Water Use and Drought Response in Cultivated and Wild Apples - Abiotic Stress: Plant Responses and Applications in Agriculture - page 256

Numerous studies support the existence of extensive cross-talk between plant hormone
signaling pathways [71-74]. It is therefore expected that both the salicylic acid and Jasmonic
acid (JA)/ethylene pathways indirectly influence the expression of genes that respond to
drought. Along these lines an Arabidopsis mutant ( rcd1 ) belonging to the ADP-(ribosyl)trans‐
ferase domain-containing subfamily of the WWE family exhibits reduced sensitivity to ABA,
ethylene and Me-JA, suggesting that it acts at an integrative node in hormonal signaling
regulating different stress-responsive genes [75]. rcd1 is just one example of many where one
gene participates in multiple pathways.
10. Studies in Malus sieversii
Genetic polymorphisms from twenty populations of M. sieversii in Xinjiang, China were
analyzed with RAPD markers to assess genetic diversity [76]. Based on the bands generated
with 42 randomly chosen primers, variation within a population (83.1%) was higher than
among populations (16.9%). The authors conclude that M. sieversii is a rich source of genetic
diversity.
Evaluation of six Malus species using a variety of morphological and physiological traits led
to the conclusion that M. toringoides and M. sieversii were the most drought tolerant of those
analyszed [77]. Measurement of root parameters indicated that M. sieversii root surface area
decreased in response to drought to ~25% of the well-watered control in contrast to M .
toringoides roots which decreased to ~ 43% of the control. On the other hand, root surface
activity (absorption) declined the least in M. toringoides (17%) compared to seiversii (33%).
When data from all the measurements were taken into consideration, M. toringoides and M.
seiversii were the top two most drought resistant species.
A study of the contribution of rootstock source to drought resistance was conducted using
'Gale Gala' apple scions grafted onto Malus sieversii or Malus hupehensis roots [78]. Differential
responses of the grafted material to drought stress were determined by a number of physio‐
logical and morphological traits. Typical reductions in biomass, growth rate and leaf area are
observed under drought conditions, but M. sieversii showed smaller reductions in these traits
during drought treatment than M. hupehensis . Furthermore, a larger increase in whole plant
WUE was measured in grafts on M. sieversii rootstocks compared to M. hupehensis .
Problem Statement: Many of our agronomically important fruit trees are derived from a rather
narrow genetic base. To provide methods for enhancing apple germplasm resistance to
drought and other dehydrative abiotic stresses it is imperative that we identify those genes
that contribute to drought survival. Once these genes are identified and characterized they can
be used in marker assisted selection strategies or altered by genetic engineering.
Application Area: The origin of the domesticated apple is believed to be in Central Asia via
the silk route through Kazakhstan [79]. The predominant species contributing to the domes‐
tication of the modern apple is believed to be Malus sieversii which is thought to be the
progenitor of M. × domestica and a possible source of resistance alleles lost during domestication