During development, cells not only adopt specialized identities but also maintain those identities. Endoreduplication is thought to maintain cell identity. High concentrations of ARABIDOPSIS THALIANA MERISTEM LAYER1 (ATML1) specify giant cell identity and induce endoreduplication in sepals. How different concentrations of ATML1 can specify different identities remains unclear. Here, we show that high concentrations of ATML1 induce the biosynthesis of both long-chain and very long-chain fatty acids (LCFAs/VLCFAs), and these fatty acids are required for the maintenance of giant cell identity. Inhibition of VLCFA biosynthesis causes endoreduplicated giant cells to resume division and lose their identity, indicating that endoreduplication is not sufficient to maintain cell identity. Structural predictions suggest that LCFA-containing lipids bind to the START domain 2 of ATML1, causing ATML1 dimerization and its auto-activation. Our data and modeling imply that ATML1 induces biosynthesis of its own lipid ligands in a positive feedback loop, shedding light on the intricate network dynamics that specify and maintain giant cell identity.Teaser: Endoreduplicated cells in Arabidopsis thaliana sepals divide and de-differentiate in the absence of VLCFA biosynthesis.### Competing Interest StatementThe authors have declared no competing interest.

Pink rib discoloration is a physiological disorder resulting in loss of quality and marketability of minimally processed and whole head lettuce (Lactuca sativa L.). Although the biochemistry of pink rib is complex and a comprehensive understanding is lacking, multiple research efforts have focused on inhibiting or reducing pink rib through exogenous application of small molecules such as L-cysteine. In this study, the effects of naturally occurring phytochemicals [serotonin (0.5 or 1 g L-1), naringenin (0.5 g L-1), L-cysteine (0.5 g L-1), and melatonin (1 g L-1)] on pink rib expression were examined in iceberg and Romaine lettuce. The exogenous application of these natural compounds was investigated for reduced pink rib discoloration in lettuce during simulated commercial storage and examined for metabolite and enzymatic activity changes. Immersing wounded midribs for 30 s in serotonin, naringenin, or L-cysteine solutions had no or only a slight inhibitory effect to inhibit pink rib discoloration while those immersed in a melatonin solution had reduced development of pink rib in both iceberg and Romaine lettuces after 5 d at 5 degrees C. Phenylalanine ammonia lyase (PAL) activity and chlorogenic acid concentration increased upon pink rib discoloration, consistent with previous reports. However, melatonin-treated samples did not result in reduced PAL activity, chlorogenic acid content, or polyphenol oxidase (PPO) activity. Untargeted metabolite analysis showed that melatonin treatment resulted in a global effect on metab-olite changes in the wounded lettuce midrib tissue. To compare the severity of pink rib among treatments, pink rib was quantified throughout storage visually and by converting digital images of the lettuce midribs to the L*a*b* color space. The a* component of L*a*b* represents redness versus greenness, thus its values were used to objectively measure pink rib severity. Melatonin was shown to be a promising compound to minimize devel-opment of pink rib discoloration at cut surfaces of iceberg and Romaine lettuce during commercial handling and storage.

Abstract Nature’s vital, but notoriously inefficient, CO2-fixing enzyme Rubisco often limits the growth of photosynthetic organisms including crop species. Form I Rubiscos comprise eight catalytic large subunits and eight auxiliary small subunits and can be classified into two distinct lineages—‘red’ and ‘green’. While red-type Rubiscos (Form IC and ID) are found in rhodophytes, their secondary symbionts, and certain proteobacteria, green-type Rubiscos (Form IA and IB) exist in terrestrial plants, chlorophytes, cyanobacteria, and other proteobacteria. Eukaryotic red-type Rubiscos exhibit desirable kinetic properties, namely high specificity and high catalytic efficiency, with certain isoforms outperforming green-type Rubiscos. However, it is not yet possible to functionally express a high-performing red-type Rubisco in chloroplasts to boost photosynthetic carbon assimilation in green plants. Understanding the molecular and evolutionary basis for divergence between red- and green-type Rubiscos could help us to harness the superior CO2-fixing power of red-type Rubiscos. Here we review our current understanding about red-type Rubisco distribution, biogenesis, and sequence–structure, and present opportunities and challenges for utilizing red-type Rubisco kinetics towards crop improvements.

The south Florida pine rocklands is a critically endangered, fire-dependent ecosystem dominated by the overstory tree Pinus densa (South Florida slash pine). Because pine recruitment in this ecosystem has proven problematic, restoration efforts need to include replanting slash pine trees. Even though ectomycorrhizal fungi are known to be critical symbionts of young pines and are necessary for the development of healthy pines, virtually nothing is known about these mutualists and their role in pine establishment and survival in the pine rocklands. One approach to improve pine establishment is to grow seedlings in a nursery before outplanting, facilitating early associations with ectomycorrhizae, and therefore improving seedling health. In this study, we compared health metrics (height, stem diameter, final needle length, root length, root colonization, needle greenness, root volume, and root:shoot ratio) of seedlings grown in soil amended with five commercially available mycorrhizal inocula versus field soil collected from three pine rockland fragments. Seedlings grown with native field soil from the pine rocklands generally performed better than those grown with commercial inoculum in all metrics except root length. According to their labels, each commercial inoculum contained between 4 and 10 ectomycorrhizal fungi species. However, no ectomycorrhizal fungi were recovered from two of the inoculum products and only three ectomycorrhizal fungi in total were recovered from the other three products. In contrast, seedlings grown with field soil are associated with ten ectomycorrhizal species. Our results highlight the importance of incorporating native ectomycorrhizal fungi into pine seedling replanting as part of restoration efforts in the pine rocklands.

Scutellaria is a genus of plants containing multiple species with well-documented medicinal effects. S. baicalensis and S. barbata are among the best-studied Scutellaria species, and previous works have established flavones to be the primary source of their bioactivity. Recent genomic and biochemical studies with S. baicalensis and S. barbata have advanced our understanding of flavone biosynthesis in Scutellaria . However, as over several hundreds of Scutellaria species occur throughout the world, flavone biosynthesis in most species remains poorly understood. In this study, we analyzed organ-specific flavone profiles of seven Scutellaria species, including S. baicalensis, S. barbata and two species native to the Americas ( S. wrightii to Texas and S. racemosa to Central and South America). We found that the roots of almost all these species produce only 4’-deoxyflavones, while 4’-hydroxyflavones are accumulated exclusively in their aerial parts. On the other hand, S. racemosa and S. wrightii also accumulated high levels of 4’-deoxyflavones in their aerial parts, different with the flavone profiles of S. baicalensis and S. barbata . Furthermore, our metabolomics and NMR study identified the accumulation of isoscutellarein 8- O -β-glucuronopyranoside, a rare 4’-hydroxyflavone, in the stems and leaves of several Scutellaria species including S. baicalensis and S. barbata , but not in S. racemosa and S. wrightii . Distinctive organ-specific metabolite profiles among Scutellaria species indicate the selectivity and diverse physiological roles of flavones.

Two natural auxins, phenylacetic acid (PAA) and indole-3-acetic acid (IAA), play crucial roles in plant growth and development. One route of IAA biosynthesis uses the glucosinolate intermediate indole-3-acetaldoxime (IAOx) as a precursor, which is thought to occur only in glucosinolate-producing plants in Brassicales. A recent study showed that overproducing phenylacetaldoxime (PAOx) in Arabidopsis increases PAA production. However, it remains unknown whether this increased PAA resulted from hydrolysis of PAOx-derived benzyl glucosinolate or, like IAOx-derived IAA, is directly converted from PAOx. If glucosinolate hydrolysis is not required, aldoxime-derived auxin biosynthesis may occur beyond Brassicales. To better understand aldoxime-derived auxin biosynthesis, we conducted an isotope-labelled aldoxime feeding assay using an Arabidopsis glucosinolate-deficient mutant sur1 and maize, and transcriptomics analysis. Our study demonstrated that the conversion of PAOx to PAA does not require glucosinolates in Arabidopsis. Furthermore, maize produces PAA and IAA from PAOx and IAOx, respectively, indicating that aldoxime-derived auxin biosynthesis also occurs in maize. Considering that aldoxime production occurs widely in the plant kingdom, aldoxime-derived auxin biosynthesis is likely to be more widespread than originally believed. A genome-wide transcriptomics study using PAOx-overproduction plants identified complex metabolic networks among IAA, PAA, phenylpropanoid and tryptophan metabolism.

Premise When plants are exposed to stress conditions, irreversible damage can occur, negatively impacting yields. It is therefore important to detect stress symptoms in plants, such as the accumulation of anthocyanin, as early as possible. Methods and Results Twenty‐two regression models in five color spaces were trained to develop a prediction model for plant anthocyanin levels from digital color imaging data. Of these, a quantile random forest regression model trained with standard red, green, blue ( sRGB ) color space data most accurately predicted the actual anthocyanin levels. This model was then used to noninvasively monitor the spatial and temporal accumulation of anthocyanin in Arabidopsis thaliana leaves. Conclusions The digital imaging–based nature of this protocol makes it a low‐cost and noninvasive method for the detection of plant stress. Applying a similar protocol to more economically viable crops could lead to the development of large‐scale, cost‐effective systems for monitoring plant health.