Maize production is threatened by drought stress worldwide. Identification of the genetic components underlying drought tolerance in maize is of great importance. Here we report a genome-wide association study (GWAS) of maize drought tolerance at the seedling stage that identified 83 genetic variants, which were resolved to 42 candidate genes. The peak GWAS signal showed that the natural variation in ZmVPP1, encoding a vacuolar-type H(+) pyrophosphatase, contributes most significantly to the trait. Further analysis showed that a 366-bp insertion in the promoter, containing three MYB cis elements, confers drought-inducible expression of ZmVPP1 in drought-tolerant genotypes. Transgenic maize with enhanced ZmVPP1 expression exhibits improved drought tolerance that is most likely due to enhanced photosynthetic efficiency and root development. Taken together, this information provides important genetic insights into the natural variation of maize drought tolerance. The identified loci or genes can serve as direct targets for both genetic engineering and selection for maize trait improvement.

Postgerminative growth of seed plants requires specialized metabolism, such as gluconeogenesis, to support heterotrophic growth of seedlings until the functional photosynthetic apparatus is established. Here, we show that the Arabidopsis thaliana fugu5 mutant, which we show to be defective in AVP1 (vacuolar H + -pyrophosphatase), failed to support heterotrophic growth after germination. We found that exogenous supplementation of Suc or the specific removal of the cytosolic pyrophosphate (PPi) by the heterologous expression of the cytosolic inorganic pyrophosphatase1 (IPP1) gene from budding yeast (Saccharomyces cerevisiae) rescued fugu5 phenotypes. Furthermore, compared with the wild-type and AVP1 Pro :IPP1 transgenic lines, hypocotyl elongation in the fugu5 mutant was severely compromised in the dark but recovered upon exogenous supply of Suc to the growth media. Measurements revealed that the peroxisomal b-oxidation activity, dry seed contents of storage lipids, and their mobilization were unaffected in fugu5. By contrast, fugu5 mutants contained ;2.5-fold higher PPi and ;50% less Suc than the wild type. Together, these results provide clear evidence that gluconeogenesis is inhibited due to the elevated levels of cytosolic PPi. This study demonstrates that the hydrolysis of cytosolic PPi, rather than vacuolar acidification, is the major function of AVP1/FUGU5 in planta. Plant cells optimize their metabolic function by eliminating PPi in the cytosol for efficient postembryonic heterotrophic growth.

In multicellular organisms, the coordination of cell proliferation and expansion is fundamental for proper organogenesis, yet the molecular mechanisms involved in this coordination are largely unexplored. In plant leaves, the existence of this coordination is suggested by compensation, in which a decrease in cell number triggers an increase in mature cell size. To elucidate the mechanisms of compensation, we isolated five new Arabidopsis (Arabidopsis thaliana) mutants (fugu1-fugu5) that exhibit compensation. These mutants were characterized together with angustifolia3 (an3), erecta (er), and a KIP-RELATED PROTEIN2 (KRP2) overexpressor, which were previously reported to exhibit compensation. Time-course analyses of leaf development revealed that enhanced cell expansion in fugu2-1, fugu5-1, an3-4, and er-102 mutants is induced postmitotically, indicating that cell enlargement is not caused by the uncoupling of cell division from cell growth. In each of the mutants, either the rate or duration of cell expansion was selectively enhanced. In contrast, we found that enhanced cell expansion in KRP2 overexpressor occurs during cell proliferation. We further demonstrated that enhanced cell expansion occurs in cotyledons with dynamics similar to that in leaves. In contrast, cell expansion was not enhanced in roots even though they exhibit decreased cell numbers. Thus, compensation was confirmed to occur preferentially in determinate organs. Flow cytometric analyses revealed that increases in ploidy level are not always required to trigger compensation, suggesting that compensation is only partially mediated by ploidy-dependent processes. Our results suggest that compensation reflects an organ-wide coordination of cell proliferation and expansion in determinate organs, and involves at least three different expansion pathways.

Size is an important parameter in the characterization of organ morphology and function. To understand the mechanisms that control leaf size, we previously isolated a number of Arabidopsis thaliana mutants with altered leaf size. Because leaf morphogenesis depends on determinate cell proliferation, the size of a mature leaf is controlled by variation in cell size and number. Therefore, leaf-size mutants should be classified according to the effects of the mutations on the cell number and/or size. A group of mutants represented by angustifolia3/grf-interacting factor1 and aintegumenta exhibits an intriguing cellular phenotype termed compensation: when the leaf cell number is decreased due to the mutation, the leaf cell size increases, leading to compensation in leaf area. Several lines of genetic evidence suggest that compensation is probably not a result of the uncoupling of cell division from cell growth. Rather, the evidence suggests an organ-wide mechanism that coordinates cell proliferation with cell expansion during leaf development. Our results provide a key, novel concept that explains how leaf size is controlled at the organ level.