The PHD finger of Arabidopsis SIZ1 recognizes trimethylated histone H3K4 mediating SIZ1 function and abiotic stress response.

Arabidopsis SIZ1 encodes a SUMO E3 ligase to regulate abiotic and biotic stress responses. Among SIZ1 or mammalian PIAS orthologs, plant SIZ1 proteins contain the plant homeodomain (PHD) finger, a C4HC3 zinc finger. Here, we investigated the importance of PHD of Arabidopsis SIZ1. The ProSIZ1::SIZ1(ΔPHD):GFP was unable to complement growth retardation, ABA hypersensitivity, and the cold-sensitive phenotype of the siz1 mutant, but ProSIZ1::SIZ1:GFP could. Substitution of C162S in the PHD finger was unable to complement the siz1 mutation. Tri-methylated histone H3K4 (H3K4me3) was recognized by PHD, not by PHD(C162S). WRKY70 was up-regulated in the siz1-2 mutant and H3K4me3 accumulated at high levels in the WRKY70 promoter. PHD interacts with ATX, which mediates methylation of histone, probably leading to suppression of ATX's function. These results suggest that the PHD finger of SIZ1 is important for recognition of the histone code and is required for SIZ1 function and transcriptional suppression.

Efficient transient protein expression in tomato cultivars and wild species using agroinfiltration-mediated high expression system.

KEY MESSAGE: The new transient protein expression system using the pBYR2HS vector is applicable to several tomato cultivars and wild species with high level of protein expression. Innovation and improvement of effective tools for transient protein expression in plant cells is critical for the development of plant biotechnology. We have created the new transient protein expression system using the pBYR2HS vector that led to about 4 mg/g fresh weight of protein expression in Nicotiana benthamiana. In this study, we validated the adaptability of this transient protein expression system by agroinfiltration to leaves and fruits of several tomato cultivars and wild species. Although the GFP protein was transiently expressed in the leaves and fruits of all tomato cultivars and wild species, we observed species-specific differences in protein expression. In particular, GFP protein expression was higher in the leaves and fruits of Micro-Tom, Solanum pimpinellifolium (0043) and S. pimpinellifolium (0049-w1) than in those of cultivars and wild species. Furthermore, Agrobacterium with GABA transaminase enhanced transient expression in tomato fruits of Micro-Tom. Taken together with these results, our system is applicable to several tomato cultivars and species as well as a model tomato, even though characteristics are often different among tomato cultivars or species. Thus, the system is an effective, simple, and valuable tool to achieve rapid transgene expression to examine gene function in tomato plant cells.

MYC-type transcription factors, MYC67 and MYC70, interact with ICE1 and negatively regulate cold tolerance in Arabidopsis.

The expression of hundreds of genes is induced by low temperatures via a cold signaling pathway. ICE1, a MYC-type transcription factor, plays an important role in the induction of CBF3/DREB1A to control cold-responsive genes and cold tolerance. To elucidate other molecular factors, a yeast 2-hybrid screening was performed. Two MYC-type transcription factors, MYC67 and MYC70, were identified as ICE1-interacting proteins. The myc mutants were more tolerant to freezing temperatures than wild type. CBF3/DREB1A and other cold-responsive genes were up-regulated in the myc mutants. Overexpression of the MYC genes increased the cold sensitivity and down-regulated the expression of cold-responsive genes. The MYC proteins interacted with the cis-elements in the CBF3/DREB1A promoter, probably to interfere interaction between ICE1 and the cis-elements. Taken together, these results demonstrate that MYC67 and MYC70, ICE1 interactors, negatively regulate cold-responsive genes and cold tolerance.

Ca2+-permeable mechanosensitive channels MCA1 and MCA2 mediate cold-induced cytosolic Ca2+ increase and cold tolerance in Arabidopsis.

Cold shock triggers an immediate rise in the cytosolic free calcium concentration ([Ca2+]cyt) in Arabidopsis thaliana and this cold-induced elevation of [Ca2+]cyt is inhibited by lanthanum or EGTA. It is suggested that intracellular calcium mainly contributes to the cold-induced [Ca2+]cyt response by entering into the cytosol. Two calcium-permeable mechanosensitive channels, MCA1 and MCA2 (mid1-complementing activity), have been identified in Arabidopsis. Here, we demonstrate that MCA1 and MCA2 are involved in a cold-induced increase in [Ca2+]cyt. The cold-induced [Ca2+]cyt increase in mca1 and mca2 mutants was markedly lower than that in wild types. The mca1 mca2 double mutant exhibited chilling and freezing sensitivity, compared to wild-type plants. Expression of At5g61820, At3g51660, and At4g15490, which are not regulated by the CBF/DREB1s transcription factor, was down-regulated in mca1 mca2. These results suggest that MCA1 and MCA2 are involved in the cold-induced elevation of [Ca2+]cyt, cold tolerance, and CBF/DREB1-independent cold signaling.