts of higher plants. These have shown that lAA is derived from the amino acid tryptophan, by the route shown in Figure 3.28. Further details can be found in recent reviews (e.g. [158]). 

The auxins, like the gibberellins, show fluctuations in level during the course of seed development. Many examples of this have been reported but only a few will be mentioned here. In apple seeds there are two peaks in auxin content, the first coinciding with the change from a free nuclear (i.e. coenocytic) endosperm to a cellular one (Sect. 3.1). Digestion of the endosperm then begins, ac-... 

Fig. 3.29 A and B. Changes in auxin content during seed development. (A) Triticum aestivum cv. Kloka. Horizontal line shows the period of anthesis. — lAA in whole grain, fresh wt. After Wheeler, 1972 [197]. (B) Pisum sativum cv. Alaska, — lAA in liquid endosperm, o---o lAA in embryo. After Eeuwens and Schwabe, 1975 [51]...

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