Auxin Biosynthesis during Seed Germination in Phaseolus vulgaris

Abstract

The relative roles of de novo biosynthesis of indoleacetic acid (IAA) and IAA conjugates stored in mature seeds (Phaseolus vulgaris L.) in supplying auxin to germinating bean seedlings were studied. Using 2H oxide and 2,4,5,6,7-[2H1L-tryptophan as tracers of IAA synthesis, we have shown that de novo biosynthesis of IAA, primarily from tryptophan, is an important source of auxin for young bean seedlings. New synthesis of IAA was detected as early as the second day of germination, at which time the seedlings began to accumulate fresh weight intensively and the total content of free IAA began to increase steadily. IAA conjugates that accumulate in large amounts in cotyledons of mature seeds may thus be considered to be only one of the possible sources of IAA required for the growth of bean seedlings.It is believed that the major source of free IAA for young seedlings are IAA conjugates stored in the seeds during their maturation. This hypothesis was developed primarily for maize in which de novo biosynthesis of IAA in the seedlings of germinating kemels is not detectable and the major source of this hormone is ester conjugates (5,19,26). Bandurski and coworkers showed that the maize seedling is essentially a closed system, in which the level of IAA is regulated by transport from the endosperm, synthesis and hydrolysis of IAA conjugates, and IAA oxidation. However, even maize includes a few cultivars that may synthesize IAA de novo during seed germination (27); thus, the general applicability of the above hypothesis remains to be tested.In legumes, the major portion of total IAA present in mature seeds is found in amide-linked conjugates (3,9,13,18). In bean (Phaseolus vulgaris L.) seeds, amide conjugates account for more than 80% of the total IAA, and they consist predominantly of a series of immunologically related IAA peptides of 3 to 25 kD (6, 7, 16). We (11) showed that amidelinked IAA conjugates declined in bean cotyledons during the first 2 d of germination; however, as germination progressed, this decline slowed down and the level of IAA conjugates remained relatively high even after almost 1 week

A gene encoding a protein modified by the phytohormone indoleacetic acid

“…However, other bean tissues contain proteins that are detected by Ab3.6K (data not shown). Previous analytical results from our laboratory (22) showed that proteins with IAA attached are present in vegetative tissues of bean, but these earlier reports did not determine the characteristics of the protein moiety. The absence of iap1 mRNA in vegetative tissues (Fig.…”

“…8). Unlike maize (20,21), bean seedlings begin de novo IAA biosynthesis within the first hours after imbibition in the dark (22).…”

Differential activation of the primary auxin response genes, PS‐IAA4/5 and PS‐IAA6, during early plant development

“…It appears that the competence to respond to exogenous auxin is under developmental control which coincides with the initiation of de novo IAA biosynthesis in the growing seedling. Bean seedlings start to synthesize IAA de novo between day 2 and day 3 after imbibition (Bialek et aL, 1992). The need for auxin by the germinating embryo is provided for by the seed.…”

Selection and Characterization of [alpha]-Methyltryptophan-Resistant Lines of Lemna gibba Showing a Rapid Rate of Indole-3-Acetic Acid Turnover

“…N, was used to reduce the volume of the pentane to about 1 niL, and the indole content was determined by GC-MS monitoring ions at m/z 117 and 123 (the molecular ions for indole and ['H,]indole, respectively). Because of the low mrichment of the internal standard, the major peak was for I 'H,]indole rather than for ['H,]indole (see Bialek et al, 1992, for calculations). The correction factor ("R value"; C'ohen et al, 1986) for indole measured at m/z 117 and the internal standard measured at m/z 123 was determined 2mpirically to be 1.65.…”

“…The methanol was then removed in vacuo and the derivatized amino acid was dissolved in distilled water. A Fisher PrepSep C,, SPE column and a Waters (Milford, MA) NovaPak C,, HPLC column (on a Varian [Sunnyvale, CAI 5000 HPLC) were used for further purification before analysis by GC-MS (Hewlett-I'ackard 5890 series I1 GC with a Hewlett-Packard 5971A mass selective detector), as described by Bialek et al (1992).…”