Light inducibility of anthocyanins in vp aleurone

Homozygous vp suppresses anthocyanin pigment in the aleurone. Dooner and Nelson (Biochem. Genet. 15:509, 1977) suggested that the Vp locus is regulatory for the anthocyanin pathway because vp Bz tissue shows reduced levels of the Bz-coded glucosyl transferase activity.

Chen and Coe (Biochem. Genet. 15:333, 1977) reported that c-p vp seeds were able to synthesize pigment on the ear when exposed to light. This interaction is unusual in view of the observations that: 1) c-p Vp seeds are unable to form anthocyanins if supplied light while still on the plant, and 2) C vp seeds are suppressed in anthocyanin synthesis. The first observation suggests that the Vp allele inhibits the utilization of the light signal by the c-p allele, and is thus antagonistic to pigment formation, while the second observation suggests that the Vp allele is required for pigment formation. A partial explanation of these interactions was suggested when it was noticed that a self-pollinated ear of C/C Vp/vp constitution had a few faintly pigmented vp seeds on a region of the ear that had accidentally been exposed to light during maturation. Further tests demonstrated that the vp suppression of anthocyanin synthesis in C seeds can be overcome with light exposure.

Dormant seeds from a C/C Vp/vp self-pollinated ear (some of which would again be heterozygous Vp/vp) were planted in the greenhouse in the 1977-1978 season and self-pollinated. At 14 days after pollination (DAP) some of the ears were stripped of husks and covered with plastic bags, to allow light exposure. From 15 DAP (when pigment first appears in this strain) until 40 DAP a record was kept of the time of pigmentation of the individual seeds of the ear and also of the viviparous seed segregation, which becomes apparent at approximately 30 DAP. Other ears in the family were left covered until harvest. Homozygous c-p vp and C vp seeds were stored in the cold after the greenhouse harvest and transplanted from the ear (where they were prematurely germinating) to the field in the summer, 1978. These plants were self-pollinated and the ears exposed to light on the plant, or left covered until harvest.

All of the seeds on light-exposed ears developed pigment in the aleurone layer by 30 DAP. The viviparous seeds on ears not exposed to light remained colorless. Most seeds on the segregating ears developed some pigment within the first few days after removing the husks. It might be expected that those seeds that will have (after 30 DAP) viviparous embryos would be late in developing pigment, since light is required for pigment formation. However, there was no obvious correlation between the time of pigment formation and the vivipary of the embryo; some seeds that pigmented on the first day were viviparous. All the seeds on the C vp ears were viviparous, whether they were exposed to light or not. Thus light has no "corrective" effect on the vivipary of the embryo, but only on the anthocyanin phenotype of the aleurone.

In order to explain the pleiotropic effect of vp, Dooner and Nelson suggested that Vp is a structural or regulatory gene directly involved in the anthocyanin pathway as well as in the synthesis of some dormancy factor other than abscisic acid. If this is so, then the light inducibility of vp could be explained by the light signal allowing a bypass of the requirement for Vp action. Possibly this light-induced shunt would open the pathway after the step where an intermediate is used for the synthesis of a dormancy factor, since the seeds are still viviparous. However, to explain the c-p vp interaction a requirement that the Vp product (or a derivative) is inhibitory to the use of the light signal by c-p aleurone would have to be invoked. The absence of this product (perhaps the putative dormancy factor) in vp tissue would allow c-p seeds to form pigment on the ear when light is supplied. Thus the light signal is coincidentally required to induce pigment synthesis and to open the shunt to allow pigment synthesis in c-p vp aleurone.

Dooner and Nelson (1977) suggested coumarin (derived from cinnamic acid) or naringenin (a flavanone) as possible dormancy factors. It is unlikely that naringenin is a dormancy factor in maize. Fresh or dormant c2 aleurone tissue is blocked in anthocyanin synthesis but can use exogenous naringenin to synthesize anthocyanin pigment (McCormick, 1978, Biochem. Genet. 16:777). If naringenin was a dormancy factor, c2 seeds might be expected to be viviparous, but they are not. The possibility of course remains that coumarins or other compounds related biosynthetically to the anthocyanins could be dormancy factors in maize.

Further tests will be necessary before an adequate explanation of the C and Vp interaction can be made. Whatever the molecular mechanism(s), it is clear that anthocyanin synthesis in vp aleurone is light-inducible, and it is plausible to suppose that this light-inducible step is at a point other than the c-p light-inducibility. Thus the vp light-inducibility supplies another "handle" for examination of the correlation between the genetic and physiological effects of pigment synthesis. Preliminary data suggest that certain pattern alleles at the R locus (e.g. R-nj) also show light inducibility.

It is of interest that the three purported regulatory genes (C, R, and Vp) all have alleles that are light-inducible for anthocyanin synthesis, whereas no alleles of the structural genes are light-inducible. If the C, R, and Vp loci do control the functions of the structural genes of the pathway then the light-inducible alleles of these loci will be useful in a practical sense for future molecular analyses with this system, because the turn-on of the structural genes can be precisely controlled with light.

Although it is premature to speculate on the molecular organization of the C locus, the following non-molecular model is proposed to account for the data. The data suggest that a quantitative variation distinguishes the C, c-p and c-n alleles from each other; the C-I allele does not fit in this quantitative series and is apparently an active antagonist of anthocyanin formation. If one assumes that the C locus is composed of two "cistrons," one with I function (the antagonistic effect) and one with C function (the activation of anthocyanin formation), then the following allelic compositions can explain the interactions at the C locus. The C-I allele has the antagonistic function but not the activation function, and thus is (Ic); the C allele has no antagonistic function, but has the activation function, and thus is (iC); the c-p allele has both the antagonistic function and the activation function, and thus is (IC); and the c-n allele has neither the antagonistic function nor the activation function and thus is (ic). Then in triploid aleurone tissue the constitution of c-p/c-p/c-p seeds is (IC)/(IC)/(IC); these seeds require light to give the activation component a competitive boost against the three doses of the antagonistic component. Colorless C-I/C/C seeds would have the constitution (Ic)/(iC)/(iC); if one further assumes that the antagonistic function is more efficient than the activation function the C-I/C/C seeds would require the action of light to out-compete the antagonistic function of the one (I) dose. Unique results of experiments designed to test these compositions at the recombinational and mutational level can be predicted. Further compoundness of the C component "cistron" would probably be required to explain the c-m2 data.

Sheila McCormick


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