LIBRO-CONGRESO-CITRUS

S07O01
Citrus developmental research: a historic, conceptual perspective
Goldschmidt E.E.
The Hebrew University of Jerusalem, The Robert H. Smith Faculty of Agriculture, Food and Environment, Israel.
goldsmit@agri.huji.ac.il
As long as microscopes were the major research tool, developmental anatomy and morphology led
the way. But then, towards the middle of the 20th century, horticulturists were enchanted by the plant
hormone concept. The hormonal thinking dominated citrus developmental physiology for a long time;
flowering, fruit development and ripening, abscission, rooting and vegetative growth were all attributed
to one, specific plant hormone or to the perplexing ‘hormonal balance’ concept. The synthetic growth
regulators were exploited in every possible way to solve all practical problems. A major representative of
this ‘Hormonal’ era was the renown Israeli citrus researcher S.P. Monselise (1920-1986). The emergence
of the source-sink concept (ca. 1980) partially replaced the hormonal hypothesis. The source-sink
hypothesis claims that carbohydrate levels control the major stages of the fruiting process –flowering, fruit
set and fruit enlargement– and are, therefore, responsible also for the ‘alternate bearing’ phenomenon.
Although the effects of girdling and fruit thinning seem to support the sink-source concept, direct control
of flowering and fruit set by carbohydrate levels was not unequivocally demonstrated. The Spanish
researcher J.L. Guardiola (1943-2010) played a major role in this debate. But then, rather soon, came
the molecular-genetic era, forcing us all to acquire a new language and follow its logic. Citrus research is
still a few steps behind Arabidopsis but, here we are, still doing our best in order to solve the remaining
mysteries of citrus.
S07O02
Stock-scion interactions in grafted citrus: a role for microRNA
Tzarfati R. 1 , Ben-Dor S. 2 , Sela I. 1 , and Goldschmidt E.E. 1
1 The Hebrew University of Jerusalem, Institute of Plant Science and Genetics in Agriculture, Israel; and 2 The Weizmann Institute,
Biological Services, Israel. goldsmit@agri.huji.ac.il
Grafting is a widely used plant propagation technique in horticultural crops, but the physiological basis
of stock-scion interactions is as yet, incompletely understood. Recent studies indicate that movement of
proteins and small RNAs through the graft union might be involved. MicroRNAs (miRNAs) are known to
play a significant role in regulation of higher plants’ developmental and metabolic traits. We would like
to hypothesize that changes in expression of mRNAs play a role in mediating the effects of grafting. The
objective of the present study was to examine this hypothesis in citrus. We have determined the expression
of a broad range of mRNAs in citrus leaf petioles, as affected by grafting. Four stock/scion combinations
(‘Merav’ mandarin and ‘Star Ruby’ grapefruit scions X ‘Troyer’ and ‘Volkamer’ rootstocks), rootstock autografts
and non-grafted rootstock controls were examined. Microarray comparisons of mRNA expression
revealed significant differences in major as well as minor mRNAs. Grafting caused a dramatic reduction
in the expression of the major miR156 (and miR157) which appear to be associated with reduction of
juvenility in woody plants. Expression of miR894 also declined after grafting. Differential rootstock and
scion-dependent expression of the minor miR397 was found in petioles of grafted scions, while in nongrafted
rootstocks, the expression of miR397 was barely detectable. Bioinformatic analysis confirmed the
presence of miR397 in the citrus genome, validated its sequence and demonstrated its ability to form
a stem loop. The differences in miR397 expression might be related to copper and other micronutrient
requirements of citrus stock-scion combinations. Thus, our results support the hypothesis, suggesting an
involvement of specific mRNAs in engendering physiological effects of grafting. The precise mechanism
remains to be elucidated.
Supported by funds of the Israeli Citrus Marketing Board.
XII INTERNATIONAL CITRUS CONGRESS 2012 - 107
S07