An Empirical Model for Leaf Expansion in Cacao in Relation to Plant Water Deficit

Cacao (Theobroma cacao L.) seedlings at the onset of a flushcycle were exposed to five different irrigation treatments.The expansion of all leaves in an emerging flush was followed,and estimates of leaf water potential(_w) were made on each daythat leaf areas were measured. The growth in area of the leaveswas fitted with a modified logistic curve of the form y = a/[1+ be ^–(ct+dt2)], and parameters of leaf growth were derivedfrom the fitted constants. A coefficient of stress exposure,S, was derived as the slope of the relation between cumulative_w and time. Three parameters of the logisitc function (a, cand d) were strongly associated with S. By fitting regressionsfor the relations between S and estimates of each parameterfor the five treatments, values of a, b, c and d may be estimatedat any level of S encompassed by these data. The effects ofwater stress on leaf expansion rate and on the final leaf areaattained by the emerging flush can be adequately predicted bythis technique. Theobroma cacao L.cacao, leaf expansion, logistic model, water stress     

Visualization of novel virulence activities of the Xanthomonas type III effectors AvrBs1, AvrBs3 and AvrBs4

Xanthomonas campestris pv. vesicatoria secretes at least 20 effector proteins through the type III secretion system directly into plant cells. In this study, we uncovered virulence activities of the effector proteins AvrBs1, AvrBs3 and AvrBs4 using Agrobacterium‐mediated transient expression of the corresponding genes in Nicotiana benthamiana, followed by microscopic analyses. We showed that, in addition to the nuclear‐localized AvrBs3, the effector AvrBs1, which localizes to the plant cell cytoplasm, also induces a morphological change in mesophyll cells. Comparative analyses revealed that avrBs3‐expressing plant cells contain highly active nuclei. Furthermore, plant cells expressing avrBs3 or avrBs1 show a decrease in the starch content in chloroplasts and an increased number of vesicles, indicating an enlargement of the central vacuole and the cell wall. Both AvrBs1 and AvrBs3 cause an increased ion efflux when expressed in N. benthamiana. By contrast, expression of the avrBs3 homologue avrBs4 leads to large catalase crystals in peroxisomes, suggesting a possible virulence function of AvrBs4 in the suppression of the plant defence responses. Taken together, our data show that microscopic inspection can uncover subtle and novel virulence activities of type III effector proteins.