It is known that volatile emissions from some beneficial rhizosphere microorganisms promote plant growth. Here we show that volatile compounds (VCs) emitted by phylogenetically diverse rhizosphere and non-rhizhosphere bacteria and fungi (including plant pathogens and microbes that do not normally interact mutualistically with plants) promote growth and flowering of various plant species, including crops. In Arabidopsis plants exposed to VCs emitted by the phytopathogen Alternaria alternata, changes included enhancement of photosynthesis and accumulation of high levels of cytokinins (CKs) and sugars. Evidence obtained using transgenic Arabidopsis plants with altered CK status show that CKs play essential roles in this phenomenon, because growth and flowering responses to the VCs were reduced in mutants with CK-deficiency (35S:AtCKX1) or low receptor sensitivity (ahk2/3). Further, we demonstrate that the plant responses to fungal VCs are light-dependent. Transcriptomic analyses of Arabidopsis leaves exposed to A. alternata VCs revealed changes in the expression of light- and CK-responsive genes involved in photosynthesis, growth and flowering. Notably, many genes differentially expressed in plants treated with fungal VCs were also differentially expressed in plants exposed to VCs emitted by the plant growth promoting rhizobacterium Bacillus subtilis GB03, suggesting that plants react to microbial VCs through highly conserved regulatory mechanisms.

A "box-in-box" cocultivation system was used to investigate plant responses to microbial volatile compounds (VCs) and to evaluate the contributions of organic and inorganic VCs (VOCs and VICs, respectively) to these responses. Arabidopsis plants were exposed to VCs emitted by adjacent Alternaria alternata and Penicillium aurantiogriseum cultures, with and without charcoal filtration. No VOCs were detected in the headspace of growth chambers containing fungal cultures with charcoal filters. However, these growth chambers exhibited elevated CO 2 and bioactive CO and NO headspace concentrations. Independently of charcoal filtration, VCs from both fungal phytopathogens promoted growth and distinct developmental changes.Plants cultured at CO 2 levels observed in growth boxes containing fungal cultures were identical to those cultured at ambient CO 2 . Plants exposed to charcoal-filtered fungal VCs, nonfiltered VCs, or superelevated CO 2 levels exhibited transcriptional changes resembling those induced by increased irradiance. Thus, in the "box-in-box" system, (a) fungal VICs other than CO 2 and/or VOCs not detected by our analytical systems strongly influence the plants' responses to fungal VCs, (b) different microorganisms release VCs with distinct action potentials, (c) transcriptional changes in VC-exposed plants are mainly due to enhanced photosynthesis signaling, and (d) regulation of some plant responses to fungal VCs is primarily posttranscriptional. KEYWORDS growth promotion, microbial volatile compounds, photosynthesis, plant-microbe interactions *Pablo García-Gómez and Goizeder Almagro contributed equally to this work.

Phosphoglucose isomerase (PGI) catalyzes the reversible isomerization of glucose-6-phosphate and fructose-6-phosphate. It is involved in glycolysis and in the regeneration of glucose-6-P molecules in the oxidative pentose phosphate pathway (OPPP). In chloroplasts of illuminated mesophyll cells PGI also connects the Calvin-Benson cycle with the starch biosynthetic pathway. In this work we isolated pgi1-3, a mutant totally lacking pPGI activity as a consequence of aberrant intron splicing of the pPGI encoding gene, PGI1. Starch content in pgi1-3 source leaves was ca. 10-15% of that of wild type (WT) leaves, which was similar to that of leaves of pgi1-2, a T-DNA insertion pPGI null mutant. Starch deficiency of pgi1 leaves could be reverted by the introduction of a sex1 null mutation impeding β-amylolytic starch breakdown. Although previous studies showed that starch granules of pgi1-2 leaves are restricted to both bundle sheath cells adjacent to the mesophyll and stomata guard cells, microscopy analyses carried out in this work revealed the presence of starch granules in the chloroplasts of pgi1-2 and pgi1-3 mesophyll cells. RT-PCR analyses showed high expression levels of plastidic and extra-plastidic β-amylase encoding genes in pgi1 leaves, which was accompanied by increased β-amylase activity. Both pgi1-2 and pgi1-3 mutants displayed slow growth and reduced photosynthetic capacity phenotypes even under continuous light conditions. Metabolic analyses revealed that the adenylate energy charge and the NAD(P)H/NAD(P) ratios in pgi1 leaves were lower than those of WT leaves. These analyses also revealed that the content of plastidic 2-C-methyl-D-erythritol 4-phosphate (MEP)-pathway derived cytokinins (CKs) in pgi1 leaves were exceedingly lower than in WT leaves. Noteworthy, exogenous application of CKs largely reverted the low starch content phenotype of pgi1 leaves. The overall data show that pPGI is an important determinant of photosynthesis, energy status, growth and starch accumulation in mesophyll cells likely as a consequence of its involvement in the production of OPPP/glycolysis intermediates necessary for the synthesis of plastidic MEP-pathway derived hormones such as CKs.