Transgenic cassava plants carrying an ipt gene from Agrobacterium controlled by a senescence promoter from Arabidopsis retained leaves longer under field conditions compared to controls. While the transgenic plants had higher foliage weight due to increased leaf size and longer petioles, their harvest index and root dry matter content were reduced. This suggests retaining leaves longer impacted yield traits. The transgenic plants also had higher levels of zeatin in senescing leaves and ABA in green tissues, likely due to altered carotenoid metabolism and transport. Further analysis is needed to better understand expression of the senescence promoter in cassava and effects of delayed leaf senescence under drought stress conditions.

1. Transgenic plants carrying an Isopentenyl Transferase (ipt) gene of Agrobacterium, under the control of a senescence-
ipt) Agrobacterium, senescence-
inducible promoter from Arabidopsis, increased foliage under field conditions
Arabidopsis,
López D1, Beltrán J1, Ceballos H1, Duque MC1, Zhang P2, Gruissem W3, Duque L4, Chavarriaga P1 and Tohme J1
Beltrá
1. Conservation and Use of Tropical Genetic Resources, CIAT, AA 6713 , Cali, Colombia. 2. Chinese Academy of Sciences.
6713 Colombia.
3. ETH-Zurich. 4. Cornell University
ETH-
INTRODUCTION Plant Quantitative Traits Control 60444(529-28)
60444(529-
Phenotypic characterization of transgenic cassava clones is vital for Plant height (cm) 251.5 255.7
breeding since many of the original traits, which are not targeted for Height first branch (cm) ** 84.3 102.5
transformation, may change during the transformation process. We Branching levels ** 3.4 3.1
performed an agronomical evaluation of the clone 60444(529-28) that was Branching angle* 36.6°
36.6° 35.1°
35.1°
transformed with an ipt gene (Isopentenyl Transferase), driven by the
Weight of aerial part (kg) 3.7 3.6
senescence-inducible promoter SAG12 from A. thaliana. Among other
characters, we wanted to evaluate the ability of this clone to retain leaves Root weight (kg/plant) 3.53 2.58
under standard field conditions, and the effect that leaf retention might Harvest index (%) ** 48.6 41.2
have on root yield indicators. Root dry matter (%) ** 42.1 40.6
We observed quantitative differences between transgenic and controls. HCN (>180ppm = bitter) 182.0 192.8
Although the transgenics had increased foliage weight, their Harvest Index Foliage weight ** 0.39 kg 0.61 kg
and Dry Matter Content was reduced, suggesting that retaining leaves Foliage proportion (%) 24.1 25.7
longer has an impact on yield traits. Amylose (%) * 20.8 20.1
MATERIALS AND METHODS *α ≤ 0.05 **α ≤ 0.01
The transgenic clone was produced by ETH-Zurich using the construct Zeatin and ABA Content. The content of zeatin was higher in
PSAG12-ipt (below), and planted in a CRBD (10 plants/clone/4 senescing leaves of the transgenic. ABA content was also higher in green
environments, below) in CIAT’s fields between September/2006 and leaves and stems of the transgenic possibly due to higher rates of
July/2007. We measured 39 agronomical descriptors (Fukuda 1998), carotene synthesis, catabolism and translocation.
including zeatin and ABA quantification (performed in Cornell
Univesrity), and checked the expression of the gene ipt by real time PCR in 60444 Control 60444 (529-28)
(529-
leaves of different senescing stages. Data were analyzed using SAS. Tissue Zeatin ABA Zeatin ABA
pmol/cm2 pmol/cm2 pmol/cm2 pmol/cm2
PSAG12-ipt Green leaf 3.60 4.37 * 3.44 7.57 *
Senescing leaves 1.56 ** 6.17 2.75 ** 4.94
Abscision point 0.20 0.20 0.21 0.33
Stem 0.37 0.36 ** 0.34 0.57 **
RESULTS *α ≤ 0.05 **α ≤ 0.01
Agro-morphological Traits. There were no differences between
clones for the 24 qualitative descriptors measured (i.e., leaf and petiole
SAG12 Specificity in cassava. Expression of the gene was detected
color, leaf-lobe width, length and shape, among others). Among the
in green and senescent leaves (G, HS, S), steam (St) and roots (R),
quantitative descriptors (Tables) assumed to be influenced by the
indicating that the promoter pSAG12 may not be tightly regulated in
expression of the ipt gene, foliage weight -the weight of all attached leaves
cassava.
at harvest, including petioles- was significantly higher in the transgenic
clone.
S (Senescent) HS (Half Senescent) G (Green) 1kb G HS S G HS S NT NT NT P H2O
Having more and/or bigger control 6044(529-28)
6044(529-
leaves did not translate
necessarily into higher dry
60444(529-28)
matter content, or harvest 1Kb G HS St R H2O NT P
index, two characters
measured as indicators of
root yield. The higher
weight of foliage in the
transgenic line could have CONCLUSIONS AND PERSPECTIVES
been partially explained by
the longer petioles and/or
wider lobes of leaves. •The transgenic clone 60444(529-28) retained its leaves longer, 14
additional days on average, so increasing foliage weight. Also the width
of its leaf-lobes and the length of petioles were higher, elevating foliage
Leaf trait Control 60444(529-28)
60444(529- mass. The expression of PSAG12-ipt may explain these findings while
Number of lobes * 7 ( 53%) or 9 ( 47%) 7 (66%) or 9 (34%) explaining, at the same time, a higher content of zeatin in senescent
Length of central lobe leaves. However, apparently, keeping the leaves attached longer resulted
19.5 19.5 in a loss of root dry matter for this particular clone, under these
(cm)
experimental conditions.
Width of central lobe
5.0 5.4
(cm) ** •We are analyzing in detail PSAG12-ipt expression to better understand
Lobe length/width ** 3.8 3.6 its onset and specificity under field conditions.
Petiole length ** 30.5 32.0 •Testing 60444(529-28) under severe drought conditions may enlighten
the role of PSAG12-ipt to reduce root dry matter loss under water stress.
*α ≤ 0.05 **α ≤ 0.01
Date prepared July 2008