The pyruvate dehydrogenase complex (PDC) is subjected to multiple interacting levels of control in plant cells. The first level is subcellular compartmentation. Plant cells are unique in having two distinct, spatially separated forms of the PDC; mitochondrial (mtPDC) and plastidial (plPDC). The mtPDC is the site of carbon entry into the tricarboxylic acid cycle, while the plPDC provides acetyl-CoA and NADH for de novo fatty acid biosynthesis. The second level of regulation of PDC activity is the control of gene expression. The genes encoding the subunits of the mt-and plPDCs are expressed following developmental programs, and are additionally subject to physiological and environmental cues. Thirdly, both the mt-and plPDCs are sensitive to product inhibition, and, potentially, to metabolite effectors. Finally, the two different forms of the complex are regulated by distinct organelle-specific mechanisms. Activity of the mtPDC is regulated by reversible phosphorylation catalyzed by intrinsic kinase and phosphatase components. An additional level of sensitivity is provided by metabolite control of the kinase activity. The plPDC is not regulated by reversible phosphorylation. Instead, activity is controlled to a large extent by the physical environment that exists in the plastid stroma.

Mutation of either arginase structural gene (ARGAH1 or ARGAH2 encoding arginine [Arg] amidohydrolase-1 and -2, respectively) resulted in increased formation of lateral and adventitious roots in Arabidopsis (Arabidopsis thaliana) seedlings and increased nitric oxide (NO) accumulation and efflux, detected by the fluorogenic traps 3-amino,4-aminomethyl-2#,7#-difluorofluorescein diacetate and diamino-rhodamine-4M, respectively. Upon seedling exposure to the synthetic auxin naphthaleneacetic acid, NO accumulation was differentially enhanced in argah1-1 and argah2-1 compared with the wild type. In all genotypes, much 3-amino,4-aminomethyl-2#,7#-difluorofluorescein diacetate fluorescence originated from mitochondria. The arginases are both localized to the mitochondrial matrix and closely related. However, their expression levels and patterns differ: ARGAH1 encoded the minor activity, and ARGAH1-driven b-glucuronidase (GUS) was expressed throughout the seedling; the ARGAH2TGUS expression pattern was more localized. Naphthaleneacetic acid increased seedling lateral root numbers (total lateral roots per primary root) in the mutants to twice the number in the wild type, consistent with increased internal NO leading to enhanced auxin signaling in roots. In agreement, argah1-1 and argah2-1 showed increased expression of the auxin-responsive reporter DR5TGUS in root tips, emerging lateral roots, and hypocotyls. We propose that Arg, or an Arg derivative, is a potential NO source and that reduced arginase activity in the mutants results in greater conversion of Arg to NO, thereby potentiating auxin action in roots. This model is supported by supplemental Arg induction of adventitious roots and increased NO accumulation in argah1-1 and argah2-1 versus the wild type.

SummaryIn plants, transitions in mating system from outcrossing to self-fertilization are common; however, the impact of these transitions on interspecific and interpopulation reproductive barriers is not fully understood. We examined the consequences of mating system transition for reproductive barriers in 19 populations of the wild tomato species Solanum habrochaites.We identified S. habrochaites populations with self-incompatible (SI), self-compatible (SC) and mixed population (MP) mating systems, and characterized pollen-pistil interactions among S. habrochaites populations and between S. habrochaites and other tomato species. We examined the relationship between mating system, floral morphology, interspecific and interpopulation compatibility and pistil SI factors.We documented five distinct phenotypic groups by combining reproductive behavior with molecular data. Transitions from SI to MP were not associated with weakened interspecific reproductive barriers or loss of known pistil SI factors. However, transitions to SC at the northern range margin were accompanied by loss of S-RNase, smaller flowers, and weakened (or absent) interspecific pollenÀpistil barriers. Finally, we identified a subset of SC populations that exhibited a partial interpopulation reproductive barrier with central SI populations.Our results support the hypothesis that shifts in mating system, followed by additional lossof-function mutations, impact reproductive barriers within and between species.