An extremely low stomatal density mutant overcomes cooling limitations at supra-optimal temperature by adjusting stomatal size and leaf thickness

Diverse projected climate change scenarios affect the physiology of broccoli plants to different extents

Climate change caused by global warming involves crucial plant growth factors such as atmospheric CO2 concentration, ambient temperature or water availability. These stressors usually co-occur, causing intricate alterations in plant physiology and development. This work focuses on how elevated atmospheric CO2 levels, together with the concomitant high temperature, would affect the physiology of a relevant crop, such as broccoli. Particular attention has been paid to those defence mechanisms that contribute to plant fitness under abiotic stress. Results show that both photosynthesis and leaf transpiration were reduced in plants grown under climate change environments compared to those grown under current climate conditions. Furthermore, an induction of carbohydrate catabolism pointed to a redistribution from primary to secondary metabolism. This result could be related to a reinforcement of cell walls, as well as to an increase in the pool of antioxidants in the leaves. Broccoli plants, a C3 crop, grown under an intermediate condition showed activation of those adaptive mechanisms, which would contribute to coping with abiotic stress, as confirmed by reduced levels of lipid peroxidation relative to current climate conditions. On the contrary, the most severe climate change scenario exceeded the adaptive capacity of broccoli plants, as shown by the inhibition of growth and reduced vigour of plants. In conclusion, only a moderate increase in atmospheric CO2 concentration and temperature would not have a negative impact on broccoli crop yields.

Singlet oxygen induces cell wall thickening and stomatal density reducing by transcriptome reprogramming

Singlet oxygen (1O2) has a very short half-life of 10-5 s; however, it is a strong oxidant that causes growth arrest and necrotic lesions on plants. Its signaling pathway remains largely unknown. The Arabidopsis flu (fluorescent) mutant accumulates a high level of 1O2 and shows drastic changes in nuclear gene expression. Only two plastid proteins, EX1 (executer 1) and EX2 (executer 2), have been identified in the singlet oxygen signaling. Here, we found that the transcription factor abscisic acid insensitive 4 (ABI4) binds the promoters of genes responsive to 1O2-signals. Inactivation of the ABI4 protein in the flu/abi4 double mutant was sufficient to compromise the changes of almost all 1O2-responsive-genes and rescued the lethal phenotype of flu grown under light/dark cycles, similar to the flu/ex1/ex2 triple mutant. In addition to cell death, we reported for the first time that 1O2 also induces cell wall thickening and stomatal development defect. Contrastingly, no apparent growth arrest was observed for the flu mutant under normal light/dim light cycles, but the cell wall thickening (doubled) and stomatal density reduction (by two-thirds) still occurred. These results offer a new idea for breeding stress tolerant plants.

Light and temperature regulation of leaf morphogenesis in Arabidopsis

Leaves are the main photosynthetic organs in plants, and their anatomy is optimized for light interception and gas exchange. Although each species has a characteristic leaf anatomy, which depends on the genotype, leaves also show a large degree of developmental plasticity. Light and temperature regulate leaf development from primordia differentiation to late stages of blade expansion. While the molecular mechanisms of light and temperature signaling have been mostly studied in seedlings, in the latest years, research has focused on leaf development. Here, I will describe the latest work carried out in the environmental regulation of Arabidopsis leaf development, comparing signaling mechanisms between leaves and seedlings, highlighting the new discoveries, and pointing out the most exciting open questions.

Growth Light Quality Influences Leaf Surface Temperature by Regulating the Rate of Non-Photochemical Quenching Thermal Dissipation and Stomatal Conductance

Significant efforts have been made to optimise spectrum quality in indoor farming to maximise artificial light utilisation and reduce water loss. For such an improvement, green (G) light supplementation to a red-blue (RB) background was successfully employed in our previous studies to restrict both non-photochemical quenching (NPQ) and stomatal conductance (gs). At the same time, however, the downregulation of NPQ and gs had the opposite influence on leaf temperature (Tleaf). Thus, to determine which factor plays the most prominent role in Tleaf regulation and whether such a response is temporal or permanent, we investigated the correlation between NPQ and gs and, subsequently, Tleaf. To this end, we analysed tomato plants (Solanum lycopersicum L. cv. Malinowy Ozarowski) grown solely under monochromatic LED lamps (435, 520, or 662 nm; 80 µmol m-2 s-1) or a mixed RGB spectrum (1:1:1; 180 µmol m-2 s-1) and simultaneously measured gs and Tleaf with an infrared gas analyser and a thermocouple or an infrared thermal camera (FLIR) during thermal imaging analyses. The results showed that growth light quality significantly modifies Tleaf and that such a response is not temporal. Furthermore, we found that the actual adaxial leaf surface temperature of plants is more closely related to NPQ amplitude, while the temperature of the abaxial surface corresponds to gs.

Complexity of SMAX1 signaling during seedling establishment

Karrikins (KARs) are small butenolide compounds identified in the smoke of burning vegetation. Along with the stimulating effects on seed germination, KARs also regulate seedling vigor and adaptive behaviors, such as seedling morphogenesis, root hair development, and stress acclimation. The pivotal KAR signaling repressor, SUPPRESSOR OF MAX2 1 (SMAX1), plays central roles in these developmental and morphogenic processes through an extensive signaling network that governs seedling responses to endogenous and environmental cues. Here, we summarize the versatile roles of SMAX1 reported in recent years and discuss how SMAX1 integrates multiple growth hormone signals into optimizing seedling establishment. We also discuss the evolutionary relevance of the SMAX1-mediated signaling pathways during the colonization of aqueous plants to terrestrial environments.

FOUR LIPS plays a role in meristemoid-to-GMC fate transition during stomatal development in Arabidopsis

Meristemoids, which are stomatal precursor cells, exhibit self-renewal and differentiation abilities. However, the only known core factor associated with meristemoid division termination and fate transition is the heterodimer formed by the basic helix-loop-helix proteins MUTE and SCREAMs (SCRMs). FOUR LIPS (FLP), a well-known transcription factor that restricts guard mother cell (GMC) division, is a direct target of MUTE. Whether FLP involves in meristemoid differentiation is unknown. Through sensitized genetic screening of flp-1, we identified a mute-like (mutl) mutant with arrested meristemoids. The mutant carried a novel allele of the MUTE locus, i.e., mute-4. Intriguingly, mute-4 is a hypomorphic allele that exhibits wild-type appearance with slightly delayed meristemoid-to-GMC transition, whereas it renders an unexpected mutl epidermis with most meristemoids arrested and very few stomata when combined with flp (flp mute-4), suggesting that FLP is a positive regulator during this transition process. Consistently, the expression of FLP increased during GMC commitment, and the number of cells at this stage was markedly increased in flp. flp scrm double mutants produced arrested meristemoids similar to mute, and FLP was able to interact physically with SCRM. Taken together, our results demonstrate that FLP functions together with MUTE and SCRMs to direct meristemoid-to-GMC fate transition.