Maize Genetics Cooperation
Newsletter vol 84 2010
Please Note: Notes
submitted to the Maize Genetics Cooperation Newsletter may be cited only with
consent of authors.
CRA –MAC
Unit� di Ricerca per la Maiscoltura,
Via
Stezzano 24, 24126 Bergamo, Italy
Analyses of o2, o7, and o2o7 mutations on amino acid metabolisms in
maize endosperm
by transcript profiling*
Hartings H., Pirona
R., Rossi V., M. Motto
Our long-term goal is comprehensive
dissection of the molecular mechanisms underlying endosperm development and
metabolisms in maize kernels. In this context, genetics has played an important role by discovering a series of opaque
endosperm mutants and demonstrating their effects on genes mediating zein deposition (see Motto et al., Maydica 54: 321-342, 2009). For example, the recessive mutation opaque-2 (o2) and opaque-7 (o7) induce a specific decrease in the
accumulation of 22- and 19-kDa α-zeins,
respectively.
The o2 mutation has been widely studied at the genetic, biochemical,
and molecular levels. It was shown that O2
encodes a basic leucine zipper (bZIP)
transcriptional regulator that is specifically expressed in the endosperm; it
also activates the expression of 22-kDa α-zein
and 15-kDa β-zein genes by
interacting with the TC-CACGT(a/c)R(a/t) and
GATGYRRTGG sequences of their promoters, therefore displaying a broad binding
specificity and recognizing a variety of target sites in several distinct genes
(Gavazzi et al., Plant Physiol. 145: 933-945, 2007). O2
also regulates directly or indirectly a number of other non-storage protein
genes, including b-32, encoding a
type I ribosome-inactivating protein, cyPPDK1,
one of the two cytosolic isoforms
of the pyruvate orthophosphate dikinase
gene, and b-70, encoding a heat shock protein 70 analogue, possibly acting as a
chaperonin during protein body formation (cf. Motto et al., Maydica 54: 321-342, 2009). O2, furthermore, regulates the levels of lysine-ketoglutarate
reductase (Brochetto-Braga et al., Plant Physiol.,
98:1139-1147, 1992 ) and aspartate
kinase1 (Azevedo et al., Phytochemistry, 46:395-419,1997).These
broad effects suggest that O2 plays
an important role in the developing grain as a coordinator of the expression of
genes controlling storage protein, and N and C metabolisms.
Although the molecular basis of o7 mutation is yet unknown, evidence
indicates that this mutation, in addition to repressing the lower molecular
weight α-zeins, drastically affects the development of maize endosperm due to a
reduction in starch content. Moreover, the high content in o7 endosperms of non-protein N has suggested the existence in o7 of a block in the synthetic route
leading to proteins similar to that observed for the starch modifying gene shrunken4 (Motto et al., Maydica 54: 321-342, 2009).
To advance our understanding of the
nature of the o2 and o7 mutations in affecting amino
acid metabolisms, we used genome-wide analyses of gene expression profiles
during kernel development. Specifically, for this study we have used mRNA
transcripts from A69Y+ and from the nearly isogenic o2 and o7 mutants, and for the double mutant combination o2o7, collected from endosperms at 14 days after pollination.
Additionally, the profile of
endosperm transcripts was obtained with the Zeastar Unigene set, based on the sequence information of >7,200
maize genes, mainly derived from maize endosperm and covering a wide
range of metabolic pathways and cellular and physiological processes
Our study indicated
that several ESTs homologous to enzymes involved in amino acid synthesis were
differentially expressed in the o2, o7, and o2o7 endosperms. In particular, ESTs homologous to enzymes involved
in tryptophan synthesis were affected in o2
endosperm. Tryptophan synthase (EC 4.2.1.20)
homologues showed a significant reduction of expression in o2 endosperms, while anthranilate phosphoribosyl transferase (EC
2.4.2.18) and anthranilate synthase
(EC 4.1.3.27) homologous ESTs were found to be differentially
expressed in all three mutant backgrounds. The former showed a
significant reduction of its expression level, while the latter appeared up-regulated by 50%.
ESTs homologous to phosphoglycerate dehydrogenase
(EC 1.1.195), cysteine synthase
(EC 2.5.1.47), methionine synthase
(EC 2.1.1.14), S-adenosylmethionine synthetase (EC 2.5.1.6), and a methyl transferase
(EC 2.1.1.37), all enzymes involved in the Ser, Gly, Cys, and Met pathways were negatively affected in the o2 endosperm. However, neither of these
showed a significantly altered expression level in the o7 and o2o7 endosperms.
Finally, the Ile, Val and Leu pathways were affected
in all three lines. ESTs homologous to acetolactate synthase (EC 2.2.1.6) and ketolacid
reductoisomerase (EC 1.1.1.86), and involved in the
biosynthesis of these amino acids were significantly reduced in expression in
all three backgrounds, while leucine dehydrogenase (EC 1.4.1.9) was significantly different from
wild-type only in the o7 endosperms.
In conclusion, the current study indicated
that the
transcription levels of various genes encoding key enzymes involved in amino
acids were significantly affected in the o2
mutant. O7 regulates the expression
of some genes of the amino acids biosynthesis, but only in few cases the mRNAs
affected are the same that are up- or down-regulated in the o2 mutant, suggesting that the O2 and O7 factors act on specific target genes. Among the pathways
affected by o2 and o7 mutants are those leading to the
synthesis of the aromatic (Phe, Trp,
and Tyr), Asp-derived, and branched chain amino acids (BCAA). These pathways
are deeply interconnected both in terms of C precursor supply and of allosteric interactions (Curien et al., Plant
Physiol. Biochem.,
46:325-339, 2008). A complex interplay of regulators controls the metabolic
flow through the aromatic, Asp and BCAA-pathways, which includes feedback
inhibitors of regulatory enzymes (Galili and H�fgen, Metabolic Eng., 4:3-11,
2002). Furthermore,
alterations in enzymes affecting amino acid metabolism have been shown to have pleiotropic effects on free amino acid levels in plant
tissues. For example, it was found that a mutation in a key enzyme in the
Asp-pathway, a feedback-insensitive aspartate kinase mutant in tobacco, not only has a higher level of
amino acids derived from the Asp pathway, but other pathways as well (Frankard et al., Plant Physiol., 99: 1285-1293, 1992). In
addition, it was reported that the alteration of Trp
and Tyr levels in transgenic tobacco leaves affects the level of Trp, as well as the aliphatic amino acids Met, Val, and Leu (Guillet et al., Plant Physiol., 122:933-943,
2000).
*Research in this laboratory was supported by
the European Communities BIOTECH Programme, as part
of the Zeastar project (2001-2005) and by Ministero per le Politiche Agricole, Alimentari e Forestali, Roma: special grant �Zeagen�.