The subtilisin-like serine protease SDD1 mediates cell-to-cell signaling during Arabidopsis stomatal development

Wild-type stomata are distributed nonrandomly, and their density is controlled by endogenous and exogenous factors. In the Arabidopsis mutant stomatal density and distribution1-1 (sdd1-1), the establishment of the stomatal pattern is disrupted, resulting in stomata clustering and twofold to fourfold increases in stomatal density. The SDD1 gene that encodes a subtilisin-like Ser protease is expressed strongly in stomatal precursor cells (meristemoids and guard mother cells), and the SDD1 promoter is controlled negatively by a feedback mechanism. The encoded protein is exported to the apoplast and probably is associated with the plasma membrane. SDD1 overexpression in the wild type leads to a phenotype opposite to that caused by the sdd1-1 mutation, with a twofold to threefold decrease in stomatal density and the formation of arrested stomata. While SDD1 overexpression was effective in the flp mutant, the tmm mutation acted epistatically. Thus, we propose that SDD1 generates an extracellular signal by meristemoids/guard mother cells and demonstrate that the function of SDD1 is dependent on TMM activity.     

Oriented asymmetric divisions that generate the stomatal spacing pattern in arabidopsis are disrupted by the too many mouths mutation

Wild-type stomata are spaced by intervening cells, a pattern disrupted in the Arabidopsis mutant too many mouths (tmm). To determine the mechanism of wild-type spacing and how tmm results in pattern violations, we analyzed the behavior of cells through time by using sequential dental resin impressions. Meristemoids are stomatal precursors produced by asymmetric division. We show that wild-type patterning largely results when divisions next to a preexisting stoma or precursor are oriented so that the new meristemoid is placed away. Because this placement is independent of cell lineage, these divisions may be oriented by cell-cell signaling. tmm randomizes this orientation and releases a prohibition on asymmetric division in cells at specific locations, resulting in stomatal clusters. TMM is thus necessary for two position-dependent events in leaves: the orientation of asymmetric divisions that pattern stomata, and the control of which cells will enter the stomatal pathway. In addition, our findings argue against most previous hypotheses of wild-type stomatal patterning.     

Cytosolic invertases contribute to cellulose biosynthesis and influence carbon partitioning in seedlings of Arabidopsis thaliana

In plants, UDP‐glucose is the direct precursor for cellulose biosynthesis, and can be converted into other NDP‐sugars required for the biosynthesis of wall matrix polysaccharides. UDP‐glucose is generated from sucrose by two distinct metabolic pathways. The first pathway is the direct conversion of sucrose to UDP‐glucose and fructose by sucrose synthase. The second pathway involves sucrose hydrolysis by cytosolic invertase (CINV), conversion of glucose to glucose‐6‐phosphate and glucose‐1‐phosphate, and UDP‐glucose generation by UDP‐glucose pyrophosphorylase (UGP). Previously, Barratt et al. (Proc. Natl Acad. Sci. USA, 106, 2009 and 13124) have found that an Arabidopsis double mutant lacking CINV1 and CINV2 displayed drastically reduced growth. Whether this reduced growth is due to deficient cell wall production caused by limited UDP‐glucose supply, pleiotropic effects, or both, remained unresolved. Here, we present results indicating that the CINV/UGP pathway contributes to anisotropic growth and cellulose biosynthesis in Arabidopsis. Biochemical and imaging data demonstrate that cinv1 cinv2 seedlings are deficient in UDP‐glucose production, exhibit abnormal cellulose biosynthesis and microtubule properties, and have altered cellulose organization without substantial changes to matrix polysaccharide composition, suggesting that the CINV/UGP pathway is a key metabolic route to UDP‐glucose synthesis in Arabidopsis. Furthermore, differential responses of cinv1 cinv2 seedlings to exogenous sugar supplementation support a function of CINVs in influencing carbon partitioning in Arabidopsis. From these data and those of previous studies, we conclude that CINVs serve central roles in cellulose biosynthesis and carbon allocation in Arabidopsis.     

Cryptochromes,Phytochromes, and COP1 Regulate Light-Controlled Stomatal Development in Arabidopsis

In Arabidopsis thaliana, the cryptochrome (CRY) blue light photoreceptors and the phytochrome (phy) red/far-red light photoreceptors mediate a variety of light responses. COP1, a RING motif–containing E3 ubiquitin ligase, acts as a key repressor of photomorphogenesis. Production of stomata, which mediate gas and water vapor exchange between plants and their environment, is regulated by light and involves phyB and COP1. Here, we show that, in the loss-of-function mutants of CRY and phyB, stomatal development is inhibited under blue and red light, respectively. In the loss-of-function mutant of phyA, stomata are barely developed under far-red light. Strikingly, in the loss-of-function mutant of either COP1 or YDA, a mitogen-activated protein kinase kinase kinase, mature stomata are developed constitutively and produced in clusters in both light and darkness. CRY, phyA, and phyB act additively to promote stomatal development. COP1 acts genetically downstream of CRY, phyA, and phyB and in parallel with the leucine-rich repeat receptor-like protein TOO MANY MOUTHS but upstream of YDA and the three basic helix-loop-helix proteins SPEECHLESS, MUTE, and FAMA, respectively. These findings suggest that light-controlled stomatal development is likely mediated through a crosstalk between the cryptochrome-phytochrome-COP1 signaling system and the mitogen-activated protein kinase signaling pathway.     

KOBITO1 encodes a novel plasma membrane protein necessary for normal synthesis of cellulose during cell expansion in Arabidopsis

The cell wall is the major limiting factor for plant growth. Wall extension is thought to result from the loosening of its structure. However, it is not known how this is coordinated with wall synthesis. We have identified two novel allelic cellulose-deficient dwarf mutants, kobito1-1 and kobito1-2 (kob1-1 and kob1-2). The cellulose deficiency was confirmed by the direct observation of microfibrils in most recent wall layers of elongating root cells. In contrast to the wild type, which showed transversely oriented parallel microfibrils, kob1 microfibrils were randomized and occluded by a layer of pectic material. No such changes were observed in another dwarf mutant, pom1, suggesting that the cellulose defect in kob1 is not an indirect result of the reduced cell elongation. Interestingly, in the meristematic zone of kob1 roots, microfibrils appeared unaltered compared with the wild type, suggesting a role for KOB1 preferentially in rapidly elongating cells. KOB1 was cloned and encodes a novel, highly conserved, plant-specific protein that is plasma membrane bound, as shown with a green fluorescent protein-KOB1 fusion protein. KOB1 mRNA was present in all organs investigated, and its overexpression did not cause visible phenotypic changes. KOB1 may be part of the cellulose synthesis machinery in elongating cells, or it may play a role in the coordination between cell elongation and cellulose synthesis.     

生长素调控植物气孔发育的研究进展

气孔是分布于植物表皮由保卫细胞围成的小孔, 是植物体与外界环境进行水分和气体交换的重要通道, 通过影响光合作用、蒸腾作用及一系列生物学过程来促进植物适应环境的变化。生长素是最早被发现的植物激素, 在植物生长发育中发挥重要作用。近年来的研究表明, 生长素通过载体蛋白-TIR1/AFB受体-AUXIN/IAA-ARFs信号通路, 调控STOMAGEN的表达; 之后, 经STOMAGEN-类LRR受体蛋白激酶ERf-MAPKs级联反应激酶-SPCH转录因子信号通路, 启动气孔的发育进程。EPF1、EPF2和类LRR受体蛋白激酶TMM不是该过程的必需元件。生长素对气孔的调控受光信号影响, 光信号通路组分E3泛素连接酶COP1位于MAPKs激酶的上游, 参与气孔的发育调控。