Aba mutant of Arabidopsis

The aba mutant of Arabidopsis is impaired in epoxy carotenoid biosynthesis and thus, makes zeaxanthin but not violaxanthin, antheraxanthin, and neoxanthin (Rock and Zeevaart, 1991 Rock et al, 1992). The pigment stoichiometries indicate a 1 1 replacement of neoxanthin and violaxanthin by zeaxanthin. The lutein content decreases compared to wildtype Arabidopsis, indicating a reduced Chi a/b antenna. Also comparing the composition of the antenna changes, there is less ofthe major LHCII and more of the minor Chi a/b complexes (Hurry et al, 1997). 

Of course, the interaction between the membrane-spanning helices A and B that is presumably stabilized by these carotenoids may in turn influence the trimer formation of the complex. However, it is also possible that the additional carotenoid(s), known to be part of the complex from biochemical data but not visible in the crystal stmcture, stabilize trimers. This can be suggested from the observation that in the aba mutant of Arabidopsis where zeaxanthin appears to replace violaxanthin and neoxanthin, the major LHCII dissociates more easily into monomers when isolated under partially denaturing conditions (Tardy and Havaux, 1996). A closer biochemical inspection of the major LHCII in various carotenoid-deficient plant and algae mutants will be necessary to assess the impact of carotenoids on the formation of trimeric LHCII. Another experimental approach will be to study how the variation of the carotenoid components influences the reconstitution of trimeric LHCII in vitro (Hobe et al, 1994). 

Rhee KH, Morris EP, Zheleva D, Hankamer B, Kiihlbrandt W and Barber J (1997) Two dimensional structure of plant Photosystem II at 8 A resolution. Nature 389 522-526 Rock CD and Zeevaart JAD (1991) The aba mutant of Arabidopsis thaliana is impaired in epoxy-carotenoid biosynthesis. Proc Natl Acad Sci USA 88 7496-7499 Rock CD, Bowlby NR, Hoffmann-Benning S and Zeevaart JAD (1992) The aba mutant of Arabidopsis thaliana (L) Heynh. has reduced chlorophyll fluorescence yields and reduced thylakoid stacking. Plant Physiol 100 1796-1801 Romer S, Humbeck K and Senger H (1990) Relationship between biosynthesis of carotenoids and increasing complexity of Photosystem I in mutant C-6D of Scenedesmus obliquus. Planta 182 216-222... 

Fig. 2. Pathway of ABA biosynthesis in higher plants. The metabolic blocks in the vpl4 mutant of maize and in the abal, aba2, and abaS mutants of Arabidopsis are indicated.

The concentration of ABA increases in late embryo development shortly before the onset of desiccation and seed dormancy [42,81], ABA inhibits precocious germination of immature embryos in culture [70]. Genetic studies have shown that ABA is involved in dormancy and ABA deficient and ABA-insensitive mutants have been isolated [44]. Reciprocal crosses of wild type and ABA-deficient aba mutants of Arabidopsis showed that there are two different pools of ABA in Arabidopsis seeds. The embryonic ABA is... 

ROCK, C. and ZEEVAART, J., The aba mutant of Arabidopsis lhaliana is impaired in epoxy-carotenoid biosynthesis., PNAS 1991,88, 7496-7499. 

In mutants blocked in ABA-ald to ABA conversion, the substrate, ABA-ald, does not accumulate, but it is reduced and isomerized to t-ABA-alc which accumulates at high levels [82]. The glucoside of r-ABA-alc has been isolated from quince fruit [83], the abal mutant of N. plumbaginifolia [84], and the aba3 mutant of Arabidopsis [72] (Fig. 3). 

Prior to 1995, only one locus affecting Xanthophyll biosynthesis in photosynthetic tissues of Arabidopsis had been identified, the ABA i locus, the mutation of which disrupts zeaxanthin deepoxidase, one of two xanthophyll cycle enzymes (Koomneef et al, 1982 Rock and Zeevaart, 1991 Rock et al., 1992). As a step toward advancing understanding of xanthophyll biosynthesis, incorporation, and function in plants, the author s laboratory has screened for and identified mutations defining two additional loci required for xanthophyll biosynthesis in Arabidopsis, LUTl and LUT2 LUT= LUTein deficient). Mutations at either locus result in defects in the synthesis of lutein, the most predominant xanthophyll in plants. Singly and in combination with the aba mutation, these lut mutations have allowed the genetic construction of five distinct mutant lines which differ dramatically in their carotenoid composition relative to wild-type Arabidopsis. In the remainder of this chapter I will first briefly discuss the aba mutation followed by a... 

Exposure to low, non-freezing temperatures induces a process, known as cold acclimation, which makes the plant able to increase its fireezing tolerance. Treatment of wheat, rye and bromegrass cell cultures with ABA for four days at 20°C bypasses the requirement for exposure to low temperature for increased freezing tolerance suggesting that cold acclimation involves ABA. The Arabidopsis aba mutant is unable to cold acclimate by incubation at 4°C but the phenotype can be reverted by treatment with ABA. This shows that adaptation to freezing temperatures requires endogenous ABA. 

However, ABA seems to be a prerequisite for gene expression but the wild type level is far in excess of the necessary concentration. This is demonstrated by the levels of gene expression in mutants with low ABA level. It is known that in the viviparous mutants of maize accumulation of the rabl 7 [67,90], rab28 [68], Em and Catl [94] is reduced but not absent. The relatively high expression in the mutants does not reflect the low level of endogenous ABA. Similar results have been found in the Arabidopsis aba mutant [21]. 

In Arabidopsis, mutants with reduced sensitivity to exogenous ABA were selected on the basis of their good growth in a solution containing 10 pM ABA [14]. The mutations were at three different loci and were termed abil, abi2 and abiS. Phenotypically these mutants resembled the aba mutants in several responses, yet they contained ABA levels similar or even somewhat higher than in the wild type. 

The reversibility by applied ABA of the reduction in growth of leaves and stems of ABA-deficient mutants suggests, at first view, a growth-stimulatory effect of ABA [26]. However, Quarrie [18] rightly remarks that all those observations [25] were made in greenhouses without a proper humidity control. At a 95% RH in a controlled environment cabinet, plants of Arabidopsis wild type and the mutants with the least serious ABA deficiency aba ) were of the same height and those of the severely deficient abal mutant were 10-12% shorter. Hence, more detailed studies are required to unravel direct and indirect effects of ABA deficiency on the growth response of stems and leaves. 

ABA produce more mature seeds showing primary dormancy than the wild-type plants. A reduced dormancy often occurs in seeds of ABA-insensitive mutants. Gibberellin A (GA) is absolutely required for germination of the wild-type Arabidopsis seeds. Seeds of ethylene-insensitive and GA-insensitive mutants are supersensitive to exogenous ABA, which suggests that in imbibed seeds ethylene and GA may directly counteract the action of ABA in dormancy maintenance. ... 

Several of the cold-regulated genes isolated so far in Arabidopsis (Kurkela Franck, 1990 Hajela et al., 1990), alfalfa (Mohapatra et al., 1988) and barley (L. Cattivelli, unpublished data), show ABA-indu-cibility, but, at least in alfalfa and in barley, not all (Mohapatra et al., 1989 L. Cattivelli, unpublished data). These results support the hypothesis that although ABA is probably essential for hardening (in fact an ABA-deficient mutant does not cold-acclimate), it is not responsible for all the modifications induced during exposure of plants to low... 

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