The polysaccharide xyloglucan is thought to play an important structural role in the primary cell wall of dicotyledons. Accordingly, there is considerable interest in understanding the biochemical basis and regulation of xyloglucan metabolism, and research over the last 16 years has identified a large family of cell wall proteins that specifically catalyze xyloglucan endohydrolysis and/or endotransglucosylation. However, a confusing and contradictory series of nomenclatures has emerged in the literature, of which xyloglucan endotransglycosylases (XETs) and endoxyloglucan transferases (EXGTs) are just two examples, to describe members of essentially the same class of genes/proteins. The completion of the first plant genome sequencing projects has revealed the full extent of this gene family and so this is an opportune time to resolve the many discrepancies in the database that include different names being assigned to the same gene. Following consultation with members of the scientific community involved in plant cell wall research, we propose a new unifying nomenclature that conveys an accurate description of the spectrum of biochemical activities that cumulative research has shown are catalyzed by these enzymes. Thus, a member of this class of genes/proteins will be referred to as a xyloglucan endotransglucosylase/hydrolase (XTH). The two known activities of XTH proteins are referred to enzymologically as xyloglucan endotransglucosylase (XET, which is hereby re-defined) activity and xyloglucan endohydrolase (XEH) activity. This review provides a summary of the biochemical and functional diversity of XTHs, including an overview of the structure and organization of the Arabidopsis XTH gene family, and highlights the potentially important roles that XTHs appear to play in numerous examples of plant growth and development.

No abstract

Contents  Summary 1 Introduction 2 Fast and fascinating – thigmonasty and thigmotropism 2 Subcellular touch‐induced movements 10 Thigmomorphogenesis 10 Discovery of the Arabidopsis TCH genes 12 Microarray identification of touch‐inducible genes 12 Regulation of TCH gene expression 13  Acknowledgements 14  References 14 Summary Perception and response to mechanical stimuli are likely essential at the cellular and organismal levels. Elaborate and impressive touch responses of plants capture the imagination as such behaviors are unexpected in otherwise often quiescent creatures. Touch responses can turn plants into aggressors against animals, trapping and devouring them, and enable flowers to be active in ensuring crosspollination and shoots to climb to sunlit heights. Morphogenesis is also influenced by mechanical perturbations, including both dynamic environmental stimuli, such as wind, and constant forces, such as gravity. Even individual cells must sense turgor and wall integrity, and subcellular organelles can translocate in response to mechanical perturbations. Signaling molecules and hormones, including intracellular calcium, reactive oxygen species, octadecanoids and ethylene, have been implicated in touch responses. Remarkably, touch‐induced gene expression is widespread; more than 2.5% of Arabidopsis genes are rapidly up‐regulated in touch‐stimulated plants. Many of these genes encode calcium‐binding, cell wall modifying, defense, transcription factor and kinase proteins. With these genes as tools, molecular genetic methods may enable elucidation of mechanisms of touch perception, signal transduction and response regulation.

No abstract

Diverse life forms have evolved internal clocks enabling them to monitor time and thereby anticipate the daily environmental changes caused by Earth's rotation. The plant circadian clock regulates expression of about one-third of the Arabidopsis genome, yet the physiological relevance of this regulation is not fully understood. Here we show that the circadian clock, acting with hormone signals, provides selective advantage to plants through anticipation of and enhanced defense against herbivory. We found that cabbage loopers (Trichoplusia ni) display rhythmic feeding behavior that is sustained under constant conditions, and plants entrained in light/dark cycles coincident with the entrainment of the T. ni suffer only moderate tissue loss due to herbivory. In contrast, plants entrained out-of-phase relative to the insects are significantly more susceptible to attack. The in-phase entrainment advantage is lost in plants with arrhythmic clocks or deficient in jasmonate hormone; thus, both the circadian clock and jasmonates are required. Circadian jasmonate accumulation occurs in a phase pattern consistent with preparation for the onset of peak circadian insect feeding behavior, providing evidence for the underlying mechanism of clock-enhanced herbivory resistance. Furthermore, we find that salicylate, a hormone involved in biotrophic defense that often acts antagonistically to jasmonates, accumulates in opposite phase to jasmonates. Our results demonstrate that the plant circadian clock provides a strong physiological advantage by performing a critical role in Arabidopsis defense.circadian rhythm | diurnal rhythm | plant-herbivore interaction | jasmonic acid | salicylic acid I n the battle between plant host and herbivore, plants appear to be disadvantaged due to their relative immobility because once herbivorous insects attack, plants cannot escape by relocating. However, it is well-documented that plants can detect attack by insect herbivores and, in response, activate defense responses (1). These defense responses include accumulation of proteins and compounds that are toxic to and/or act to deter feeding of herbivores, thereby reducing herbivore performance and increasing plant resistance. Jasmonate hormones are critical for plant herbivore defense (1-3) and for the regulation of both herbivore and wound-responsive gene expression (4).Remarkably, many genes regulated in expression by wounding also have strong circadian regulation. Over 40% of genes whose expression is induced by wounding have peak circadian expression at subjective dusk, and over 80% of genes down-regulated by wounding have peak expression at subjective dawn (5). However, it remained to be determined whether this circadian regulation of wound-inducible genes enables plants to anticipate herbivore attack through a cyclical activation of defense response. Here we demonstrate that Arabidopsis plants that are entrained such that their subjective day is in-phase with Trichoplusia ni subjective day have increased resistance to herbivory. In contrast,...